Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

DOI： 10.1109/lpt.2023.3315781

Publishing type：Research paper (scientific journal)   Publisher：AIP Publishing  

Free-space electro-optic (EO) modulators operating at gigahertz and beyond are attractive for a wide range of emerging applications, including high-speed imaging, free-space optical communication, microwave photonics, and diffractive computing. Here, we experimentally demonstrate a high-speed plasmonic metasurface EO modulator operating in a near-infrared wavelength range with a gigahertz modulation bandwidth. To achieve efficient intensity modulation of reflected light from an ultrathin metasurface layer, we utilize the bimodal plasmonic resonance inside a subwavelength metal–insulator–metal grating, which is precisely tuned to satisfy the critical coupling condition. As a result, perfect absorption of −27 dB (99.8%) and a high quality (Q) factor of 113 are obtained at a resonant wavelength of 1650 nm. By incorporating an EO polymer inside the grating, we achieve a modulation depth of up to 9.5 dB under an applied voltage of ±30 V. The 3-dB modulation bandwidth is confirmed to be 1.25 GHz, which is primarily limited by the undesired contact resistance and the output impedance of the driver. Owing to the high electrical conductivity of metallic gratings and a compact device structure with a minimal parasitic capacitance, the demonstrated device can potentially operate at several tens of gigahertz, which opens up exciting opportunities for ultrahigh-speed active metasurface devices in various applications.

DOI： 10.1063/5.0173216

Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

DOI： 10.1109/psc57974.2023.10297153

Publishing type：Research paper (scientific journal)  

In this study, the impact of various growth parameters, such as wafer temperature, device structures, and substrate misorientation, on the quality of InGaP epitaxial layers and solar cells grown in a planetary MOVPE reactor using TBP as a P source were investigated and optimized. Results showed that the carrier lifetime in unintentionally n-doped InGaP significantly decreased as the growth temperature increased from 560 to 590 °C. At temperatures above 600 °C, the growth of InGaP was unsuccessful due to extremely rough surface morphology. The study revealed that InGaP rear homojunction and rear heterojunction solar cells using n-type InGaP base layers displayed improved photo-carrier collection and performance compared to traditional front junction solar cells with p-type InGaP base layers. Notably, InGaP grown on GaAs (001) with a 5° miscut towards (111)A was found to have better quality, while InGaP grown on GaAs (001) with a 6° tilt towards (111)B had worse results in terms of carrier lifetime, PL intensity, and solar cell performance.

DOI： 10.1016/j.solmat.2023.112402

Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

DOI： 10.1109/jlt.2023.3276003

Publishing type：Research paper (scientific journal)  

The p-Al0.35Ga0.65As/n-InGaP heterojunction solar cells are promising competitors compared with conventional InGaP or AlGaAs solar cells as the heterogeneous combination can overcome the demerits of each material. However, InGaP has an optimized growth temperature much lower than that of AlGaAs in metal-organic vapor phase epitaxy (MOVPE) growth. Therefore, a challenge arises from oxygen contamination at the hetero-interface during temperature adjustment for obtaining MOVPE-grown high-quality p-AlGaAs/n-InGaP heterojunction. Here we report that a temperature-graded layer (TGL) of AlGaAs can solve the issue of growth temperature mismatch, and significantly improve the voltage performance of AlGaAs/InGaP solar cells. Absolute electroluminescence measurement and atomic force microscopy confirmed a smooth interface with less non-radiative recombination after TGL was applied. The temperature-graded growth is verified able to improve the AlGaAs/InGaP interface quality and provides a scalable method for AlGaAs-based heterogeneous growth.

DOI： 10.35848/1347-4065/acd9b8

Publishing type：Research paper (scientific journal)   Publisher：Optica Publishing Group  

Polarimetric self-coherent systems using direct-detection-based Stokes-vector receivers (SVRs) are a promising technology to meet both the cost and capacity requirements of short-reach optical interconnects. However, conventional SVRs require a number of optical components to detect the state of polarization at high speed, resulting in substantially more complicated receiver configurations compared with the current intensity-modulation–direct-detection counterparts. Here, we demonstrate a simple and compact polarimetric self-coherent receiver based on a thin dielectric metasurface and a photodetector array (PDA). With a single 1.05-µm-thick metasurface device fabricated on a compact silicon-on-quartz chip, we implement functionalities of all the necessary passive components, including a 1×3 splitter, three polarization beam splitters with different polarization bases, and six focusing lenses. Combined with a high-speed PDA, we demonstrate self-coherent transmission of 20-GBd 16-ary quadrature amplitude modulation and 50-GBd quadrature phase-shift keying signals over a 25-km single-mode fiber. Owing to the surface-normal configuration, it can easily be scaled to receive spatially multiplexed channels from a multicore fiber or a fiber bundle, enabling compact and low-cost receiver modules for future highly parallelized self-coherent systems.

DOI： 10.1364/optica.484318

Publishing type：Research paper (scientific journal)  

It is highly challenging to grow high-quality gallium nitride (GaN) layers on silicon (Si) substrates due to the intrinsic mismatching of their structural and thermal properties. Aluminum nitride (AlN) interlayers have been used to induce a compressive strain to GaN layers during growth, which compensates for the tensile strain in these layers on Si substrates during cooling. In this study, we investigated the effect of the growth temperature and layer structure of the AlN interlayer to understand the relationship between surface flatness and relaxation ratio of the AlN interlayer and the compressive strain in the overlying GaN layer. Low-temperature (LT) growth enhanced lattice relaxation of the AlN interlayer, whereas the AlN surface was atomically flat at high temperature (HT). We also examined a two-step growth to combine the advantages of LT- and HT-AlN. This approach resulted in a surface with multiple flat regions separated by grooves, which had the largest compressive strain in the overlying GaN layer at the early stages of growth. At later stages, the strain was the largest on the HT-AlN interlayer. In both cases, the experimentally measured compressive strain exceeded simulated predictions. Finally, possible solutions for inducing a larger compressive strain in the GaN layer using interlayers were discussed.

DOI： 10.1063/5.0143985

Publishing type：Research paper (international conference proceedings)  

We present a broadband and compact coherent receiver in a surface-normal configuration using wire-grid polarizers. Demodulation of 64-Gbaud QPSK and 50-Gbaud 16QAM signals as well as ultra-wideband operation covering 1260-1630 wavelength bands are demonstrated experimentally.

Publishing type：Research paper (scientific journal)  

Ultrathin GaAs solar cells have gained attention because they are cost-effective and exhibit high carrier-collection efficiency and open-circuit voltage compared with cells with thicker absorber layers. However, ultrathin GaAs solar cells have a low current density due to their thin light-absorbing layers. To improve the photoabsorption of ultrathin GaAs solar cells, light-trapping structures with textured front or rear surfaces are necessary, and easy methods for fabricating textured structures are required. In this study, a new method to create light-scattering textured surfaces using block copolymers is developed. This method only requires spin coating, annealing, and wet etching, making it easily applicable to large-area devices such as solar cells. An ultrathin GaAs solar cell is fabricated with a rear-textured structure using block copolymers as masks for wet etching. The results demonstrate that the short-circuit current density is enhanced by 1.39 mA cm−2, which improves the energy conversion efficiency by 0.71 percentage points compared with a reference cell without a rear-textured structure.

DOI： 10.1002/pssa.202300586

Publishing type：Research paper (international conference proceedings)  

We present a broadband and compact coherent receiver in a surface-normal configuration using wire-grid polarizers. Demodulation of 64-Gbaud QPSK and 50-Gbaud 16QAM signals as well as ultra-wideband operation covering 1260-1630 wavelength bands are demonstrated experimentally.

DOI： 10.1364/CLEO_AT.2023.SM4I.3

Publishing type：Research paper (international conference proceedings)  

Spatially scalable metasurface-based optical hybrid for dual-polarization (DP) multi-core coherent receiver is proposed and experimentally demonstrated. Simultaneous homodyne detection and demodulation of DP-64QAM signals from a 4-core fiber are achieved with an error vector magnitude of 2.3% for all spatial/polarization channels at 1550 nm.

DOI： 10.1049/icp.2023.2655

Publishing type：Research paper (scientific journal)  

AlGaAs is promising absorber material for multi-junction solar cells (MJSC) for its tunable bandgap to match various bandgap combination designs. However, the path to achieving high-quality AlGaAs for solar cell application remains to be determined. In this study, we grow and characterize carbon-doped p-Al0.375Ga0.625As on exact and vicinal GaAs(0 0 1) substrates, using metal-organic vapor phase epitaxy (MOVPE). Low-temperature photoluminescence (PL) measurement is performed in the range from 6 K to 300 K. The Arrhenius plot of PL intensities reveals an increasing trend for non-radiative recombination in conjunction with increasing miscut angle. The presence of atomic ordering that inversely correlates to the miscut angle is proposed to explain the PL linewidth and peak energy dependency on substrate orientation. The conflicting tendency of non-radiative recombination and ordering with miscut angle are further validated using solar cell devices. A slightly vicinal substrate is recommended for AlGaAs solar cells according to our observation.

DOI： 10.1002/pssa.202300529

Publishing type：Research paper (international conference proceedings)  

Front hetero-junction (FHJ) and rear hetero-junction (RHJ) InGaAs solar cells were grown by metal-organic vapor phase epitaxy (MOVPE). Compared with RHJ solar cell, FHJ InGaAs solar cell shows a better short circuit current density (Jsc), but a worse open circuit voltage (Voc). Based on simulation results, non-radiative recombination in FHJ and RHJ InGaAs solar cells has been compared. FHJ has a better carrier diffusion length, but greater carrier loss at the interface resulted to worse Voc. RHJ has a less carrier loss in the interface but worse carrier diffusion length resulted to worse Jsc.

DOI： 10.1109/PVSC48320.2023.10359579

Publishing type：Research paper (scientific journal)  

Indium gallium phosphide (InGaP) solar cells are widely used as top subcells in multi-junction solar cells, however, there are large open-circuit voltage (VOC) losses in InGaP solar cells. Improving the radiative efficiency by inserting an InGaP multiple quantum well (MQW) into the InGaP solar cell structure is a promising approach to enhancing the VOC of InGaP solar cells, but such InGaP MQW solar cells are not demonstrated. Moreover, the critical parameters, such as the V/III ratio for an InGaP MQW, are not investigated. This study investigates the optimal V/III ratio for enhancing the radiative efficiency of an InGaP MQW. Doublehetero structures are fabricated, comprising a 330 nm core layer with 55 periods of an InGaP MQW grown under varying V/III ratios. The InGaP MQW grown under V/III = 62 gives the highest photoluminescence intensity among all InGaP structures. An InGaP MQW solar cell grown at V/III = 62 exhibits a 2 mV VOC loss suppression compared with an InGaP bulk solar cell. Therefore, an InGaP MQW grown under proper growth conditions contributes to enhance the energy conversion efficiency of InGaP solar cells. This study provides relevant insights into InGaP MQW crystal growth and the performance improvement in InGaP solar cells.

DOI： 10.1002/pssa.202300660

Publishing type：Research paper (international conference proceedings)  

The four-junction solar cell was fabricated by surface activated wafer bonding (SAB) method with the inverted metamorphic (IMM) triple-junction InGaP/GaAs/In0.35Ga0.65As integrated on the In0.53Ga0.47As single junction cell. Applying the direct integrated p+-AlGaAs/n+-InGaAs tunnel junction at the bonding interface, electrically connected 4-junction solar cell improved the open-circuit voltage Voc = 3.24 V was realized. The current density at the maximum power point was restricted by photocurrent in the In0.53Ga0.47As 4th subcell compared with 11 mA/cm2 of short-circuit current density (Jsc) expected for the IMM-3J under the AM1.5G illumination.

DOI： 10.1109/PVSC48320.2023.10359831

Publishing type：Research paper (international conference proceedings)  

This paper presents a study comparing GaAs solar cells with different structures grown by metal-organic vapor phase epitaxy (MOVPE). The impacts of absorber type and junction position are individually analyzed regarding the solar cell performance. When using the same structures, the n-type absorber samples had enhanced open-circuit voltage (Voc) and radiative efficiency than the p-type absorber ones, indicating superior crystal quality of n-GaAs. Besides, when using the same type of absorber, front-junction (FJ) samples showed an all-around boosted performance than rear-junction (RJ) ones, refreshing our understanding of the role of RHJ structure. Based on these observations, a p-on-n FJ structure using an n-GaAs absorber is recommended for high-performance solar cells.

DOI： 10.1109/PVSC48320.2023.10360070

Publishing type：Research paper (scientific journal)   Publisher：Institute of Electronics, Information and Communications Engineers (IEICE)  

DOI： 10.1587/transele.2022ocp0003

Publishing type：Research paper (international conference proceedings)   Publisher：Optica Publishing Group  

Spatially scalable surface-incidence dual-polarization (DP) coherent receiver with a dielectric metasurface and photodetector array is proposed. The designed metasurface is fabricated on a silicon-on-quartz chip to demonstrate ideal wave-mixing functionalities required for DP coherent detection.

DOI： 10.1364/cleo_si.2023.sm4g.1

Publishing type：Research paper (scientific journal)  

A method for measuring light absorption using photothermal deflection spectroscopy (PDS) is investigated. Typical reflectance and transmittance (R & T) absorption measurements have low spectral resolution (SR) with standard setups. It is believed that absorption measurements using PDS solve this problem because of its high sensitivity, even at a high SR. However, accurate absorption measurements have not been obtained using conventional PDS. The reflectance of the sample must be considered in the absorptance calculations. Various absorption measurement methods are compared and a novel calculation method for accurate PDS absorption measurements is proposed. A sample with quantum structures is fabricated and PDS measurements are performed. Exciton absorption peaks are also observed. The high SR of PDS is demonstrated and the measured exciton absorption peaks are confirmed using piezoelectric photothermal measurements. Furthermore, it is confirmed that the absolute values of absorptance derived from the R & T and PDS measurements agree well. The new PDS absorption measurement technique enables to obtain accurate absorption characteristics, which are indispensable for designing and optimizing optoelectronic devices.

DOI： 10.1002/pssa.202300585

Publishing type：Research paper (scientific journal)   Publisher：Optica Publishing Group  

An optical phased array (OPA) is a compact high-speed wavefront modulation device that is promising for next-generation optical sensing systems. In particular, speckle-based single-pixel imaging (SSPI) using OPA is an attractive scheme since precise tuning of optical phases is unnecessary. In this work, we present a comprehensive analysis of SSPI using an OPA with phase shifters by comparing two classes of OPAs: uniformly spaced OPA (UOPA) and non-redundant OPA (NROPA). Through singular value decomposition analysis of the illumination patterns generated from the OPA, we clarify the theoretical limit of the imaging resolution for each case. As a result, the number of resolvable points can be as large as for the case of NROPA. This is in clear contrast to the case of UOPA, where the number of resolvable points can only be as large as . Finally, imaging results of a test target are compared to study the impact of the array layout in OPA-based SSPI. Our work provides theoretical understanding of OPA-based SSPI and reveals the effectiveness of SSPI using NROPA.

DOI： 10.1364/josaa.476683

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

There are several methods for the evaluation of the performance of solar cells through the voltage loss. However, the advantages and disadvantages of each method have not been thoroughly discussed nor elucidated. This study compares these various methods to clarify their advantages and disadvantages. The bandgap offset W-OC has been used for the evaluation of the voltage loss. However, it was found that such approach gives inaccurate assessment of the voltage loss in thin-bulk and quantum-structure solar cells. Our study discusses the reasons for such inaccuracy and proves that we can accurately evaluate the nonradiative voltage loss in quantum-structure solar cells using the optoelectronic reciprocity theorem and detailed-balance theory. Our findings enable a fair, accurate, and accessible evaluation of the nonradiative voltage loss in solar cells which facilitates the development of high-efficiency quantum-structure solar cell.

DOI： 10.1109/JPHOTOV.2022.3229186

Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society (ACS)  

DOI： 10.1021/acsphotonics.2c00759

Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

DOI： 10.23919/oecc/psc53152.2022.9849960

Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

DOI： 10.23919/oecc/psc53152.2022.9850150

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

A highly reliable, broadband, in-vehicle optical network is proposed. The fragile laser diodes are placed only in the master and optical packets are transmitted between the master and slave nodes in a ring topology network.

DOI： 10.23919/oecc/psc53152.2022.9849883

Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

DOI： 10.23919/oecc/psc53152.2022.9850133

Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

DOI： 10.23919/oecc/psc53152.2022.9850138

Language：English   Publishing type：Research paper (scientific journal)   Publisher：WILEY-V C H VERLAG GMBH  

InGaAs/GaAsP multiple quantum wells (MQWs) are used in electronic devices because the high radiative efficiency of MQWs improves the device performance. To realize high radiative efficiency, a GaAs interlayer is inserted to decrease the defects at the heterointerface between the well and barrier of the MQWs. However, it is unclear how the interlayer thickness and insertion position affect the radiative efficiency of the MQWs. Herein, the influence of the GaAs interlayer thickness on the radiative efficiency by measuring the photoluminescence of MQWs with different interlayer thicknesses is investigated. A thicker GaAs interlayer below the InGaAs enhances the radiative efficiency. Moreover, the GaAs interlayer above the InGaAs must be thicker than approximate to 6 nm to enhance the radiative efficiency. The initial growth of GaAs above the InGaAs degrades the surface morphology, which may be associated with the carry-over of indium on the InGaAs surface and may generate defects between the InGaAs and GaAsP layers. The surface of sufficiently thick GaAs interlayers has a flatter morphology and is free from the degradation of radiative efficiency. The results suggest inserting of interlayers with a smooth surface morphology between strained layers during epitaxial growth is an effective method to suppress nonradiative recombination.

DOI： 10.1002/pssa.202100426

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

An optical unitary converter (OUC) that can convert a set of N mutually orthogonal optical modes into another set of arbitrary N orthogonal modes is expected to be a key device in diverse applications, including optical communication, deep learning, and quantum computing. While various types of OUC have been demonstrated on photonic integration platforms, the sensitivity against a slight deviation in waveguide dimensions has been the crucial issue in scaling N. Here, we demonstrate that an OUC based on the concept of multiplane light conversion (MPLC) shows outstanding robustness against waveguide deviations. Moreover, it becomes more and more insensitive to fabrication errors as we increase N, which is in clear contrast to the conventional OUC architecture, composed of 2 x 2 Mach-Zehnder interferometers. The physical origin behind this unique robustness and scalability is studied by considering a generalized OUC configuration. As a result, we reveal that the number of coupled modes in each stage plays an essential role in determining the sensitivity of the entire OUC. The maximal robustness is attained when all-to-all-coupled interferometers are employed, which are naturally implemented in a MPLC-based OUC.

DOI： 10.1103/PhysRevApplied.17.024071

Other Link： http://arxiv.org/pdf/2103.14782v1

Language：English   Publishing type：Research paper (scientific journal)   Publisher：WILEY-V C H VERLAG GMBH  

The growth of high-crystal-quality GaAs on InGaP without sacrificing the carrier lifetime and shunt resistance of the InGaP layer is reported, utilizing a 10 nm-thick low-temperature-grown InGaP cover layer (LTGCL). Electronic devices with a GaAs/InGaP heterostructure such as solar cells, high-electron-mobility transistors (HEMTs), and lasers have attracted considerable attention because of their superior characteristics compared with conventional homostructure devices. However, the device performance has scope for improvement, particularly, the interface quality. To realize high-performance GaAs/InGaP heterostructure devices, high-crystal-quality GaAs needs to be grown on high-temperature-grown InGaP, which has a long carrier lifetime and high shunt resistance. High-temperature-grown InGaP, however, has a rough surface, which degrades the crystal quality of the overlying GaAs. To solve this problem, the use of high-temperature-grown InGaP with an LTGCL is proposed, which has an atomic-scale smooth surface. The advantages of using an LTGCL are discussed with a heterostructure solar cell as an example. The findings of this study indicate that the LTGCL can contribute to further improvement of the performance of heterostructure devices.

DOI： 10.1002/pssb.202100305

Publishing type：Research paper (scientific journal)   Publisher：Optica Publishing Group  

Symmetric polarization rotator-splitter (PRS) is proposed and experimentally demonstrated on InP for the first time. Instead of integrating a mode-selective splitter, we employ a symmetric multimode-interference (MMI) splitter at the output of an adiabatic taper section to extract the linear superpositions of the transverse-electric (TE) and the transverse-magnetic (TM) components of the input signal. As a result, the entire device functions as a PRS with its basis on the S₂-S₃ plane of the Poincaré sphere, whereas we can fully eliminate complicated asymmetric structures that are challenging to fabricate on InP. Moreover, the adiabatic taper, which operates as a mode-evolution-based polarization converter, is designed judiciously to minimize the overall length. The designed InP PRS with a total length of 750 µm is fabricated by a simple single-etching process. A polarization extinction ratio of more than 16.3 dB and a polarization-dependent loss of 0.67 dB are demonstrated experimentally at a 1550-nm wavelength.

DOI： 10.1364/oe.445358

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

InP-based polarization manipulating devices are attracting continuous interest to realize compact polarization multiplexed transceivers with monolithically integrated active components. Here, we propose and experimentally demonstrate a novel type of monolithic InP polarization rotator (PR) based on a tapered half-ridge structure. Owing to the principle of mode-evolution-based adiabatic rotation of the polarization axis along propagation, the demonstrated device inherently exhibits excellent robustness against fabrication errors. Error-tolerant conversion with an efficiency over 97% is obtained experimentally across the entire C-band. With the simple self-aligned process and a thick InP upper cladding, the demonstrated device would be suitable for low-loss integration with other InP-based active waveguide components, such as lasers, amplifiers, and modulators.

DOI： 10.1109/LPT.2022.3141518

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We propose a robust silicon photonic Stokes-vector receiver based on fully symmetric waveguides without a mode-selective directional coupler. By using a fabricated receiver, we experimentally demonstrate 30-Gb/s 64-QAM self-coherent transmission over a 25-km single-mode fiber. (C) 2022 The Author(s)

Publishing type：Research paper (scientific journal)   Publisher：Laser Society of Japan  

DOI： 10.2184/lsj.50.8_463

Publishing type：Research paper (international conference proceedings)  

We propose a surface-normal optical modulator using an InP high-contrast grating embedded with electro-optic polymer. The feasibility of broad-bandwidth (>40 GHz) and low-loss (<0.22 dB) modulation is numerically demonstrated, thanks to the high electron mobility and the small optical absorption of n- InP.

DOI： 10.23919/ISLC52947.2022.9943427

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

Recent progress of developing universal optical unitary processors (OUPs) based on the concept of multi-plane light conversion (MPLC) is reviewed. The inherent redundancy of MPLC provides unique scalability and excellent robustness against fabrication imperfectness, enabling large-scale OUPs integrated on silicon and InP platforms. (C) 2022 The Author(s)

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Optica Publishing Group  

Integrated optical unitary converters (OUCs) are vital devices for various emerging applications such as mode-multiplexed optical communication, optical neural networks, and quantum computing. In order to realize large-scale OUCs in a limited footprint, the number of elements, as well as the size of each element, is important. In this work, we present a novel type of OUC using half-integer multimode interferometers (MMIs) based on the multi-plane light conversion (MPLC) concept. A half-integer MMI enables unitary coupling among the multiple input and output ports, while requiring only half the length of a conventional uniform MMI. Although the splitting ratio is not uniform across the ports, we show both numerically and experimentally that arbitrary unitary operation can still be achieved with comparable performance. We fabricate 4x4 OUC with half-integer MMIs on the monolithic InP platform and experimentally demonstrate reconfigurable 4-mode sorting and switching with a significantly reduced footprint compared with the conventional OUCs using uniform MMIs. (C) 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

DOI： 10.1364/OE.443227

Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society ({ACS})  

DOI： 10.1021/acs.jpcc.1c07110

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers Inc.  

High-speed and robust optical beam-steering device will be the key component for various applications, such as LiDAR (light detection and ranging) and free-space optical communication. Optical phased arrays (OPAs) integrated on semiconductor chips have recently received increasing attention due to the high-speed operation, compactness, and low cost. In particular, indium phosphide (InP)-based OPAs are advantageous for high-output-power applications at 1.55- \\mu \\text{m} eye-safe wavelength, owing to their capability of monolithically integrating active components, such as high-power lasers and optical amplifiers. In this letter, we design and fabricate, to the best of our knowledge, the largest-scale InP-based OPA and experimentally demonstrate its beam-steering operations. The fabricated OPA consists of 100 waveguides with carrier-injection-based phase shifters, densely integrated on 7 mm \\times5 mm footprint. A focused beam width of 0.11° is steered across the free spectral range of 8.88°, corresponding to more than 80 resolvable points. This is the largest number of resolvable points obtained by a monolithic InP OPA. The response time of the OPA is confirmed to be less than 16 ns, which is limited by the driver circuit. This work paves the way for realizing compact beam-steering modules for high-speed and high-output-power imaging applications.

DOI： 10.1109/LPT.2021.3107277

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers Inc.  

Recent studies have shown that the self-coherent systems, including the bi-directional, the Stokes-vector-modulation direct-detection (SVM-DD), and the Kramers-Kronig (KK) schemes, have the potential to reduce the cost of short-reach networks. One of the most attractive features of the self-coherent systems is that they may eliminate the necessity of a narrow-linewidth laser, which is mandatory in the conventional full coherent systems employing high-order quadrature-amplitude-modulation (QAM) formats. On the other hand, it is also recognized that the laser phase noise may have a significant impact on the system performance if there exists a large length mismatch between the signal and the CW tone in the transmission paths. In this paper, we analyze the impact of laser phase noise on the self-coherent systems numerically and experimentally by considering the bi-directional system as an example case. As a result, we show that the performance of the self-coherent system can be described efficiently by using a limited number of normalized parameters. We then reveal the existence of a distinctive threshold on the path length mismatch that influences the bit-error-rate (BER) performance
the penalty increases in a stepwise manner as the length mismatch exceeds the threshold and converges to a constant value. Based on the results, we suggest the feasibility of employing high-order QAM formats to transmit &gt
800 Gb/s signals per polarization using a 10-MHz-linewidth laser. Finally, proof-of-concept experimental results are presented to verify the theoretical and numerical analyses.

DOI： 10.1109/JLT.2021.3097092

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers Inc.  

The optical-phased array (OPA) has gained special interest in recent years as a high-speed and compact imaging device. While large-scale OPAs have been demonstrated in single-pixel imaging, the complexity of the driver circuit is becoming a crucial problem as the number of phase shifters increases. Here, we investigate the phase shift requirement of OPA for single-pixel imaging and demonstrate, for the first time, that full 2π phase shifts are not mandatory to generate a set of illumination patterns with a sufficient degree of randomness required to reconstruct the image. Using a silicon photonic OPA chip with 128 carrier-depletion-based phase shifters, we experimentally confirm this property by successfully retrieving an image under a maximum phase shift of only ∼1.5π without affecting the quality of the image. Consequently, the input voltage can be reduced significantly. Since the carrier-depletion phase shifters generally require high driving voltages, this finding paves the way to high-speed OPA-based imaging with a minimum device requirement.

DOI： 10.1109/JPHOT.2021.3113925

Language：English   Publishing type：Research paper (scientific journal)   Publisher：OPTICAL SOC AMER  

Optical phased arrays (OPAs) are promising beam-steering devices for various applications such as light detection and ranging, optical projection, free-space optical communication, and optical switching. However, previously reported OPAs suffer from either an insufficient number of resolvable points, or complicated control requirements due to an extremely large number of phase shifters. To solve this issue, we introduce the non-redundant array (NRA) concept to the OPA devices. Based on this design, we can realize high-resolution OPAs whose number of resolvable points scales quadratically with the number of antennas N. In contrast, that of traditional OPAs scales only linearly with N. Thus, a significant reduction in the number of required phase shifters can be attained without sacrificing the number of resolvable points. We first investigate the impact of employing the NRA theoretically by considering the autocorrelation function of the array layout. We then develop a Costas-array-based silicon OPA and experimentally demonstrate 2D beam steering with similar to 19,000 resolvable points using only 127 phase shifters. To the best of our knowledge, this corresponds to the largest number of resolvable points achieved by an OPA without sweeping the wavelength. (C) 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

DOI： 10.1364/OPTICA.437453

Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

DOI： 10.1109/ecoc52684.2021.9606038

Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society (ACS)  

DOI： 10.1021/acsphotonics.1c00419

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP Publishing Ltd  

Single-pixel imaging provides numerous advantages over the conventional pixelated-array imaging as it paved the way to operate at a wider range of wavelengths. Optical phased arrays (OPAs) have been recently used as fast and compact devices to generate the random illumination patterns required for this type of imaging. In this work, we investigate optimal tuning of these speckle patterns generated by an OPA for single-pixel imaging, and we employ the condition number of the illumination patterns matrix as a figure of merit for their evaluation. We show that the quality of the reconstructed image is greatly enhanced by minimizing the condition number of the illumination patterns matrix, which suppresses the effect of noise on the quality of the image.

DOI： 10.35848/1347-4065/ac0cb9

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

To improve the performance of semiconductor photoelectrodes for water splitting, the amount of band bending in the depletion layer of a semiconductor should be accurately ascertained, since it determines the splitting efficiency of photogenerated carriers. Band bending has been determined by X-ray photoelectron spectroscopy (XPS) from the valence band maximum (VBM), which has been calculated from the Ga 3d peak using the energy difference between VBM and Ga 3d (ΔEVBM-3d). This work validates several values for ΔEVBM-3d which have been reported previously, by analyzing the spectrum around the VBM and its distance from Ga 3d for the n-GaN(0001) surface under both ultrahigh vacuum (UHV) and ambient H2O. ΔEVBM-3d is estimated to be between 17.36 and 17.55 eV. By adopting 17.5 eV as ΔEVBM-3d, the amounts of band-bending were 0.5 eV under UHV and 0.1 eV under a relative humidity of 46%, respectively. For the latter condition, a surface photovoltage of 20 meV was observed upon Xe lamp irradiation, confirming the existence of band bending even with H2O adsorption on the surface. The origin of such band bending seems to be Fermi level pinning to the subsurface states which cannot be compensated by H2O.

DOI： 10.1021/acs.jpcc.0c11174

Language：English   Publishing type：Research paper (scientific journal)   Publisher：The Optical Society  

Carrier-injection-based efficient polarization controller with a strained InGaAsP multiple-quantum-well (MQW) layer is demonstrated on a regrowth-free InP platform. We employ a straight-line device configuration by cascading an asymmetric polarization rotator (PR) to provide a fixed polarization conversion and a polarization-dependent phase shifter (PD-PS) to enable tunable polarization rotation. Based on a novel design concept, both the PR and PD-PS sections are integrated monolithically without active-passive integration. Using the fabricated device, we experimentally demonstrate efficient polarization conversion over the entire Poincaré sphere with a total current of less than 40 mA. With the capability of monolithically integrating other InP-based active components, the demonstrated scheme should be attractive for various applications, such-as low-cost coherent communication, microwave photonics, and quantum key distribution.

DOI： 10.1364/OE.414387

Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

DOI： 10.1109/jlt.2020.3008968

Language：English   Publishing type：Research paper (scientific journal)   Publisher：OPTICAL SOC AMER  

We propose and demonstrate a simple integrated dual-polarization (DP) coherent receiver that does not require a polarization splitter-rotator (PSR). Based on a novel concept, a DP coherent signal is mixed with the local-oscillator (LO) waves inside a single interferometer and detected by five single-ended photodetectors. The signal-signal and LO-LO beat noises are eliminated through differential detection. We design and fabricate a proof-of-concept device on InP and experimentally demonstrate complete retrieval of DP quadrature phase-shift keyed signals. Requiring minimal number of optical components without a PSR, the demonstrated scheme would be attractive particularly for the InP and thick-silicon photonic platforms due to its significantly reduced footprint and ease of fabrication. (C) 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

DOI： 10.1364/OE.413310

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：The Optical Society  

We experimentally demonstrate single-pixel imaging using a carrier-depletion silicon optical phased array and reveal that high-quality images can be obtained even with a limited phase shift. This tapers off the device requirement for high-speed imaging.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We fabricate and demonstrate a monolithic InP-based optical unitary converter (OUC) with nonuniform multimode interference (MMI) couplers. Reconfigurable 4-mode sorting is experimentally demonstrated with a significantly reduced footprint compared with the conventional OUCs using uniform MMI couplers.

DOI： 10.1109/IPC48725.2021.9592993

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We propose and experimentally demonstrate a monolithic InP/InGaAsP mode-evolution-based polarization rotator. Using the adiabatic mode transition inside a tapered half-ridge waveguide, fabrication-tolerant conversion with an efficiency over 97.7% is obtained over the entire C-band. (c) 2021 The Authors

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We propose and demonstrate a dual-polarization (DP) homodyne coherent receiver with a single optical hybrid without any polarization splitter-rotator. Using a proof-of-concept device fabricated on InP, DP quadrature phase-shift keying signal is retrieved successfully. (c) 2021TheAuthor(s)

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electronics Information Communication Engineers  

Coherent transceivers are now regarded as promising candidates for upgrading the current 400 Gigabit Ethernet (400GbE) transceivers to 800G. However, due to the complicated structure of a dual-polarization IQ modulator (DP-IQM) with its bulky polarization-beam splitter/comber (PBS/PBC), the increase in the transmitter size and cost is inevitable. In this paper, we propose a compact PBS/PBC-free transmitter structure with a straight-line configuration. By using the concept of polarization differential modulation, the proposed transmitter is capable of generating a DP phase-shift-keyed (DP-PSK) signal, which makes it directly applicable to the current coherent systems. A detailed analysis of the system performance reveals that the imperfect equalization and the bandwidth limitation at the receiver are the dominant penalty factors. Although such a penalty is usually unacceptable in long-haul applications, the proposed transmitter can be attractive due to its significant simplicity and compactness for short-reach applications, where the cost and the footprint are the primary concerns.

DOI： 10.1587/TRANSCOM.2020EBP3124

Publishing type：Research paper (scientific journal)  

© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement Integrated reconfigurable optical unitary converters (OUCs) are crucial in realizing all-optical spatial mode demultiplexing for mode-division-multiplexed transmission systems and programmable photonic processing for optical neural networks. In this work, we present the first experimental demonstration of 4×4 OUC monolithically integrated on InP. To avoid the difficulty of integrating a large number of Mach-Zehnder interferometer couplers on the InP platform, we apply the concept of multi-plane light conversion and use cascaded stages of 4-port multimode interference couplers, which are more scalable and easier to fabricate on InP. By optimizing the phase shifters, we demonstrate reconfigurable 4-mode sorting as well as error-free switching of 40-Gbit/s signal.

DOI： 10.1364/OE.399366

Publishing type：Research paper (scientific journal)  

© 2020 The Japan Society of Applied Physics. Monolithically integrated high-speed polarization modulators are finding emerging applications in the fields of optical communication, quantum key distribution, and radio-frequency photonics. Here, we design an efficient polarization modulator with InGaAsP asymmetrically coupled quantum wells (ACQWs) on InP. By using two quantum wells separated by a thin barrier layer, the behaviors of the heavy holes and the light holes can be controlled independently under an external bias. As a result, we can induce an anomalous quantum-confined Stark effect, having a strong polarization dependence. With an optimized ACQW design, we numerically demonstrate a large polarization-dependent refractive index modulation of 7.1 10-3 at an applied field of 1.8 V μm-1. Using a 1 mm long device, we estimate that 2π rotation of the Stokes vector can be achieved with negligible absorption at a driving voltage of only 0.75 V.

DOI： 10.35848/1347-4065/aba3f7

Publishing type：Research paper (scientific journal)  

© 2020 International Solar Energy Society Due to the tunable optical absorption threshold and several intriguing physical properties, multiple-quantum-well (MQW) structures have been playing a crucial role in achieving ultra-high efficiency in tandem photovoltaic cells and various optoelectronic technologies. However, devices incorporating such nano-structures suffer from poor perpendicular carrier transport, which hinders device performance. On the other hand, designing MQWs is challenging. Simultaneously optimizing the structural parameters and constituent materials suggests a high design complexity with a large parameter space. In this report, on the basis of the quantitative analysis on the degradation of carrier transport, we propose a general design guideline for MQWs by which the structures and constituent materials can both be optimized targeting at a particular absorption threshold. We firstly demonstrate our design flow by optimizing an InGaP/InGaP strain-balanced (SB) MQW at 1.91 eV to replace In0.49Ga0.51P bulk serving as the top cell of dual-MQW triple-junction solar cells. Secondly, we apply the design concept to a conventional InGaAs/GaAsP SBMQW targeting at 1.23 eV, which is significant for not only high-efficiency single-junction photovoltaics but also current-matched tandem cells. The results illustrate the possibility of further improving current devices in terms of their constituent materials for QWs. Our design optimization method is considered to be applicable to any device based on QW structures.

DOI： 10.1016/j.solener.2020.06.037

Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

Due to the tunable optical absorption threshold and several intriguing physical properties, multiple-quantum-well (MQW) structures have been playing a crucial role in achieving ultra-high efficiency in tandem photovoltaic cells and various optoelectronic technologies. However, devices incorporating such nano-structures suffer from poor perpendicular carrier transport, which hinders device performance. On the other hand, designing MQWs is challenging. Simultaneously optimizing the structural parameters and constituent materials suggests a high design complexity with a large parameter space. In this report, on the basis of the quantitative analysis on the degradation of carrier transport, we propose a general design guideline for MQWs by which the structures and constituent materials can both be optimized targeting at a particular absorption threshold. We firstly demonstrate our design flow by optimizing an InGaP/InGaP strain-balanced (SB) MQW at 1.91 eV to replace In0.49Ga0.51P bulk serving as the top cell of dual-MQW triple-junction solar cells. Secondly, we apply the design concept to a conventional InGaAs/GaAsP SBMQW targeting at 1.23 eV, which is significant for not only high-efficiency single-junction photovoltaics but also current-matched tandem cells. The results illustrate the possibility of further improving current devices in terms of their constituent materials for QWs. Our design optimization method is considered to be applicable to any device based on QW structures.

DOI： 10.1016/j.solener.2020.06.037

Publishing type：Research paper (scientific journal)  

© 2011-2012 IEEE. Wire on well (WoW) structure, which is formed on a vicinal substrate, has been under consideration for a bandgap adjustor of multijunction cells. The photocurrent of a single-junction cell containing WoW is higher than that of a cell containing planar superlattice (PSL). This work provides direct evidence for the superiority of WoW over PSL in terms of the transport of photogenerated carriers. The time of flight (ToF) of the carriers that were photoexcited near the top surface of a cell was measured with photoluminescence by embedding a probe well beneath the nanostructures (WoW or PSL). The effective mobility of carriers in the nanostructures was obtained from the relationship between the ToF and the electric field applied via external bias. It was revealed that the carrier mobility in WoW was higher than that of PSL. The merit of enhanced mobility for WoW is discussed with device simulation and the potential for higher conversion efficiency is evidenced.

DOI： 10.1109/JPHOTOV.2020.2981911

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

Wire on well (WoW) structure, which is formed on a vicinal substrate, has been under consideration for a bandgap adjustor of multijunction cells. The photocurrent of a single-junction cell containing WoW is higher than that of a cell containing planar superlattice (PSL). This work provides direct evidence for the superiority of WoW over PSL in terms of the transport of photogenerated carriers. The time of flight (ToF) of the carriers that were photoexcited near the top surface of a cell was measured with photoluminescence by embedding a probe well beneath the nanostructures (WoW or PSL). The effective mobility of carriers in the nanostructures was obtained from the relationship between the ToF and the electric field applied via external bias. It was revealed that the carrier mobility in WoW was higher than that of PSL. The merit of enhanced mobility for WoW is discussed with device simulation and the potential for higher conversion efficiency is evidenced.

DOI： 10.1109/JPHOTOV.2020.2981911

Publishing type：Research paper (scientific journal)  

© 1989-2012 IEEE. Highly efficient and fabrication-error-resistant InP/ InGaAsP polarization rotator based on the half-ridge waveguide is experimentally demonstrated. A thin InAlAs layer is embedded inside the InGaAsP core layer to provide 20-fold etching selectivity with respect to InGaAsP, enabling precise control of the dry-etched core layer thickness. The designed device is fabricated to show a high polarization conversion efficiency of more than 95% with a large etching time tolerance of ±16%. Demonstrated scheme offers a practical path towards realizing compact and low-cost InP-based monolithic dual-polarization transceivers with on-chip polarization (de)multiplexers.

DOI： 10.1109/LPT.2020.2991450

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

Photocatalytic water splitting takes place at the semiconductor/electrolyte interface. Although the reactions are strongly affected by the subtle changes in the interface structure, little is known about the interface from an atomistic point of view. In this study, we investigate the GaN(0001)/water interface structure by combining first-principles calculation and ambient pressure X-ray photoelectron spectroscopy (AP-XPS). In particular, the relationship between the geometric and electronic structure of the interface is revealed. First, the evolution of the GaN/water interface structure upon water adsorption is predicted from firstprinciples calculations. Computational results indicate that (1) at low coverage (below 3/4 monolayer), the Fermi level is pinned to the surface states originating from surface Ga atom dangling bonds, and water adsorbs dissociatively, forming oxygen atoms as well as hydroxyl groups, and (2) at higher coverage (above 3/4 monolayer), the Fermi level becomes free from the pinning, and adsorption of intact water becomes dominant. AP-XPS measurements were carried out for the water coverage ranging from submonolayer (low coverage) to several monolayers (high coverage). The core-level binding energies calculated from first-principles were used successfully to assign the adsorbate species to experimental O is peaks. Both the electronic and geometric structures predicted by the first-principles calculation explain well the experimental spectra obtained by the AP-XPS measurements. The results demonstrate that the combined spectroscopic and first-principles computational approach offers a detailed atomic level understanding of the solid/liquid interface structures.

DOI： 10.1021/acs.jpcc.0c02192

Publishing type：Research paper (international conference proceedings)  

© 2020 OSA. Universal linear processing circuit based on multi-plane light conversion, comprising multiport directional couplers, is demonstrated to exhibit excellent fabrication tolerance even at increased scale, owing to its unique all-to-all unitary coupling at every layer.

Publishing type：Research paper (international conference proceedings)  

© 2020 OSA. We demonstrate that single-pixel imaging resolution of optical phased array can be enhanced by transmitting through a multimode fiber. Using only 128 phase shifters, >1000 points are resolved, determined by the number of fiber modes.

Publishing type：Research paper (scientific journal)  

© 2019 John Wiley & Sons, Ltd. To realize high efficiency in parallel with low cost, a light-trapping rear texture was proposed to be implemented in substrate-free thin-film multijunction (MJ) cells. A detailed-balance theory was formulated taking account of the finite light absorption in thin subcells. Such presented absorption model is general and useful to optimize the subcell thickness for MJ solar cells with light-trapping design. It is applied for InGaP/GaAs/InGaAs triple-junction solar cells to simulate subcell photocurrents and to obtain the current-matching (minimum requisite) subcell thicknesses combinations. Furthermore, the detailed-balance conversion efficiency was estimated for both radiative limit and the cases with below-unity internal radiative efficiency. For InGaP/GaAs/InGaAs MJ cells with InGaP subcell thickness less than 600 nm, adding a random-textured rear reflector can enhance light absorption so significantly that over 90% of InGaAs-cell thickness and even 50% of GaAs-cell thickness would be cut without any penalty in conversion efficiency, compared with the subcell thicknesses in traditional MJ cells with flat rear reflectors. Additionally, the thickness combination, (InGaP, GaAs, and InGaAs) = (450 nm, 333 nm, and 26 nm), is recommended to achieve both high conversion efficiency and low material cost. This work provides a very important theoretical guidance for the development on low-cost and high-efficiency MJ devices.

DOI： 10.1002/pip.3217

Language：English   Publishing type：Research paper (scientific journal)   Publisher：WILEY  

To realize high efficiency in parallel with low cost, a light-trapping rear texture was proposed to be implemented in substrate-free thin-film multijunction (MJ) cells. A detailed-balance theory was formulated taking account of the finite light absorption in thin subcells. Such presented absorption model is general and useful to optimize the subcell thickness for MJ solar cells with light-trapping design. It is applied for InGaP/GaAs/InGaAs triple-junction solar cells to simulate subcell photocurrents and to obtain the current-matching (minimum requisite) subcell thicknesses combinations. Furthermore, the detailed-balance conversion efficiency was estimated for both radiative limit and the cases with below-unity internal radiative efficiency. For InGaP/GaAs/InGaAs MJ cells with InGaP subcell thickness less than 600 nm, adding a random-textured rear reflector can enhance light absorption so significantly that over 90% of InGaAs-cell thickness and even 50% of GaAs-cell thickness would be cut without any penalty in conversion efficiency, compared with the subcell thicknesses in traditional MJ cells with flat rear reflectors. Additionally, the thickness combination, (InGaP, GaAs, and InGaAs) = (450 nm, 333 nm, and 26 nm), is recommended to achieve both high conversion efficiency and low material cost. This work provides a very important theoretical guidance for the development on low-cost and high-efficiency MJ devices.

DOI： 10.1002/pip.3217

Publishing type：Research paper (scientific journal)  

© 2020 IEEE. Impacts of reactor pressure and growth temperature on the qualities and properties of InGaP epitaxial layers grown in a narrow-channel horizontal metalorganic vapor-phase epitaxy (MOVPE) reactor were investigated with a growth rate of 10 μm/h. The roughness of InGaP was improved remarkably when the pressure was increased from 6 to 10 and 15 kPa. The reason is most likely related to the absolute pressure of P and the migrations of In and Ga adatoms on the wafer surface. Owing to the gas-phase reactions and desorption of P from InGaP, the surface of InGaP became rough, and the pit dislocation was increased with an increase in growth temperature. Although a low temperature was beneficial in order to obtain a smooth InGaP surface, the carrier lifetime in InGaP was shortened as a drawback. Consequently, InGaP n-on-p solar cells grown at a low temperature suffered from the degradations of both short-circuit current density and open-circuit voltage. Background impurities and defect densities in low temperature grown InGaP were attributed to these deteriorations. At the optimized growth temperature of 650 °C and the reactor pressure of 15 kPa, the growth rate of InGaP was accelerated to 20 and 30 μm/h. InGaP solar cells were successfully fabricated with the rate of 30 μm/h that opened up the way for the fabrication of III-V multijunction solar cells in the high-speed MOVPE reactor.

DOI： 10.1109/JPHOTOV.2020.2964994

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

Impacts of reactor pressure and growth temperature on the qualities and properties of InGaP epitaxial layers grown in a narrow-channel horizontal metalorganic vapor-phase epitaxy (MOVPE) reactor were investigated with a growth rate of 10 mu m/h. The roughness of InGaP was improved remarkably when the pressure was increased from 6 to 10 and 15 kPa. The reason is most likely related to the absolute pressure of P and the migrations of In and Ga adatoms on the wafer surface. Owing to the gas-phase reactions and desorption of P from InGaP, the surface of InGaP became rough, and the pit dislocation was increased with an increase in growth temperature. Although a low temperature was beneficial in order to obtain a smooth InGaP surface, the carrier lifetime in InGaP was shortened as a drawback. Consequently, InGaP n-on-p solar cells grown at a low temperature suffered from the degradations of both short-circuit current density and open-circuit voltage. Background impurities and defect densities in low temperature grown InGaP were attributed to these deteriorations. At the optimized growth temperature of 650 degrees C and the reactor pressure of 15 kPa, the growth rate of InGaP was accelerated to 20 and 30 mu m/h. InGaP solar cells were successfully fabricated with the rate of 30 mu m/h that opened up the way for the fabrication of III-V multijunction solar cells in the high-speed MOVPE reactor.

DOI： 10.1109/JPHOTOV.2020.2964994

Publishing type：Research paper (international conference proceedings)  

© 2020 OSA. Carrier-injection-based polarization controller with strained MQW layer is demonstrated. Based on novel design concept, both polarization-rotating and phase-shifting sections are integrated monolithically on regrowth-free InGaAsP/InP platform to achieve efficient conversion over the entire Poincaré sphere.

Publishing type：Research paper (international conference proceedings)  

© 2020 OSA. We experimentally demonstrate single-pixel imaging using a multimode fiber attached with optical phased-array chip. By driving 128 integrated phase shifters, speckle patterns are generated from the fiber to realize clear imaging with 490 resolvable points.

Publishing type：Research paper (scientific journal)  

© 1983-2012 IEEE. Direct-detection (DD) Stokes vector receiver (SVR) has rapidly gained interest for the high-speed datacenter and short-reach links, owing to its capability of recovering 3D optical signals without using expensive coherent detection. In order to reduce the size and cost of SVR, we have recently proposed and demonstrated a simple and compact device with single-ended photodetectors (S-PDs), which can be monolithically integrated on InP. In this paper, we provide comprehensive analysis on the receiver sensitivity for different SVR configurations. From rigorous theoretical investigations, we derive optimal design of SVR to maximize the sensitivity as well as explicit analytical expression of the bit-error-rate assuming an unamplified thermal-noise-limited case. From the obtained results, we find that the optimized 4-port S-PD-based SVR exhibits a moderate penalty of 2.1 dB with respect to the conventional more complicated SVR configuration based on 3-port balanced PDs. Since the proposed S-PD-based SVR can reduce the number of PDs, and requires neither a polarization-beam splitter nor precisely tuned optical interferometer, it may be an attractive candidate in realizing a low-cost and compact fully integrated DD-based receiver for the future >Tb/s short-reach systems, where the power budget is not the primary concern.

DOI： 10.1109/JLT.2019.2952980

Publishing type：Research paper (scientific journal)  

© 2019 IEEE. An integrated optical unitary converter (OUC), which can realize arbitrary N × N unitary transformation on chip, is promising for widespread applications in various areas, such as optical communication, quantum information processing, and optical neural networks. Most of the integrated OUCs demonstrated to date comprise cascaded 2 × 2 Mach-Zehnder interferometers (MZIs) based on Reck's scheme or its variation, or cascaded multimode interference (MMI) couplers based on multi-plane light conversion (MPLC). However, these schemes are sensitive to fabrication errors, making it difficult to realize high-performance large-scale OUCs. In this work, we experimentally demonstrate a silicon photonic 4 × 4 OUC that is inherently robust to fabrication errors, based on a novel structure. The proposed structure consists of cascaded multiport directional couplers (DCs) and phase shifter arrays. In this scheme, the multiport DC assures unitary transformation with inherent robustness against fabrication errors. By optimizing all the phase shifters using a simulated annealing algorithm, we experimentally demonstrate reconfigurable 4-mode demultiplexing and error-free demultiplexing of 40-Gb/s non-return-to-zero (NRZ) signals.

DOI： 10.1109/JLT.2019.2943116

Publishing type：Research paper (international conference proceedings)  

OFC 2020 © OSA 2020 © 2020 The Author(s) Carrier-injection-based polarization controller with strained MQW layer is demonstrated. Based on novel design concept, both polarization-rotating and phase-shifting sections are integrated monolithically on regrowth-free InGaAsP/InP platform to achieve efficient conversion over the entire Poincaré sphere.

DOI： 10.1364/OFC.2020.T4H.4

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

This study focuses on how the performances of III-V cells with high-speed metalorganic vapor phase epitaxy (MOVPE) are affected by the cell structure: a conventional non-p (a thin highly doped n-emitter on a thick lightly-doped p-base), a rear junction (RJ) n-on-p (a thick lightly-doped n-emitter on a thin highly-doped p-base) and a rear hetero junction (RHJ; a thick lightly-doped n-emitter on a highly-doped p-BSF). For the 120-mu m/h grown GaAs solar cells, dark current density (J(dark)) corresponding to non-radiative recombination in a space charge region (n=2) and the consequent open circuit voltage (V-oc) were significantly improved with RJ and RHJ structures. The reason was the lower defect density, most likely native point defects, in n-GaAs compared with that in p-GaAs. However, both short circuit current density and fill-factor were decreased in RJ and RHJ samples, and more accented in the latter. In the case of 30-mu m/h grown InGaP solar cells, the RHJ (n-InGaP/p-InAlGaP) structure showed no advantage over the normal n-on-p device in terms of J(dar)k and V-oc, probably due to the abruptness/roughness of hetero interface. Nevertheless, the RHJ structure has a good potential for high speed MOVPE growth of multiple junction solar cells because of its simple structure with smaller number of interfaces.

Publishing type：Research paper (international conference proceedings)  

© 2020 SPIE. Stokes-vector modulation (SVM) formats have recently drawn attention for realizing low-cost and high-capacity optical transceivers. However, SVM transmitters reported so far are still complicated because they are based on optical components employed mainly in coherent systems. In order to reduce the device size, cost and power consumption, a more compact transmitter configuration is desired. In this invited paper, we first experimentally demonstrate transmission of 30-Gb/s 8-ary SVM signal over 50 km using the proposed configuration. Then we numerically investigate system performance of our recently-proposed compact transmitter for the 8-ary SVM format.

DOI： 10.1117/12.2547308

Publishing type：Research paper (international conference proceedings)  

© COPYRIGHT SPIE. Downloading of the abstract is permitted for personal use only. Solar cells based on multiple quantum wells have attracted a great deal of attention in recent years. Due to the superior radiative nature, the output voltage is expected to be more ideal over solar cells made of bulk materials. In this research, based on the effective map analysis technique we proposed previously to ensure a fair comparison, we investigate quantum well solar cells and their bulk counterpart in terms of radiative efficiency, and compare several significant properties at a consistent absorption edge wavelength. This provides a good insight into the factors which limit the efficiency of quantum well solar cells.

DOI： 10.1117/12.2546279

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

Universal linear processing circuit based on multi-plane light conversion, comprising multiport directional couplers, is demonstrated to exhibit excellent fabrication tolerance even at increased scale, owing to its unique all-to-all unitary coupling at every layer. (C) 2020 The Author(s)

Publishing type：Research paper (international conference proceedings)  

© OSA 2020 © 2020 The Author(s) We experimentally demonstrate single-pixel imaging using a multimode fiber attached with optical phased-array chip. By driving 128 integrated phase shifters, speckle patterns are generated from the fiber to realize clear imaging with 490 resolvable points.

Publishing type：Research paper (scientific journal)  

© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement An integrated Stokes vector receiver (SVR) that can retrieve state of polarization of light in the three-dimensional (3D) Stokes space has widespread applications, such as short-reach communication links, polarization-sensitive imaging, and sensing. While various approaches have been demonstrated to date, monolithic integration of polarization components on InP has been a challenging issue. In this paper, we develop a novel 4-port SVR circuit integrated on a compact InP chip to retrieve complete Stokes parameters of incoming light with various intensity and degree-of-polarization. By judiciously designing the lengths and positions of asymmetric waveguide sections, we demonstrate that the SV of signal can be projected onto four vertices of a regular tetrahedron inscribed in the Poincaré sphere. Additionally, we employ this device in decoding 10-Gbaud 4-ary and 8-ary Stokes-vector-modulated signals in the 3D Stokes space.

DOI： 10.1364/OE.27.036449

Publishing type：Research paper (scientific journal)  

© 1995-2012 IEEE. Gain-coupled 4 × 25 Gb/s electroabsorption modulated laser (EML) array is implemented by an identical epitaxial layer integration scheme. Gain-coupling is incorporated via absorptive gratings to improve the single mode yield of the device. The EML array exhibits excellent single-mode performance, high extinction ratio, and a modulation bandwidth over 28 GHz. Clear eye opening under 28-Gb/s nonreturn-to-zero modulation has been demonstrated for all four channels. Transmission performance of the EML array over 20- and 40-km single-mode fiber is tested, confirming its potential for 100 GbE applications.

DOI： 10.1109/JSTQE.2019.2911417

Publishing type：Research paper (international conference proceedings)  

Asia Communications and Photonics Conference (ACP) © OSA 2019 In this invited paper, we report our recent progresses for SVM-based systems. The results show that the technologies for the SVM format can simplify not only the advanced DD systems, but also the coherent systems.

Publishing type：Research paper (scientific journal)  

© 2019 John Wiley & Sons, Ltd. Multijunction solar cells are currently leading in efficiency among all types of solar cells. However, the efficiency of those cells under concentrated illumination is limited by the trade-off between shadowing by the contact grid and series resistances. In this paper, we provide a method using luminescence to image the current transport efficiency for a precise and rapid measurement of the resistance effect. Light beam induced current (LBIC) measurement is used for the first time to calculate current transport efficiency in comparison with that of the luminescence method. Profiles of transport efficiency in both means are in good agreement. Furthermore, a quasi-3-D simulation using SPICE is used to explain the transport efficiency image. We find that the carrier transport efficiency distribution is primarily influenced by the perimeter recombination of the middle cell.

DOI： 10.1002/pip.3140

Publishing type：Research paper (scientific journal)  

© 2019 IEEE. We demonstrate an offset-quantum-well-based Stokes vector modulator (SVM) on InP with compressively strained InGaAsP/InGaAsP multiple-quantum wells (MQWs). Tuning the applied voltages to the phase modulators, arbitrary state-of-polarization (SOP) is generated at the output. A single metal-organic chemical vapor deposition (MOCVD) regrowth step is employed for integrating a pair of passive polarization converters (PCs) and active MQW-based polarization-dependent phase modulator (PD-PM) sections. All components are concatenated in series on an offset-quantum well (OQW) platform. A 3.5-mm-long SVM has a PD-PM with half-wave voltage Vπ of 14 V and VπL of 2.1 V.cm.

DOI： 10.1109/LPT.2019.2923102

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

We demonstrate an offset-quantum-well-based Stokes vector modulator (SVM) on InP with compressively strained InGaAsP/InGaAsP multiple-quantum wells (MQWs). Tuning the applied voltages to the phase modulators, arbitrary state-of-polarization (SOP) is generated at the output. A single metal-organic chemical vapor deposition (MOCVD) regrowth step is employed for integrating a pair of passive polarization converters (PCs) and active MQW-based polarization-dependent phase modulator (PD-PM) sections. All components are concatenated in series on an offset-quantum well (OQW) platform. A 3.5-mm-long SVM has a PD-PM with half-wave voltage V-pi of 14 V and V pi L of 2.1 V.cm.

DOI： 10.1109/LPT.2019.2923102

Publishing type：Research paper (international conference proceedings)  

© 2019 The Institute of Electronics, Information and Communication Engineers (IEICE). We propose and demonstrate reconfigurable optical unitary converter with non-uniform multimode interference (MMI) couplers. By using compact 182-\mu \mathrm{m} -long non-uniform MMIs, desired 8\times 8 unitary operation is obtained with the crosstalk equivalent to that with 364-\mu \mathrm{m} -long uniform-MMIs.

DOI： 10.23919/PS.2019.8817811

Publishing type：Research paper (international conference proceedings)  

© 2019 The Institute of Electronics, Information and Communication Engineers (IEICE). We propose novel integrated polarization modulator with InGaAsP three-step asymmetrically coupled quantum wells. By using abnormal quantum-confined Stark effect, we numerically demonstrate more than one-order improvement in modulation efficiency from the conventional uniform quantum-well design.

DOI： 10.23919/PS.2019.8818063

Publishing type：Research paper (international conference proceedings)  

© 2019 The Institute of Electronics, Information and Communication Engineers (IEICE). We fabricate and experimentally demonstrate a 4-port Stokes vector receiver with on-chip photodetectors monolithically integrated with InP-based polarization-converting circuit. Full Stokes parameters of intensity-varied signals are successfully retrieved from photocurrent signals of compressively-strained multiple-quantum-well photodetectors.

DOI： 10.23919/PS.2019.8818167

Publishing type：Research paper (scientific journal)  

© 2019 To suppress wafer bowing and crack generation of GaN on Si substrates, we investigated the effects of the Al content and thickness of the AlGaN interlayer on the compressive strain in the overlying GaN layer theoretically and experimentally. In the simulation, AlGaN relaxes gradually over the critical thickness. Therefore, the relaxation ratio of AlGaN at the top surface can be defined as a function of Al content and thickness. Too high Al content or too thick AlGaN interlayer induced too large initial strain in the upper GaN layer, which caused rapid and succeeding gradual relaxation, i.e., decrease of strain, of the GaN layer during growth because of generation of threading dislocations. Conversely, low Al content or thin AlGaN interlayer could induce constant but only small strain in the GaN layer. Therefore, the ideal relaxation ratio of the AlGaN surface exists to apply the maximal constant compressive strain in the GaN layer. The relaxation ratios of AlGaN interlayers determined in experiments were much smaller than those calculated in the simulation. Although the measured compressive strain in the GaN layer was smaller than expected, its decrease rate was small when grown on AlGaN interlayers with an almost ideal relaxation ratio.

DOI： 10.1016/j.jcrysgro.2019.01.013

Publishing type：Research paper (international conference proceedings)  

© 2019 The Author(s) 2019 OSA. We experimentally demonstrate a simple straight-line configuration of Stokes vector modulator with two cascaded phase modulators. Three-dimensional 8-ary Stokes-vector-modulated signal is generated at 30 Gb/s and transmitted over a 50-km dispersion-compensated single-mode fiber.

DOI： 10.23919/CLEO.2019.8749402

Publishing type：Research paper (international conference proceedings)  

© 2019 The Author(s) 2019 OSA. A surface-normal optical modulator using electro-optic polymer embedded inside a 570-nm-thick silicon high-contrast-grating resonator is fabricated. With a voltage applied to silicon grating, we obtain > 20% intensity modulation of the transmitted light at 30 MHz.

DOI： 10.23919/CLEO.2019.8750627

Publishing type：Research paper (international conference proceedings)  

© 2019 IEEE. We propose a compact horn-shaped metal-clad cavity modulator integrated on an InP waveguide, operating in 1.5-μm wavelength range. Through 3D finite difference time-domain simulation, we show that the Q factor of the resonant transmission dip can be enhanced drastically from 152 to 408 by introducing a horn-shaped SiO2 insulation layer to the cavity with an optimized slope angle. As a result, efficient modulation with the extinction ratio of 3.2 dB and insertion loss of 1.5 dB is obtained in a 2.3-μm-long modulator under a refractive index change of Δn = 0.01.

DOI： 10.1109/ICIPRM.2019.8819298

Publishing type：Research paper (international conference proceedings)  

© 2019 IEEE. To promote InGaP solar cell efficiency toward the theoretical limit, one promising approach is to incorporate multiple quantum wells (MQWs) into the InGaP host and improve its open-circuit voltage by facilitating radiative carrier recombination owing to carrier confinement. In this research, we demonstrate numerically that a strain-balanced (SB) In1-xGaxP/In1-yGayP MQW enhances confined carrier density while degrades the effective carrier mobility. However, a smart design of the MQW structure is possible by considering quantitatively the trade-off between carrier confinement effect and carrier transport, and MQW can be advantageous over the InGaP bulk material for boosting photovoltaic efficiency.

DOI： 10.1109/ICIPRM.2019.8819121

Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

To suppress wafer bowing and crack generation of GaN on Si substrates, we investigated the effects of the Al content and thickness of the AlGaN interlayer on the compressive strain in the overlying GaN layer theoretically and experimentally. In the simulation, AlGaN relaxes gradually over the critical thickness. Therefore, the relaxation ratio of AlGaN at the top surface can be defined as a function of Al content and thickness. Too high Al content or too thick AlGaN interlayer induced too large initial strain in the upper GaN layer, which caused rapid and succeeding gradual relaxation, i.e., decrease of strain, of the GaN layer during growth because of generation of threading dislocations. Conversely, low Al content or thin AlGaN interlayer could induce constant but only small strain in the GaN layer. Therefore, the ideal relaxation ratio of the AlGaN surface exists to apply the maximal constant compressive strain in the GaN layer. The relaxation ratios of AlGaN interlayers determined in experiments were much smaller than those calculated in the simulation. Although the measured compressive strain in the GaN layer was smaller than expected, its decrease rate was small when grown on AlGaN interlayers with an almost ideal relaxation ratio.

DOI： 10.1016/j.jcrysgro.2019.01.013

Publishing type：Research paper (scientific journal)  

DOI： 10.1063/1.5086321

Publishing type：Research paper (international conference proceedings)  

© 2019 OSA. A novel Stokes vector (SV) receiver circuit with 4-port configuration integrated on a compact InP chip is fabricated to demonstrate SV retrieval with various intensity and degree-of-polarization as well as application to in-band OSNR monitoring.

DOI： 10.1364/ofc.2019.w3b.6

Publishing type：Research paper (international conference proceedings)  

© 2019 OSA. We develop large-scale optical phased array (OPA) with 128 phase shifters integrated on a compact silicon chip for single-pixel ghost imaging application. By using speckle illumination pattern generated from OPA, calibration-free robust imaging is demonstrated.

DOI： 10.1364/ofc.2019.m4e.2

Publishing type：Research paper (international conference proceedings)  

© 2019 OSA. We demonstrate novel silicon photonic 4×4 reconfigurable optical unitary converter, comprising multiport directional couplers and phase shifter arrays. By optimizing the phase shift with simulated annealing algorithm, reconfigurable mode sorting and switching are experimentally realized.

DOI： 10.1364/ofc.2019.th1e.3

Publishing type：Research paper (scientific journal)  

© 2019 Optical Society of America. Since the conventional coherent transceiver is costly to be deployed in short-reach networks due to its complicated receiver structure, it is desired to simplify the structure itself. In this paper, we propose a simple polarization-diversity coherent receiver structure by exploiting the concept of the Stokes analyzer. Compared to the conventional architecture, the number of the photodiodes (PDs) is reduced from eight to six without relying on complicated analog circuits. In addition, splitters and combiners for dual-polarization (DP) signals can be replaced with only one polarization beam splitter or combiner (PBS/C). For evaluation of the proof-of-concept (PoC), we develop a prototype of the receiver using free-space optical components. We demonstrate the transmission of 120-Gb/s DP quadrature phase-shift keying (QPSK) and DP 8-ary quadrature-amplitude modulation (8QAM) signals over a 100-km single-mode fiber (SMF). We believe that the demonstrated architecture could potentially be integrated monolithically on silicon-photonic or InP platforms to realize compact and low-cost coherent transceivers for short-reach applications.

DOI： 10.1364/OE.27.009080

Publishing type：Research paper (scientific journal)  

© 2018 Elsevier B.V. We present the feasibility of metalorganic vapor phase epitaxy (MOVPE) with extremely high-speed growth of GaAs for solar cell applications. The growth rate was increased up to 120 μm/h, and exhibited an almost linear relationship with the amount of supplied trimethylgallium (TMGa). The thickness variation and doped carrier concentration of GaAs grown at 90 μm/h were comparable to those of conventionally grown GaAs at a lower growth rate. The potential for reducing the V/III supply ratio was investigated to reduce the material cost. Non-doped GaAs wafers were grown at the accelerated growth rate of 90 μm/h, with various V/III ratios. The growth rate of GaAs increased by 20% when the V/III ratio was decreased from 40 to 5. In low temperature photoluminescence (PL) measurement, significant change in PL spectra was not observed, indicating that there was no significant change in quality. The light-power conversion efficiency was almost comparable for V/III ratios from 10 to 40 whereas that at the lowest V/III ratio of 5 was degraded. Photovoltaic (PV) solar cells of GaAs were fabricated with various growth condition. It is found that the performance of the cells grown at 90 μm/h were comparable with previous results.

DOI： 10.1016/j.jcrysgro.2018.12.004

Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

We present the feasibility of metalorganic vapor phase epitaxy (MOVPE) with extremely high-speed growth of GaAs for solar cell applications. The growth rate was increased up to 120 mu m/h, and exhibited an almost linear relationship with the amount of supplied trimethylgallium (TMGa). The thickness variation and doped carrier concentration of GaAs grown at 90 mu m/h were comparable to those of conventionally grown GaAs at a lower growth rate. The potential for reducing the V/III supply ratio was investigated to reduce the material cost. Nondoped GaAs wafers were grown at the accelerated growth rate of 90 mu m/h, with various V/III ratios. The growth rate of GaAs increased by 20% when the V/III ratio was decreased from 40 to 5. In low temperature photoluminescence (PL) measurement, significant change in PL spectra was not observed, indicating that there was no significant change in quality. The light-power conversion efficiency was almost comparable for V/III ratios from 10 to 40 whereas that at the lowest V/III ratio of 5 was degraded. Photovoltaic (PV) solar cells of GaAs were fabricated with various growth condition. It is found that the performance of the cells grown at 90 mu m/h were comparable with previous results.

DOI： 10.1016/j.jcrysgro.2018.12.004

Publishing type：Research paper (scientific journal)  

© 2019 American Physical Society. Two reciprocity theorems are important for fundamental understanding of the solar cell operation and applications to device evaluation: (1) the carrier-transport reciprocity connecting the dark-carrier injection with the short-circuit photocarrier collection and (2) the optoelectronic reciprocity connecting the electroluminescence with the photovoltaic quantum efficiency at short circuit. These theorems, however, fail in devices with thick depletion regions such as p-i-n junction solar cells. By properly linearizing the carrier-transport equation in such devices, we report that the dark-carrier injection is related to the photocarrier collection efficiency at the operating voltage, not at short circuit as suggested in the original theorem. This leads to the general form of the optoelectronic reciprocity relation connecting the electroluminescence with the voltage-dependent quantum efficiency, providing a correct interpretation of the optoelectronic properties of p-i-n junction devices. We also discuss the validity of the well-known relation between the open-circuit voltage and the external luminescence efficiency. The impact of illumination intensity and device parameters on the validity of the reciprocity theorems is quantitatively investigated.

DOI： 10.1103/PhysRevApplied.11.024029

Publishing type：Research paper (scientific journal)  

© 2018 Elsevier B.V. High growth rate of GaAs more than 90 μm/h is achieved in the attempt of the productivity improvement of solar cells. The design concept is derived from the MOVPE reactor for nitrides. The design criteria for nitride MOVPE reactor were found to be very useful for GaAs MOVPE. Originally, MOVPE reactors for nitrides were developed on the basis of the reactor design for classical III-V materials. Today, the principle of reactor design to cope with the singularity of nitrides brings about essential feedback to the next-generation reactors for classical III-V materials for improved productivity. At first, design criteria for GaN MOVPE reactor are described in light of the vapor phase reaction. Secondly, cost analysis of a 19 µm-thick GaN layer on a bulk GaN substrate for a vertical diode is briefly discussed in light of a trade-off between a high purity and a high growth rate of GaN. In GaAs-based solar cells, the thickness of light absorber is a couple of micrometers, accompanied by a thin (Al)InGaP cap, the growth time of which can be reduced to a few minutes if the growth rate is on the order of a few tens µm/h. In consideration of these factors, cost analysis of MOVPE for GaAs solar cells will be discussed in comparison with GaN on GaN diode.

DOI： 10.1016/j.jcrysgro.2018.11.011

Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

High growth rate of GaAs more than 90 mu m/h is achieved in the attempt of the productivity improvement of solar cells. The design concept is derived from the MOVPE reactor for nitrides. The design criteria for nitride MOVPE reactor were found to be very useful for GaAs MOVPE. Originally, MOVPE reactors for nitrides were developed on the basis of the reactor design for classical III-V materials. Today, the principle of reactor design to cope with the singularity of nitrides brings about essential feedback to the next-generation reactors for classical III-V materials for improved productivity.At first, design criteria for GaN MOVPE reactor are described in light of the vapor phase reaction. Secondly, cost analysis of a 19 mu m-thick GaN layer on a bulk GaN substrate for a vertical diode is briefly discussed in light of a trade-off between a high purity and a high growth rate of GaN.In GaAs-based solar cells, the thickness of light absorber is a couple of micrometers, accompanied by a thin (Al) InGaP cap, the growth time of which can be reduced to a few minutes if the growth rate is on the order of a few tens mu m/h. In consideration of these factors, cost analysis of MOVPE for GaAs solar cells will be discussed in comparison with GaN on GaN diode.

DOI： 10.1016/j.jcrysgro.2018.11.011

Publishing type：Research paper (scientific journal)  

© 2019 IOP Publishing Ltd. By enhancing the transport of gas precursors though the chemical boundary layer using a narrow-channel reactor, the growth rate (GR) of GaAs by metal-organic vapor-phase epitaxy has been increased to 120 μm h -1 without saturation with an increase in TMGa supply. The minority hole lifetime in n-GaAs was clearly shortened with an increase in GR from 20 to 120 μm h -1 . Consequently, the short-circuit current density as well as conversion efficiency slightly decreased with an increase in GR. Nevertheless, a conversion efficiency of 24.48% could be realized for a GaAs cell with the n-type base layer grown at 120 μm h -1 .

DOI： 10.1088/1361-6463/aaf850

Publishing type：Research paper (international conference proceedings)  

© 2019 The Author(s) We experimentally demonstrate a simple straight-line configuration of Stokes vector modulator with two cascaded phase modulators. Three-dimensional 8-ary Stokes-vector-modulated signal is generated at 30 Gb/s and transmitted over a 50-km dispersion-compensated single-mode fiber.

DOI： 10.1364/CLEO_SI.2019.SM3G.4

Publishing type：Research paper (international conference proceedings)  

© OSA 2019 © 2019 The Author(s) A novel Stokes vector (SV) receiver circuit with 4-port configuration integrated on a compact InP chip is fabricated to demonstrate SV retrieval with various intensity and degree-of-polarization as well as application to in-band OSNR monitoring.

Publishing type：Research paper (international conference proceedings)  

© 2019 The Author (s) A surface-normal optical modulator using electro-optic polymer embedded inside a 570-nm-thick silicon high-contrast-grating resonator is fabricated. With a voltage applied to silicon grating, we obtain >20% intensity modulation of the transmitted light at 30 MHz.

DOI： 10.1364/CLEO_AT.2019.JTh2A.48

Publishing type：Research paper (international conference proceedings)  

© 2019 SPIE. InGaP top cell often limits the efficiency of multi-junction solar cells. Its efficiency is expected to increase by introducing strain-balanced In1-xGaxP/In1-yGayP multiple-quantum-wells since the concentration of photoexcited carriers into the wells enhances radiative recombination. To keep carrier collection efficiency to be similar to the case of a bulk InGaP cell, effective mobilities of carriers through the cascaded quantum wells are evaluated. The critical mobilities for maintaining good carrier transport as a photovoltaic device are discussed, which provides a general design principle for the multiplequantum- well architecture.

DOI： 10.1117/12.2508592

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

Language：Japanese   Publishing type：Research paper (scientific journal)  

Publishing type：Research paper (international conference proceedings)  

Asia Communications and Photonics Conference (ACP) © OSA 2019 © 2019 The Author(s) In this invited paper, we report our recent progresses for SVM-based systems. The results show that the technologies for the SVM format can simplify not only the advanced DD systems, but also the coherent systems.

Publishing type：Research paper (international conference proceedings)  

© OSA 2019 © 2019 The Author(s) We demonstrate novel silicon photonic 4×4 reconfigurable optical unitary converter, comprising multiport directional couplers and phase shifter arrays. By optimizing the phase shift with simulated annealing algorithm, reconfigurable mode sorting and switching are experimentally realized.

Publishing type：Research paper (international conference proceedings)  

© OSA 2019 © 2019 The Author(s) We develop large-scale optical phased array (OPA) with 128 phase shifters integrated on a compact silicon chip for single-pixel ghost imaging application. By using speckle illumination pattern generated from OPA, calibration-free robust imaging is demonstrated.

Publishing type：Research paper (international conference proceedings)  

© 2019 Institution of Engineering and Technology. All rights reserved. We fabricate and experimentally demonstrate 4u4 optical unitary mode converter monolithically integrated on InP. Based on the multi-plane light conversion concept, reconfigurable 4-mode sorting and switching are demonstrated. We also observed clear eye opening of 40-Gb/s non-return-to-zero signal.

Publishing type：Research paper (scientific journal)  

Publishing type：Research paper (international conference proceedings)  

OFC 2019 © OSA 2019 © 2019 The Author(s) Stokes-vector modulation direct-detection (SVM-DD) formats are expected as cost-effective methods to transmit multi-level signals in short-reach links. In this talk we review our novel SV modulator and receiver circuits realized on monolithic InP platforms, which possess compact non-interferometric configurations, relatively simple fabrication procedures, and compatibility with other active photonic components.

Publishing type：Research paper (scientific journal)  

© 2018 Elsevier B.V. Low-defect III-V integration on Si(1 0 0) requires atomically well-ordered heterointerfaces. To this end, we study the interaction of As with Si(1 0 0) surfaces and the formation of dimerized Si(1 0 0):As surfaces in situ with reflection anisotropy spectroscopy during metalorganic chemical vapor deposition (MOCVD). Control of surface processes allows us to intentionally ‘rotate’ the dimer orientation and choose between predominantly (1 × 2) and (2 × 1) reconstructed Si(1 0 0):As surfaces both for low and higher surface misorientation. We also verify that the (2 × 1) surface reconstruction becomes energetically more favorable for vicinal Si(1 0 0):As surfaces. We explain the preparation of Si(1 0 0):As surfaces displaying an almost single-domain, (1 × 2) reconstructed surface with an RMS roughness ≤1 Å and exhibiting broad, atomically flat terraces as well as regular double-layer steps. This surface is highly suitable for subsequent low-defect III-V integration.

DOI： 10.1016/j.apsusc.2018.07.181

Publishing type：Research paper (scientific journal)  

DOI： 10.1063/1.5054964

Publishing type：Research paper (scientific journal)  

© 2018 The Author(s). Semiconductor devices capable of generating a vortex beam with a specific orbital angular momentum (OAM) order are highly attractive for applications ranging from nanoparticle manipulation, imaging and microscopy to fiber and quantum communications. In this work, an electrically pumped integrated OAM emitter operating at telecom wavelengths is fabricated by monolithically integrating an optical vortex emitter with a distributed feedback laser on the same InGaAsP/InP epitaxial wafer. A single-step dry-etching process is adopted to complete the OAM emitter, equipped with specially designed top gratings. The vortex beam emitted by the integrated device is captured and its OAM mode purity characterized. The integrated OAM emitter eliminates the external laser required by silicon- or silicon-on-insulator-based OAM emitters, thus demonstrating great potential for applications in communication systems and the quantum domain.

DOI： 10.1038/s41467-018-05170-z

Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

Low-defect III-V integration on Si(1 0 0) requires atomically well-ordered heterointerfaces. To this end, we study the interaction of As with Si(1 0 0) surfaces and the formation of dimerized Si(1 0 0): As surfaces in situ with reflection anisotropy spectroscopy during metalorganic chemical vapor deposition (MOCVD). Control of surface processes allows us to intentionally 'rotate' the dimer orientation and choose between predominantly (1x2) and (2x1) reconstructed Si(1 0 0): As surfaces both for low and higher surface misorientation. We also verify that the (2x1) surface reconstruction becomes energetically more favorable for vicinal Si(1 0 0): As surfaces. We explain the preparation of Si(1 0 0): As surfaces displaying an almost single-domain, (1x2) reconstructed surface with an RMS roughness <= 1 angstrom and exhibiting broad, atomically flat terraces as well as regular double-layer steps. This surface is highly suitable for subsequent low-defect III-V integration.

DOI： 10.1016/j.apsusc.2018.07.181

Publishing type：Research paper (international conference proceedings)  

© 2018 IEEE. In order to avoid formation of antiphase domains at the III-V/Si interface, which significantly can reduce the performance of photovoltaic devices, the Si(100) surface requires precise, double-atomic-step preparation. Here, we study the interaction of Si(100) surfaces with As, which is present in most application-relevant III-V MOCVD reactors, in dependence of the miscut magnitude. Combining optical in situ spectroscopy with ultra-high-vacuum-based surface analysis, we yield control over the dimer orientation on the Si:As surfaces. We demonstrate both (1x2) and (2x1) reconstructed surfaces, and atomically smooth Si(100):As 0.1° surfaces with evenly spaced, doubleatomic steps, which are highly suitable for subsequent III-V integration.

DOI： 10.1109/PVSC.2018.8547817

Publishing type：Research paper (international conference proceedings)  

© 2018 IEEE. According to the prior researches, the Carrier Collection Efficiency (CCE) of the solar cell containing Wire on Well (WoW) is higher than that of containing Multiple Quantum Well (MQW), and the carrier lifetime of WoW is longer than that of MQW; few studies, however, have focused on the effective carrier mobility of WoW. We evaluated the mobility of nano-structures and revealed that the carrier mobility of WoW is higher than that of MQW.

DOI： 10.1109/PVSC.2018.8548262

Publishing type：Research paper (international conference proceedings)  

© 2018 IEEE. Effects of V/III ratio on the quality of GaAs and performance of p-n solar cells grown at extremely high growth rates have been investigated. Under a growth rate of 90 μm/h, the decrease in V/III ratio from 40 to 20, 10 and 5 did not obviously affect the quality of GaAs; no change was detectable by AFM, XRD, and time-resolved PL as the extent of degradation might be out of detectable range. However, the background p-type doping concentration, most likely due to carbon impurity increased from 2.1x10 16 to 3.2x10 16 and 8.4x10 16 cm -3 with reduces in V/III ratio from 20 to 10 and 5, respectively. Meanwhile, the sample grown with the V/III ratio of 40 showed a semi-insulating behavior. In addition, the performance of GaAs p-on-n cells was remarkably reduced when the V/III ratio during the growth of 90-μm /h grown n-base layer was decreased from 20 to 10 and 5. Background impurities including carbon were considered as a reason for this degradation. The uniformity of cell efficiency was also degraded with a lower V/III ratio.

DOI： 10.1109/PVSC.2018.8547263

Publishing type：Research paper (international conference proceedings)  

© 2018 IEEE. Multi-junction solar cells are currently the efficiency leaders among all types of solar cells. However, efficiencies of multi-junction solar cells are limited by various series resistance when they are illuminated under high concentrations. In this paper, we provide a robust method to obtain the current transport efficiency mapping for a precise and rapid measurement of resistance effect using luminescence characterization. We verified the method on a triple-junction solar cell with illuminations on all subcells. The results successfully matched with both electrical measurement and simulation results. Light beam induced current (LBIC) measurement is used for the first time to compare with the luminescence mapping. Both methods show a similar profile of transport efficiency.

DOI： 10.1109/PVSC.2018.8548208

Publishing type：Research paper (international conference proceedings)  

© 2018 IEEE. Gain-coupled 4 ×25 Gb/s EML array is implemented by identical epitaxial layer integration scheme. The device exhibits excellent single-mode performance, high extinction ratio, and a modulation bandwidth over 28 GHz, making it suitable for 100GbE applications.

DOI： 10.1109/ISLC.2018.8516163

Publishing type：Research paper (international conference proceedings)  

© 2018 IEEE. We propose a new approach of coupling light into a waveguide from a wavelength-scale capsule-shaped metal-clad laser. Numerical analysis shows that coupling efficiency can be readily tuned by changing the width of the adjacent taper waveguide. Spontaneous emission is observed from a waveguide of a fabricated device.

DOI： 10.1109/ISLC.2018.8516222

Publishing type：Research paper (scientific journal)  

© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Surface gratings are widely used in photonic integrated circuits (PICs). In this paper, the authors study the fabrication of surface gratings on indium phosphide (InP) based materials by inductively coupled plasma (ICP) dry etching. The etching parameters are optimized to achieve high aspect ratio while maintaining excellent surface morphology. A novel processing technique is presented for surface grating formation on top of a deeply etched ridge waveguide by taking advantages of the lag effect. Fine structures with non-uniform height can be completed in a single-step ICP dry etching, thus greatly simplifying the fabrication by eliminating the demanding overlay process. The method is believe to be valuable for the fabrication of a variety of PICs containing hyperfine structures.

DOI： 10.1002/pssa.201800406

Publishing type：Research paper (international conference proceedings)  

© 2018 The Japan Society of Applied Physics. The effects of wafer (surface) temperature on the surface morphologies of InP, InGaAsP, and InGaAs grown in a planetary metalorganic vapor phase epitaxy (MOVPE) reactor using tertiarybutylarsine (TBA) and tertiarybutylphosphine (TBP) as group V precursors were investigated. Compared with small horizontal reactors, InP growths require a relatively low temperature to prevent TBP predecomposition and parasitic gas-phase reactions. However, such a low temperature can prevent the surface migration of In adatoms and degrade surface morphology, especially when a substantial amount of Ga precursors is present. Hence, growths of InGaAs require a higher temperature than that of InP and InGaAsP. In this work, 1.04 eV In0.79Ga0.21As0.46P0.54 and 0.74 eV In0.53Ga0.47As single-junction cells as well as InGaAsP/InGaAs dual-junction solar cells have been successfully fabricated. The cell performances reveal that InGaAs layers grown at 590 °C exhibit great crystal quality, while 550-°C-grown InGaAsP layers contain a noticeable extent of defect density and lattice imperfection.

DOI： 10.7567/JJAP.57.08RD09

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

The effects of wafer (surface) temperature on the surface morphologies of InP, InGaAsP, and InGaAs grown in a planetary metalorganic vapor phase epitaxy (MOVPE) reactor using tertiarybutylarsine (TBA) and tertiarybutylphosphine (TBP) as group V precursors were investigated. Compared with small horizontal reactors, InP growths require a relatively low temperature to prevent TBP predecomposition and parasitic gas-phase reactions. However, such a low temperature can prevent the surface migration of In adatoms and degrade surface morphology, especially when a substantial amount of Ga precursors is present. Hence, growths of InGaAs require a higher temperature than that of InP and InGaAsP. In this work, 1.04 eV In0.79Ga0.21As0.46P0.54 and 0.74 eV In0.53Ga0.47As single-junction cells as well as InGaAsP/InGaAs dual-junction solar cells have been successfully fabricated. The cell performances reveal that InGaAs layers grown at 590 degrees C exhibit great crystal quality, while 550-degrees C-grown InGaAsP layers contain a noticeable extent of defect density and lattice imperfection. (C) 2018 The Japan Society of Applied Physics

DOI： 10.7567/JJAP.57.08RD09

Publishing type：Research paper (international conference proceedings)  

© Copyright 2018 The Institute of Electronics, Information and Communication Engineers. To meet the demand for continuous increase in data traffic, full usage of polarization freedom of light is becoming inevitable in the next-generation optical communication and datacenter networks. In particular, Stokes-vector modulation direct-detection (SVM-DD) formats are expected as potentially cost-effective method to transmit multi-level signals without using costly coherent transceivers in the short-reach links. For the SVM-DD formats to be practical, both the transmitter and receiver need to be substantially simpler, smaller, and lower-cost as compared to coherent counterparts. To this end, we have recently proposed and demonstrated novel SV modulator and receiver circuits realized on monolithic InP platforms. With compact non-interferometric configurations, relatively simple fabrication procedures, and compatibility with other active photonic components, the proposed devices should be attractive candidate in realizing low-cost monolithic transceivers for SVM formats. In this paper, we review our approaches as well as recent progresses and provide future prospects.

DOI： 10.1587/transele.E101.C.594

Publishing type：Research paper (scientific journal)  

© 2017 Elsevier B.V. The combination of tunable direct bandgap III-V absorbers with active Si substrates promises high-efficiency tandem solar cells. To yield the ideal III-V bandgaps between 1.6 and 1.8 eV which is considered to achieve current matching with the Si bottom-cell, however, either a big lattice-mismatch or dilute nitrides had to be considered so far. Here, we propose a new structure for a two-terminal III-V-on-Si solar cell which bases on strain-balanced III-V multi-quantum wells (MQWs) embedded in a metamorphic GaAsP top-cell matrix. Strain-balanced MQWs extend the absorption edge to a longer wavelength and enable a reduction of the As content in the GaAsP metamorphic top-cell matrix. And accumulation of carriers in MQWs favors radiative recombination, which is beneficial for high efficiency while deep quantum wells lower the charge carrier collection efficiency. Here, we predict solar energy conversion efficiencies over 42.6% with an entire MQW as thin as 500 nm. The applied model takes into account the drawbacks of MQWs, such as limited light absorption and the bottleneck of charge carrier collection from the confinement.

DOI： 10.1016/j.solmat.2017.06.060

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier B.V.  

Realization of high efficiency III-V-on-Si solar cells requires preparation of an oxygen free Si substrate surface, which allows for subsequent antiphase domain free III-V epitaxy. For GaAsP/Si tandem cells, the impact of As on established Si processing in metal organic vapor phase epitaxy (MOVPE) is essential. Here, we study the interaction of As with vicinal Si(100) surfaces, the formation of atomically well-ordered As-modified Si(100) surfaces, and its impact on subsequently grown GaP epilayers. We apply optical in situ spectroscopy during MOVPE and show that process routes strongly affect the formation of double-layer steps on Si(100). We demonstrate that exposure to As, together with HF pre-treatment, enables processing of suitable Si(100) 2° and 6° offcut surfaces at temperatures below 820 °C. Subsequently grown GaP epilayers exhibit smooth surfaces free of antiphase disorder and distinct Laue oscillations in X-ray diffraction. Such low temperature processing, which is controllable in situ, is promising for further metamorphic GaAsP integration.

DOI： 10.1016/j.solmat.2017.07.032

Publishing type：Research paper (international conference proceedings)  

© 2018 OSA. We demonstrate reconfigurable all-optical MIMO demultiplexing on a compact silicon chip. 3-mode demultiplexing is realized experimentally with wavelength-dependent loss less than 3 dB and modal crosstalk less than -13 dB over 26-nm optical bandwidth.

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Physics  

Thin-film solar cells fabricated using the epitaxial lift-off (ELO) process enable considerable cost reduction as well as light-trapping. A polyimide film was used as a support substrate for thin film devices to address the poor temperature tolerance of poly(ethylene terephthalate) films. Samples were stuck to polyimide films by an Au–Au bonding process. It was found that the insertion of a wetting layer enhanced the adhesion between a polyimide film and an Au layer. The achieved bonding was strong enough to carry out the ELO process, and no degradation was observed in cell performance. Thin-film solar cells with strain-balanced multiple quantum wells were also fabricated using the ELO process to benefit from the light-trapping effect. Quantum efficiency enhancement was observed in the long-wavelength range of 850–980 nm compared to the non-ELO-processed devices. The photo-absorption enhancement was due to the Fabry–Perot resonance between the surface and the rear Au electrodes.

DOI： 10.7567/JJAP.57.08RF03

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier B.V.  

The high growth rate of epitaxial GaAs was investigated using a novel horizontal metalorganic vapor phase epitaxy (MOVPE) reactor, from the point of view of realizing low-cost photovoltaic (PV) solar cells. The GaAs growth rate exhibited an approximately linear relationship with the amount of trimethylgalium (TMGa) supplied, up to a rate of 90 μm/h. The distribution of growth rate was observed for a two-inch wafer, along the flow direction, and the normalized profile of the distribution was found to be independent of the precursor input, from 20 to 70 μm/h. These tendencies indicated that significant parasitic prereaction did not occur in the gaseous phase, for this range of growth rate. GaAs p-n single-junction solar cells were successfully fabricated at growth rates of 20, 60, and 80 µm/h. The conversion efficiency of the cell grown at 80 µm/h was comparable to that of the 20 µm/h cell, indicating the good quality and properties of GaAs. The epitaxial growth exhibited good uniformity, as evidenced by the uniformity of the cell performance across the wafer, from the center to the edge. The result indicated the potential of high-throughput MOVPE for low-cost production, not only for PV devices but also for other semiconductor applications.

DOI： 10.1016/j.jcrysgro.2018.02.033

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE Electron Devices Society  

Epitaxial growth of GaAs in a horizontal metalorganic vapor phase epitaxy reactor was investigated to realize ultrafast growth rate, which has been demonstrated to be 90 μm/h at maximum. The GaAs growth rate had more or less a linear relationship with the amount of trimethylgallium supply. With extremely fast deposition &gt
70 μm/h, slight saturation of growth rate was observed, which is most likely due to parasitic gas-phase reactions and/or limitation in bubbling. The impact of growth rates was examined using GaAs p-n solar cells, with the 2-μm-thick n-GaAs base layers grown at 20, 60, and 80 μm/h. The studies have not evidenced a significant change in performance as a function of growth rate. The base thickness was expanded to 3.5 μm with the growth rate of 80 μm/h to increase light absorption. The growth rate of 80 μm/h did not induce significant degradation in the efficiency of the cells as compared with the ones grown at 20 and 60 μm/h. The average efficiency of 23.6% (16.3% on the basis of a mesa area) under AM1.5G was realized in the cells with 3.5-μm-thick bases grown at 80 μm/h.

DOI： 10.1109/JPHOTOV.2018.2814919

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Institute of Electrical and Electronics Engineers Inc.  

We fabricate and experimentally demonstrate a compact direct-detection-based Stokes vector (SV) receiver with on-chip photodetector (PD). Compressively strained multiple-quantum-well PD array is monolithically integrated on InP-based passive polarization-converting circuit to retrieve SV of input light from the photocurrent signals.

DOI： 10.1109/ECOC.2017.8345838

Language：English   Publishing type：Research paper (scientific journal)   Publisher：OSA - The Optical Society  

We present, to the best of our knowledge, the first experimental demonstration of reconfigurable all-optical on-chip multi-input-multi-output three-mode demultiplexing based on multi-plane light conversion. The demultiplexer consists of cascaded phase shifter arrays and multimode interference couplers integrated on a compact silicon chip. By optimizing the phase shifters, reconfigurable three-mode demultiplexing is experimentally realized with wavelength-dependent loss of less than 3 dB and modal crosstalk of less than −10 dB over a 23 nm optical bandwidth. Error-free mode demultiplexing of 40 Gbps non-return-to-zero signal is also demonstrated.

DOI： 10.1364/OL.43.001798

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1080/14686996.2018.1433439

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers Inc.  

Stokes vector modulation direct-detection (SVM-DD) formats have attained special attention in the short-reach communication links to transmit multilevel signals without using costly coherent receivers. In this paper, we fabricate and experimentally demonstrate a potentially low-cost DD-based SV receiver with on-chip photodetectors (PD) on a compact InP chip. Compressively strained multiple-quantum-well PD array is monolithically integrated with the SV projection circuit, which consists of multimode interference splitter and SV rotators based on self-aligned half-ridge structure. From the photocurrent signals at three on-chip PDs, the SV of input light is retrieved successfully over the entire Poincaré sphere. With potentially broad operating bandwidth of InP-based PDs, the demonstrated device may be an attractive candidate to realize low-cost and high-speed receivers for the future short-reach SVM-DD systems.

DOI： 10.1109/JLT.2017.2780905

Publishing type：Research paper (scientific journal)  

© 2018 IEEE. Of-late research on Stokes vector receivers for the short-reach optical communications has gained significant attention due to its capability to decode information in multidimension using cost-effective direct-detection technique. Here, we consider the use of InP for potentially low-cost direct-detection-based Stokes vector (SV) receiver. It consists of a 1 × 4 multimode interference power splitter with different combination of polarization converters on output arms, so that input SV is projected onto distinct vectors on the Stokes space. By using the fabricated device, we experimentally demonstrate decoding of two-level and four-level SV-modulated signals at 3 Gbaud. With the potential of monolithically integrating high-speed InP photodetector array on the same chip, it paves the way to realize compact and low-cost SV receivers for multilevel SV modulation formats in future short-reach optical communication links.

DOI： 10.1109/JLT.2018.2791623

Publishing type：Research paper (scientific journal)  

© 2017 Elsevier B.V. Solar cells employing quantum wells can enhance the light absorption but suffer from the difficulty in photocarrier extraction. Here, we analyzed the spectral response and the photocarrier collection mechanism of p-i-n multiple quantum well (MQW) solar cells using the effective-mobility model. Both the simulation and experimental results imply that the spatial profiles of electron and hole densities in MQWs play an important role in the carrier collection process. By considering the recombination increment under illumination, our findings suggest that the concept of the majority/minority carriers is important even in the intrinsic region: photogenerated electrons and holes only experience significant recombination when passing through the hole-rich and electron-rich regions, respectively. This can accurately explain the photocurrent behavior in cells with background doping, background illumination, and different MQW positions. Based on the experimental findings, we derived analytical formulae for carrier collection efficiency, which directly show the impact of each cell parameter and can be used for the systematic cell design.

DOI： 10.1016/j.solmat.2017.08.036

Language：English   Publishing type：Research paper (international conference proceedings)  

DOI： 10.1117/12.2296574

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

The effective mobility model is proposed as a convenient tool for the analysis and design of multiple quantum well (MQW) solar cells. The analytical formula for MQW effective mobility and its lower limit for solar cell application are discussed. By plotting MQW bandgap-mobility mappings, the feasible bandgap range of MQWs, in terms of carrier transport, can be analytically estimated for the first time. In this way, the structure optimization of MQW for narrower bandgap, which has been carried out through trail-and-error experiments so far, becomes available through a simple calculation procedure.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We fabricate and experimentally demonstrate a novel metallic metasurface modulator with electro-optic (EO) polymer. By using the resonance of a metal-insulator-metal mode, we efficiently trap surface-normal incident light inside a 540-nm-thick EO polymer layer. Electro-optic modulation of the reflected light is experimentally observed at 1630 nm.

Publishing type：Research paper (scientific journal)  

DOI： 10.7567/JJAP.57.070305

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：OSA - The Optical Society  

We demonstrate a monolithically integrated Stokes vector modulator using quantum-confined Stark effect in a simple straight-line configuration. Efficient polarization modulation over Poincare´ sphere is observed by applying reverse-bias voltages to MQW-based polarization-dependent phase modulators.

DOI： 10.1364/CLEO_SI.2018.SM4B.6

Publishing type：Research paper (international conference proceedings)  

© OSA 2018. We demonstrate reconfigurable all-optical MIMO demultiplexing on a compact silicon chip. 3-mode demultiplexing is realized experimentally with wavelength-dependent loss less than 3 dB and modal crosstalk less than -13 dB over 26-nm optical bandwidth.

DOI： 10.1364/OFC.2018.W1E.3

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Institute of Electrical and Electronics Engineers Inc.  

We propose a novel reconfigurable integrated optical mode (de)multiplexer using cascaded multiport directional couplers and phase shifter arrays. Large fabrication tolerance and scalability up to 16-mode (de)multiplexing are demonstrated.

DOI： 10.1109/OECC.2017.8114996

Language：English   Publishing type：Research paper (scientific journal)   Publisher：OPTICAL SOC AMER  

Electrically tunable metasurfaces have gained special interest as they can realize ultrathin surface-normal modulators in planar geometries. In this paper, we demonstrate a novel metasurface modulator based on electro-optic (EO) polymer that utilizes bimodal resonance inside a metallic subwavelength grating to increase the modulation efficiency. When two metal-insulator-metal (MIM) resonant modes are excited simultaneously inside the grating, they couple strongly to generate a sharp dip in the reflected spectrum. As a result, efficient intensity modulation with 15-dB extinction ratio can be obtained at the resonant wavelength under a small refractive index change of 8.5 x 10(-3), corresponding to modulation voltage of less than 10 V. Due to the low parasitic capacitance of EO polymer and high conductivity of metallic gratings which is also used as the electrodes, the RC bandwidth of the device should easily exceed 100 GHz, potentially applicable to high-speed surface-normal modulators. (C) 2017 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

DOI： 10.1364/OE.25.030304

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

We demonstrate a waveguide-based polarization controller that monolithically incorporates an active InGaAlAs/InAlAs multiple-quantum-well phase shifter and a passive half-ridge polarization converter on InP. The butt-joint active/passive integration technique and self-aligned waveguide fabrication steps are employed. Using the fabricated device, efficient rotation of the Stokes vector over pi radian is obtained on the Poincare sphere by injecting 14-mA current to a 1.4-mm-long phase shifter. Dynamic response time below 5 ns is also confirmed, demonstrating the feasibility of low-power high-speed integrated polarization controller on InP.

DOI： 10.1109/LPT.2017.2757526

Language：English   Publishing type：Research paper (scientific journal)   Publisher：OPTICAL SOC AMER  

A self-seeded reflective semiconductor optical amplifier (RSOA) has been viewed as an attractive candidate for the low-cost colorless transmitter in wavelength-division-multiplexed passive-optical networks (WDM-PON). In this paper, we propose and demonstrate the use of two-section RSOA with segmented electrodes to simultaneously improve the modulation bandwidth and extinction ratio in a self-seeded operation. We first find the optimal driving condition to induce strong modulation cancellation by analyzing the gain dynamics inside the RSOA. In addition, we experimentally confirm the improvement in direct modulation bandwidth by segmenting the electrodes. Finally, we employ the two-section RSOA in a self-seeded system and successfully demonstrate transmission over a 25-km standard single-mode fiber at 5 Gb/s as well as improved back-to-back modulation at 10-Gb/s in C-band by using the optimal driving condition. (C) 2017 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

DOI： 10.1364/OE.25.028547

Publishing type：Research paper (scientific journal)  

DOI： 10.1039/c7se00352h

Publishing type：Research paper (scientific journal)  

© 2017 American Physical Society. We investigate a theoretical model for effective carrier mobility to comprehensively describe the behavior of the perpendicular carrier transport across multiple quantum well (MQW) structures under applied electric field. The analytical expressions of effective mobilities for thermionic emission, direct tunneling, and thermally assisted tunneling are derived based on the quasithermal equilibrium approximation and the semiclassical approach. Effective electron and hole mobilities in InGaAs/GaAsP MQWs predicted from our model are in good agreement with the experimental results obtained from the carrier time-of-flight measurement near room temperature. With this concept, the complicated carrier dynamics inside MQWs can be simplified to an effective mobility, an equivalent parameter that is more straightforward to handle and can be easily incorporated in the conventional drift-diffusion model.

DOI： 10.1103/PhysRevB.96.075441

Publishing type：Research paper (scientific journal)  

© 2017 The Japan Society of Applied Physics. Single-junction GaAs solar cells were grown by metalorganic vapor-phase epitaxy (MOVPE) at various input V/III ratios. All growth parameters other than V/III ratio were carefully controlled for an accurate comparison. Nearly identical cell performance characteristics including short-circuit current density (J sc) and open-circuit voltage (V oc) indicate that cell performance is independent of V/III ratio. To determine the relationship between the electrically measured V oc and V/III ratio in a more precise manner, photoluminescence (PL) was applied as a potent optical measurement tool, which does not depend on device processing and contacting issues. We also evaluated the projected cell performance under low-concentration sunlight by electroluminescence (EL) analysis. Similarly to electrical measurement, optical measurement showed no obvious degradation owing to a low V/III ratio. This study strongly demonstrates that low-cost high-efficiency GaAs solar cells can be realized by MOVPE using a low V/III ratio.

DOI： 10.7567/JJAP.56.08MC06

Publishing type：Research paper (scientific journal)  

© 2017 The Japan Society of Applied Physics. To improve the superlattice (SL) solar cell performance, we carried out an accurate estimation of transition energies and miniband widths and focused on understanding of the optical properties of the SL structure using piezoelectric photothermal (PPT), photoreflectance (PR), and photoluminescence (PL) methods. Solar cell structure samples with different barrier thicknesses from 2.0 to 7.8 nm in quantum wells were prepared. From the PR and theoretical calculation, the formation of a miniband was confirmed. The PL peak showed a redshift and a decrease in signal intensity with decreasing barrier thickness, which were explained by carrier separation as a consequence of electron transportation through the miniband without recombination. The PPT signal intensities of the SL were still large even for the 2.0-nm-barrier-thickness sample. It is conceivable that the multiple-phonon emission during carrier transport through the miniband was detected. The usefulness of multidimensional investigation by using the above three methods is clearly demonstrated.

DOI： 10.7567/JJAP.56.08MC07

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

Several GaAs tunneling junctions (TJs) and p-i-n single junction solar cells grown using a planetary metalorganic vapor phase epitaxy (MOVPE) reactor utilizing various dopant species including Zn, C, S, and Te were investigated. The incorporation of Te atoms into GaAs was approximately two orders larger than that of S atoms. Although only 30% of Te atoms could be electrically activated, a carrier concentration of 10(19)cm(-3) was achieved. Highly C-doped GaAs was successfully obtained by decreasing the growth temperature and increasing the amount of H-2 carrier gas in order to prevent the predecomposition of CBr4 dopant gas. A hole concentration of about 10(20)cm(-3) was realized with a growth temperature of 450 degrees C. The C-Te-doped GaAs TJ exhibited the best ohmic tunneling behavior with a resistivity of 12.5 m Omega center dot cm(2), while the others had diode characteristics. The GaAs solar cell grown with the Zn-S dopant showed the highest conversion efficiency ascribed to a longer minority carrier lifetime. (C) 2017 The Japan Society of Applied Physics

DOI： 10.7567/JJAP.56.08MC11

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

We fabricate a capsule-shaped metal-clad wavelength-scale InP/InGaAs cavity and demonstrate room-temperature lasing at a wavelength of similar to 1550 nm under pulsed optical pumping. By introducing an optimized curvature to the sidewalls of a conventional rectangular metal-clad laser, we effectively reduce the metallic loss of the transverse-electric mode and experimentally confirm a clear improvement in the side mode suppression ratio and slope efficiency. Published by AIP Publishing.

DOI： 10.1063/1.5000246

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

We propose a wavelength-scale metal-clad III-V semiconductor cavity coupled to silicon-on-insulator (SOI) waveguide with integrated feedback stub for external tuning of the Q factor. We consider a typical wavelength-scale metal-clad InP/InGaAs cavity integrated on a SOI waveguide and demonstrate more than 5-fold Q-factor improvement in agreement with that predicted by the coupled-mode theory. From additional thermal analysis, we prove that improved heat dissipation can be realized by engineering the layer thicknesses without sacrificing much of the optical performance. (C) 2017 The Japan Society of Applied Physics

DOI： 10.7567/JJAP.56.082201

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

Clarification and control of the hydrogen etching of GaN is essential to achieving high-quality GaN-on-Si virtual substrates and devices based on it produced by metalorganic vapor phase epitaxy. This phenomenon and its effects on GaN-on-Si stress control were studied in this work. Without deliberate protection, voids with lateral sizes on the micrometre level underneath AlN interlayers emerged in GaN. Such voids made stress balancing in GaN-on-Si systems by inserting AlN interlayers less efficient. By flowing a protective large flow rate of ammonia, voids were eliminated while GaN decomposition still happened, which led to AlGaN alloy in the interlayers. Interfaces of Ga incorporated AlN interlayers grown under large-scale varied conditions were characterised by scanning transmission electron microscopy and energy dispersive spectrometry. Higher growth temperatures caused more Ga in the interlayers and weakened their capacity to induce compressive stress in the overlying GaN.

DOI： 10.1088/1361-6641/aa6b86

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Physics  

InGaAs and GaNAs were selected as components of a multiple quantum well (MQW) with a free-barrier conduction band (FB-CB) in which the quantum confinement for electrons was eliminated. Since the calculation demonstrated that the energy gap of a strain-balanced FB-CB InGaAs/GaNAs MQW could be decreased to 1.2 eV with lattice matching to Ge, this structure was expected as a potential absorber of the middle cell of a three-junction solar cell based on the Ge bottom cell. Additionally, the InGaAs/GaNAs MQW could mitigate detrimental impacts of the short lifetime of GaNAs because of the preferential existence of holes in InGaAs, and it can realize more efficient carrier transport than bulk GaInNAs. The time-resolved photoluminescence (TRPL) results demonstrated that the InGaAs/GaNAs MQW cell provided a significantly longer lifetime than the GaInNAs thin-film cell. The open-circuit voltage of the InGaAs/GaNAs MQW cell was superior to that of the GaInNAs thin-film cell.

DOI： 10.7567/JJAP.56.08MA04

Publishing type：Research paper (international conference proceedings)  

© 2017 JSPS 191st Committee on Innovative Interface Bonding Technology. A thickness controlled dual junction GaAs//Si solar cell for current matching was fabricated and demonstrated. The optically thin GaAs top cell grown by metal-organic vapor phase epitaxy (MOVPE) was directly integrated on the Si bottom cell by surface-activated bonding (SAB) method. Owing to the optically thin (∼300 nm) GaAs top sub-cell, the operation of current-matched dual junction cell was observed under the standard 1 SUN illumination.

DOI： 10.23919/LTB-3D.2017.7947449

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

Reconfigurable unitary optical mode converters that can convert multiple orthogonal spatial modes into different orthogonal modes without fundamental optical loss are important for mode-division-multiplexed (MDM) optical communication, imaging, and quantum computation. In this letter, we propose a novel integrated reconfigurable unitary optical mode converter, which consists of cascaded multimode interference couplers and phase shifter arrays. We numerically investigate its ability to realize arbitrary unitary mode conversion and demonstrate its applicability to mode unscrambling as well as mode switching in MDM transmission systems.

DOI： 10.1109/LPT.2017.2700619

Publishing type：Research paper (international conference proceedings)  

© 2017 Optical Society of America. Compact and robust photonic-integrated circuit for low-cost direct-detection-based Stokes vector (SV) receiver is presented. A proof-of-concept device is fabricated on InP to demonstrate successful decoding of multilevel SV-modulated signal at 1 Gbaud.

Language：English   Publishing type：Research paper (scientific journal)   Publisher：OPTICAL SOC AMER  

Stokes vector modulation and direct detection (SVM/DD) has immense potentiality to reduce the cost burden for the next-generation short-reach optical communication networks. In this paper, we propose and demonstrate an InGaAsP/InP waveguide-based polarization-analyzing circuit for an integrated Stokes vector (SV) receiver. By transforming the input state-of-polarization (SOP) and projecting its SV onto three different vectors on the Poincare sphere, we show that the actual SOP can be retrieved by simple calculation. We also reveal that this projection matrix has a flexibility and its deviation due to device imperfectness can be calibrated to a certain degree, so that the proposed device would be fundamentally robust against fabrication errors. A proof-of-concept photonic integrated circuit (PIC) is fabricated on InP by using half-ridge waveguides to successfully demonstrate detection of different SOPs scattered on the Poincare sphere. (C) 2017 Optical Society of America

Publishing type：Research paper (scientific journal)  

© 2016 Elsevier B.V. Material quality is a critical factor which determines the performance, particularly the open-circuit voltage, of multiple quantum well (MQW) solar cells. In this study, we report an electroluminescence-based characterization technique for evaluating luminescence efficiency and Shockley-Read-Hall recombination lifetime in MQW structures as a measure of the material quality. As a demonstration, various structures of InGaAs/GaAsP MQWs inserted in GaAs solar cells are investigated. The complete compensation of strain and the insertion of GaAs interlayers between heterointerfaces result in significant improvement of electroluminescence homogeneity, external luminescence efficiency, and lifetime, agreeing well with the tendency of the open-circuit voltage. We show that this characterization technique can detect even subtle degradations, which are not easily detectable by other typical techniques, such as in-situ reflection, X-ray diffraction, and spectral and transient photoluminescence, but still have a significant impact on the performance of solar cells.

DOI： 10.1016/j.jcrysgro.2016.10.056

Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

n-type GaN photoanodes used for water splitting have stability problems. One means of resolving this is loading NiO catalyst on the n-type GaN surface. Aqueous electrolytes H2SO4, HCl, KOH, and NaOH are usually used for photoelectrochemical water splitting. However, suitable electrolytes for the NiO-loading on n-type GaN photoelectrode have not yet been evaluated. Therefore, we investigated the effects of changing electrolytes used for NiO-loading in this study. The photocurrent of NiO-loading on n-type GaN increased when KOH and NaOH electrolytes were used. In addition, the surfaces showed no corrosion after reaction when these electrolytes were used. However, the photocurrent was not stable using KOH electrolyte. Interestingly, stable photocurrent was observed with when the NaOH electrolyte was used. In the case of H2SO4, the photocurrent of GaN did not change with and without NiO. The surface morphologies became rough because of GaN corrosion, and NiO dissolved in the H2SO4 electrolyte. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.ijhydene.2016.12.141

Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

The control of the dopant profile in 3-dimentional InGaAs microdiscs on Si (111) is essential for their device applications. However, such profiles can never be controlled by simply changing the supply of dopant precursors during the growth of microdiscs. This is because a variety of crystal planes, such as (111), {-110}, and irregular planes near the corners, surround a hexagonal pillar of InGaAs and the incorporation efficiency of dopant elements depends significantly on the kind of planes involved. We here observed the distributions of sulfur and zinc in p-i-n InGaAs microdiscs by both cross-sectional scanning capacitance microscopy (SCM) and secondary ion mass spectrometry using focused ion beam (NanoSIMS). Even though the InGaAs shell was grown on the microdiscs using dimethylzinc (DMZn), no p-type region was found on the top of the microdiscs and the p-type region existed on the sidewall of the discs alone. This result suggested that the zinc incorporation efficiency on InGaAs (111) plane is much lower than that on {-110} planes. Complete encapsulation of the microdiscs with p type region was possible by the post-diffusion of zinc during exposure to a mixture of tertiarybutylarsine (TBAs) and DMZn after the growth of InGaAs microdiscs.

DOI： 10.1016/j.jcrysgro.2016.12.003

Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

Crystal properties of low-temperature grown AIN (LT-AlN) combined with low temperature GaN (LT-GaN) grown by metal organic vapor phase epitaxy (MOVPE) were investigated to obtain a high quality GaN/AlN/GaN structure with a few-nm-thick AIN layer. LT-AIN suppresses unintentional Ga incorporation and can be pseudomorphically grown on GaN with a relatively smooth surface morphology. The lattice of LT-AlN coherent to GaN, however, was found to relax after reactor conditions were changed to grow the subsequent GaN layer at higher temperature. The top GaN layer grown on the relaxed LT-AIN, thus, exhibited a rough surface morphology and a threading dislocation density (TDD) higher than 109 cm(-2) estimated from an X-ray diffraction measurement. An LT-GaN capping layer was found to be highly effective for avoiding such lattice relaxation of LT-AlN. The combination of LT-AIN and LT-GaN enables us to obtain a GaN/AIN/GaN junction with high Al content, a low TDD, and abrupt interfaces. As a result, introducing an LT-GaN layer improved the photoelectrochemical (PEC) property of a polarization engineered un-doped GaN/A1N/n-type GaN (u-GaN/ AIN/n-GaN) photocathode for water splitting.

DOI： 10.1016/j.jcrysgro.2016.12.005

Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

For the purpose of controlling the wafer bow of GaN-on-Si structure, in situ curvature transient during the growth of a GaN layer on an AlN interlayer was investigated systematically by estimating the compressive strain applied to the GaN layer with the progress of the layer growth. The compressive strain was dependent on the morphology of the GaN surface prior to the growth of the AIN interlayer. It was found that the transition sequence from GaN growth to AIN growth induces roughening of the GaN surface and both high NH3 partial pressure and the short transition time were effective for reducing the roughness of the GaN surface beneath the interlayer. The improved transition sequence increased the compressive strain in GaN by a factor of 2.5. The AlN grown at the same temperature as that of GaN was beneficial in both better surface morphology and the reduction of the transition time between GaN growth and AN growth. With this high-temperature AN interlayer, its thickness is another important factor governing the compressive strain in GaN. To get AIN relaxed for applying the compressive strain to GaN, the AIN layer should be thicker but too thick layer after relaxation results in surface roughening, which in turn introduces defects to the overlying GaN layer and reduces the compressive strain by partial lattice relaxation of GaN.

DOI： 10.1016/j.jcrysgro.2016.12.090

Publishing type：Research paper (scientific journal)  

© 2017 Elsevier B.V. Wire on Well (WoW) structure embedded in a matrix is naturally formed by growing InxGa1−xAs/GaAs1−yPy strained multiple quantum wells (MQW) on vicinal substrates and employing triethylgallium (TEGa) as a precursor in low-temperature MOVPE. The structure is useful for the subcell in current-matched mult-junction solar cells with lattice-matched materials because of its ability of band-gap tuning. In this research, high density and uniform In0.30Ga0.70As/GaAs0.6P0.4 WoW was obtained up to 200 stacks and its structure was analyzed by X-ray diffraction reciprocal space mapping, atomic force microscopy and scanning transmission electron microscopy. The structure of the wire can be controlled by changing the equivalent layer thicknesses of In0.30Ga0.70As and GaAs0.6P0.4. The photoluminescence peak from the WoW shifted according to the size of InGaAs wires and the intensity was dependent on the accumulation of lattice-mismatch stress.

DOI： 10.1016/j.jcrysgro.2016.11.087

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier B.V.  

Some of the most advanced laptop computers have their trackpads made to detect both force and position precisely. These trackpads are made of intricately designed force sensors, taptic engines and capacitive glass surfaces. In this study, we have modified simple Nylon® fabric which senses both force and position. A commercially available Nylon® fabric was coated with reduced graphene (rGO) in a layer by layer fashion such that the individual fibers of the fabric get wrapped by rGO and the fabric looks like dyed with grey color. The SEM images of the twisted individual rGO coated fibers show that the coating remains undamaged until a twist angle of about 1800°. An in situ applied pressure in compression, on the fabric, of about 2500 kPa changes its resistance 8 kΩ relative to its original resistance with a durability of up to 6000 cycles. Single rGO coated fibers quarantined and tested for their strain sensitivity show that an in situ bending radius of 1 mm changes its resistance to 326 ± 21 kΩ relative to its original resistance. With the help of a read-out circuit it was also showed that quarantined rGO coated fibers arranged in a 2 × 2 grid format sense the position of the applied force.

DOI： 10.1016/j.snb.2016.09.088

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

Self-seeded reflective semiconductor optical amplifier (RSOA) has been drawing special attention as potentially low-cost colorless wavelength-division-multiplexed transmitter. In this paper, we present numerical and experimental analyses of RSOA to find the optimal condition to induce strong modulation canceling effect overwide temperature range. Three InGaAlAs/InP multiple-quantum-well RSOAs with different length are compared numerically and experimentally to reveal the existence of critical RSOA length to induce efficient modulation-cancelling effect. By using the optimal RSOA length of 1 mm, we experimentally demonstrate 2.5-Gb/s self-seeded transmission at 1550-nm wavelength over a 25-km standard single-mode fiber at both 25 and 50 degrees C. In addition, we demonstrate feasibility of higher-temperature (70 degrees C) as well as higher-bitrate (10 Gb/s) operation by employing the amplified self-seeded configuration with appropriately selected RSOAs.

DOI： 10.1109/JLT.2016.2633719

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：SPIE-INT SOC OPTICAL ENGINEERING  

Multijunction solar cells are currently the devices offering the largest conversion efficiencies of the solar radiation, which could be further increased by limiting their series resistances. A clear evaluation of the impact of those resistances is therefore required, and provided in this paper by introducing a mapping method of the current transport efficiency from luminescence images. This method brings finer information on the cell than electroluminescence methods, widely used so far for multi-junction cells, and offers much faster acquisition time than what could be obtained with a light beam induced current setup. While it has been theoretically and experimentally developed for single junction solar cells, its application to multijunction cells remains to be demonstrated. The purpose of this communication is to assess its validity and to explain some results that can be counterintuitive at a first sight. Two different triple-junction architectures are investigated and successfully compared with electrical measurements and calculations.

DOI： 10.1117/12.2250866

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

Multiple quantum wells (MQWs) help enhance the light absorption but suffer from their poor carrier transport. Here, we propose a design rule for the cell optimization, particularly focusing on the optimal position and stack number for a given MQW structure. According to the effective-mobility model, the carrier collection from MQW is efficient when the carrier density profile is balanced and the total thickness is kept below the drift length. Based on this finding, we propose the modified detailed-balance calculation which includes the carrier collection behavior in MQW solar cells.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

A new structure for III-V on Si two-junction solar cells is proposed with the use of multi-quantum wells (MQWs), reducing the lattice constant of the GaAsP-based top cell and minimizing the thickness of a metamorphic buffer layer between the top cell and the GaP seed layer on Si. The strain-balanced MQWs helps the reduction of As content in the top-cell matrix while extending the absorption edge to a longer wavelength by narrow-gap quantum wells. The efficiency is predicted to be as large as 40% with an entire MQW thinner than 800 nm. The model takes into account the drawbacks of MQWs such as limited light absorption and the bottleneck of carrier collection from the confinement states. This work addresses a new direction toward high-efficiency III-V on Si cells.

Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

Aiming at the realization of high efficiency and low cost simultaneously, we modeled light absorption by subcells for a substrate-free thin-film multi-junction cell with a rear texture. The model evaluated subcell photocurrents and the requisite thicknesses at current matching. For InGaP/GaAs/InGaAs metamorphic 3-junction solar cells, the rear texture significantly enhanced light absorption in the InGaAs bottom cell and GaAs middle subcells, and the requisite thickness of InGaAs is almost less than 10% of that with flat rear reflector, even GaAs is also less than half of that without rear texture, if InGaP-subcell is not greater than 600 nm. The reduced lower subcell thickness by the texture rear surface will mitigate the difficulty of metamorphic growth and contribute to the cost reduction of multi-junction solar cells.

Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELECTROCHEMICAL SOC INC  

Recently, extensive investigations have been carried out on the electrochemical and photoelectrochemical reductions of CO2 into hydrocarbons to convert renewable energies into chemical fuels. In these researches, aqueous solutions of bicarbonate salts, such as NaHCO3 and KHCO3, are widely used as the supporting electrolyte. However, the effects of HCO3- on the CO2 reduction have not been studied in detail. In this research, the effects of KHCO3 concentration on CO2 reduction were studied at different applied potentials on a Cu working electrode using a potentiostatic method. The concentration of dissolved CO2 ([CO2]) and ratio of [CO2]/[K+] in the KHCO3 solution after bubbling with CO2 were studied to determine the relationship between KHCO3 concentration and CO2 reduction faradaic efficiency. The results suggest that high [CO2]/[K+] ratio in solutions with low KHCO3 concentration is probably a key factor to promote CO2 reduction faradaic efficiency. A model for the effect of [CO2]/[K+] ratio on CO2 reduction efficiency was also proposed. (C) 2017 The Electrochemical Society. All rights reserved.

DOI： 10.1149/2.0601709jes

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

III-V integration on Si processed in MOCVD ambient which contains As opens up new opportunities for high-efficiency multi-junction solar cells. Here, we study the interaction of As with vicinal Si(100) surfaces, the formation of atomically well-ordered, As-modified Si(100) surfaces and its impact on subsequently grown GaP epilayers. We combine optical in situ spectroscopy with surface science techniques in ultra-high vacuum to understand the As-modified Si(100) surface and the III-V/Si interface at atomic scale. We demonstrate that depending on dimer orientation on the Si(100) surface, we are able to control the sublattice orientation of subsequently grown GaP.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We propose inherently robust and low-cost imaging scheme based on integrated optical phased array (OPA) driven by random control patterns. A proof-of-concept inonolithic 1nP-based OPA is fabricated to demonstrate high-speed one-dimensional scanning without need for timeconsuming calibration.

Publishing type：Research paper (international conference proceedings)  

© 2017 IEEE. We investigate the role of carrier distribution inside the i-region in the photocurrent collection mechanism of pi-n quantum well solar cells. The device simulation indicates that the increment of Shockley-Read-Hall recombination after illumination strongly depends on the type of photocarriers and i- region majority carriers. As a result, a solar cell with n-type background doping results in efficient collection of photogenerated electrons from quantum wells even at low field, and this has been confirmed with the results of time-of-flight measurement. Based on this finding, we demonstrate a model for estimating carrier collection efficiency which includes the effect of carrier distribution.

DOI： 10.1109/PVSC.2017.8366378

Publishing type：Research paper (international conference proceedings)  

© 2017 IEEE. Optical evaluation of the GaAs single junction solar cell with InGaAs/GaAsP strain balanced multiple quantum wells (SB-MQWs) was attempted. In order to estimate the photon recycling (PR) effect for different effective optical path lengths in optically-thin MQWs, a quantitative analysis of electroluminescence (EL) was applied for the MQWs solar cells with and without light trapping texture on the rear surface. By external quantum efficiency measurement, a clear improvement in MQWs absorption was obtained by rear surface texturing of the semi-insulating GaAs substrate with epitaxially grown PV active layer. Upon EL measurement under carrier injection, the cell with light trapping texture showed increased EL photon emission, indicating the enhancement of V OC . This is because the enhanced optical absorption by light trapping effect facilitated more vigorous photon recycling in the active region of the cell.

DOI： 10.1109/PVSC.2017.8366256

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We fabricate a capsule-shaped metal-clad wavelength-scale cavities on InP and demonstrate room-temperature lasing at 1550-nm wavelength. By introducing an optimized curvature to the sidewalls of a conventional rectangular metal-clad laser, we effectively reduce the optical loss of transverse-electric (TE) mode and experimentally confirm, for the first time, clear improvement in side mode suppression ratio and slope efficiency of the laser under pulsed optical pumping.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：OSA - The Optical Society  

We propose a novel reconfigurable integrated MIMO optical mode demultiplexer consisting of cascaded MMI couplers. By configuring phase shifters adaptively, large-scale (&gt
10 modes), low-crosstalk (&lt
-20 dB) demultiplexing can be realized on chip.

DOI： 10.1364/CLEO_AT.2017.JTh2A.116

Publishing type：Research paper (international conference proceedings)  

© 2016 IEEE. To investigate the miniband and carrier transport properties in the superlattice structure embedded in the p-i-n GaAs solar cells, photoreflectance, photoluminescence, and photothermal spectroscopies were adopted. Two critical energies corresponding to the energy differences between mini-Brillouin zone edges were estimated for thinnest quantum barrier thickness sample by PR. From the PPT and PL, only broad peaks corresponding to the quantum levels were observed and we concluded that a built-in electric field inhibited the formation of miniband.

DOI： 10.1109/NMDC.2016.7777105

Publishing type：Research paper (international conference proceedings)  

© 2016 IEICE-ES. We propose a novel design of sub-wavelength metallic-cavity InP/InGaAs laser with unidirectional output on a silicon-on-insulator waveguide. By engineering the length and shape of the integrated optical feedback stub, formed on the non-output end of the waveguide, we improve the overall Q factor while maintaining confinement factor.

Publishing type：Research paper (international conference proceedings)  

Copyright © 2016 John Wiley & Sons, Ltd. A quantum wire-on-well (WoW) structure, taking advantage of the layer undulation of an InGaAs/GaAs/GaAsP superlattice grown on a vicinal substrate, was demonstrated to enhance the carrier collection from the confinement levels and extend the carrier lifetime (220 ns) by approximately four times more than a planar reference superlattice. Strained InGaAs/GaAs/GaAsP superlattices were grown on GaAs substrates under exactly the same conditions except for the substrate misorientation (0 and 6 ° off). The growth on a 6 ° off substrate induced significant layer undulation as a result of step bunching and non-uniform precursor incorporation between steps and terraces, whereas the growth on a substrate without miscut resulted in planar layers. The undulation was the most significant for InGaAs layers, forming periodically aligned InGaAs nanowires on planar wells, a WoW structure. As for the photocurrent corresponding to the sub-bandgap range of GaAs, the light absorption by the WoW was extended to longer wavelengths and weakened as compared with the planar superlattice. Almost the same photocurrent was obtained for both the WoW and the planar superlattice. Open-circuit voltage for the WoW was not affected by the longer-wavelength absorption edge, and the same value was obtained for the two structures. Furthermore, the superior carrier collection in the WoW, especially under forward biases, improved fill factor compared with the planer superlattice. Copyright © 2016 John Wiley & Sons, Ltd.

DOI： 10.1002/pip.2769

Publishing type：Research paper (international conference proceedings)  

© 2016 IEEE. Layer undulation of InGaAs/GaAs/GaAsP strainbalanced quantum well superlattice forms InGaAs nanowires along the bunching steps on a vicinal substrate embedded in the GaAs/GaAsP matrix. When it is used as an absorber in a GaAs single-junction cell, it assists carrier escape from narrow-gap InGaAs and extends photoluminescence lifetime as compared with a planar superlattice. Such a wire structure can be superior to existing quantum wells as a band-gap adjuster of a middle cell for improved current matching and efficiency.

DOI： 10.1109/NANO.2016.7751576

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEICE-INST ELECTRONICS INFORMATION COMMUNICATIONS ENG  

We propose novel silicon-based surface-normal optical modulator using electro-optic (EO) polymer. The EO polymer is embedded inside a thin silicon subwavelength grating layer, which is used as both the interdigitated electrodes for effective poling of the EO polymer, and as high-Q resonant structure for the incident light to enable efficient modulation. We numerically demonstrate 10-dB intensity modulation at 1550-nm wavelength under a refractive index change of only 3.8 x 10(-4), corresponding to the driving voltage below 1 V. Total-reflectance phase modulator is also demonstrated by adding a backside reflector. With inherently high-speed response over several tens of GHz, scalability to dense 2-D array integrated with CMOS driver circuitry, and relatively easy and low-cost fabrication without epitaxial process, the proposed device may find versatile applications in optical interconnects, free-space optical communications, imaging and sensing.

DOI： 10.1587/elex.13.20160595

Language：English   Publishing type：Research paper (international conference proceedings)  

Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.jechem.2016.01.019

Language：English   Publishing type：Research paper (scientific journal)   Publisher：WILEY-V C H VERLAG GMBH  

A series of wavelength-scale capsule-shaped metallic cavity structures with InGaAs/InGaAsP multiple quantum wells are fabricated and characterized by photoluminescence measurement for the first time. By introducing cylindrical facets with optimal curvature to a conventional rectangular cavity, the electric field of the resonant mode is pushed effectively into the center of the mesa. As a result, the mode distribution at the metallic sidewalls and the plasmonic losses are reduced dramatically. We experimentally confirm up to fourfold enhancement in the Q factor of the fabricated capsule-shaped cavity compared with that of a rectangular structure with same dimension. The measured results are consistent with the 3D finite-difference time-domain simulation, and validate the effectiveness of the scheme.

DOI： 10.1002/pssa.201532561

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

We propose a novel ultra compact electro-absorption optical modulator based on a silicon rib waveguide and numerically demonstrate its performance. The proposed design employs two types of transparent conductive oxide (TCO) layers with different carrier densities to achieve both high modulation efficiency and low optical insertion loss. The thin TCO layer with high carrier density enables efficient modulation through the metal-oxide-semiconductor structure. On the other hand, the upper TCO layer with low carrier density allows low-resistance electrical contact for the top electrode without large optical loss. Using an indium tin oxide bilayer with optimized carrier densities, we numerically demonstrate a 4.3 dB extinction ratio and a 2.6 dB optical insertion loss with 1 mu m device length. We estimate that the modulator operates under a low driving voltage of 1.3V, exhibiting an ultra low energy consumption of 22.5 fJ/bit and a broad RC modulation bandwidth of over 40 GHz. (C) 2016 The Japan Society of Applied Physics

DOI： 10.7567/JJAP.55.042201

Language：English   Publishing type：Research paper (international conference proceedings)  

DOI： 10.1117/12.2209611

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

A recently suggested reciprocity relation states that the current transport efficiency from the junction to the cell terminal can be determined by differentiating luminescence images with respect to the terminal voltage. The validity of this relation is shown experimentally in this paper, by comparison with simultaneously measured electrical currents and simulations. Moreover, we verify that the method is applicable under various light concentrations and applied voltages, which allows us to investigate the cell in relevant conditions. Results evidence several kinds of series resistances affecting the current transport efficiencies. We show that the relative contribution of those different resistances to the loss in current collection is a function of the illumination intensity.

DOI： 10.1109/JPHOTOV.2016.2516249

Publishing type：Research paper (international conference proceedings)  

© 2016 The Electrochemical Society. Quantitative evaluation of crystal defects introduced by fast atom beam (FAB) treatment was developed for surface-activated wafer bonding. The surface of n-GaAs was treated with the FAB using Ne, Ar and Kr, and Au Schottky electrodes were formed on the surfaces. Capacitance of a Schottky diode as a function of both probe frequency and DC bias allowed us to characterize both energy depth of the defects and their density profile along the physical depth from the GaAs surface. The results indicated that atoms with the smaller diameter generate high-density defects to the deeper region from the surface. When the defect density exceeding the doping level of GaAs spreads to wider than 5 nm, significant Schottky characteristics appeared in the interfacial current-voltage characteristics, as suggested by simulations. Such a tendency was semi-quantitatively in good agreement with the measured current-voltage characteristics.

DOI： 10.1149/07509.0033ecst

Language：English   Publishing type：Research paper (international conference proceedings)  

DOI： 10.1109/PVSC.2016.7750051

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：SPIE-INT SOC OPTICAL ENGINEERING  

We review our recent studies on capsule-shaped InP/InGaAs metallic-cavity lasers. By introducing an optimal curvature at the metallic sidewalls of conventional rectangular metallic lasers, the electric fields of the resonant mode are pushed effectively into the center of the mesa, which allows dramatic reduction of the plasmonic loss. The validity of the scheme is verified both numerically and experimentally. From three-dimensional finite-difference time-domain simulation and rate-equation analysis, we estimate that the threshold current can be reduced to as low as 60 mu A with the effective modal volume of 0.45 mu m(3). Up to 4-fold increase in Q value is confirmed experimentally for the cavity structure with an optimal curvature.

DOI： 10.1117/12.2230652

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We present the design of an electrically pumped metallic cavity laser based on optical, electrical and thermal simulations. By inserting an InAlAs blocking layer, leakage current is effectively suppressed, leading to 50% reduction of threshold current together with better thermal stability. Through combined optical and electrical analyses, the insulation layer thickness is set to be 60 nm, where the device shows optimized overall performance.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

The incorporation of Ga at the transition stage from InAs vertical growth to InGaAs lateral growth was investigated for the InGaAs microdiscs on a patterned Si (111) substrate by metal-organic vapor-phase epitaxy (MOVPE). X-ray diffraction reciprocal space mapping (XRD-RSM) of (331) plane clarified the extent of InGaAs lattice relaxation and solid Ga content as a function of the partial pressure of trimethyl-gallium (P-TMGa), which were correlated to the extent of lateral growth of the microdiscs. When P-TMGa was small, two types of InGaAs discs emerged neighboring with each other, extremely-lateral ones with partially-relaxed lattice and vertical ones with pseudo-morphic InGaAs to InAs. On the other hand, sufficiently-large P-TMGa resulted in uniform and moderately lateral discs with relaxed lattice. The results suggested the threshold of P-TMGa to initiate lateral growth of InGaAs in a coherent manner on all the InAs hexagonal pillars which were grown uniformly on the Si windows.

Publishing type：Research paper (international conference proceedings)  

© 2016 The Authors. We investigated the effect of built-in electric field on miniband formation and carrier nonradiative recombination in a superlattice structure by using photoreflectance (PR) and photoluminescence (PL) spectroscopy for a strain-balanced InGaAs/GaAsP superlattice inserted in p-i-n GaAs solar cell and n-n GaAs structures. Two critical energies were obtained in the PR spectra, which corresponded to the energy differences between the Γ and π points in the mini-Brillouin zone of the first electron level and the first heavy hole level. The obtained miniband widths of both samples were same, which were smaller than the calculated values by using a flat-band structure model without a built-in electric field. From these results, it was deduced that the built-in electric field does not affect the miniband width. The PL signal intensity of the p-i-n structure samples did not change, whereas that of the n-n structure samples decreased with decreasing barrier thickness. We concluded that the radiative recombination probability decreases and the non-radiative recombination probability increases due to miniband formation.

DOI： 10.1016/j.egypro.2016.11.326

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

Stokes-vector modulation formats are gaining increasing interest for next-generation highspeed optical communication networks. We propose and demonstrate potentially low-cost monolithically integrated InP/InGaAsP waveguide-based Stokes vector analyzer to retrieve the state-of-polarization without using coherent detection.

Publishing type：Research paper (international conference proceedings)  

© 2016 SPIE. Multiple Quantum wells (MQWs) have been studied as one promising material for high-efficiency nextgeneration solar cells. However, a portion of photo-excited carriers recombine in MQWs, resulting in the degradation of cell performance. Super-lattice (SL) structures, where quantum states in neighboring quantum wells strongly couple with each other, have been proposed for the carrier collection improvement via the tunneling transport through mini-bands. Therefore, it is important to characterize mini-band formation in various types of SL structures. We examined p-i-n GaAs-based solar cells whose i layers contain 20 stacks of InGaAs/GaAsP MQW structures with 2.1-nm GaAsP barriers (thin-barrier cell), with 2.1-nm barriers and 3-nm GaAs interlayers in between GaAsP barriers and InGaAs wells (stepbarrier cell), and with 7.8-nm barriers (thick-barrier cell). We investigated the optical absorption spectra of the SL solar cells using piezoelectric photo-thermal (PPT) spectroscopy. In the thick-barrier cell, one exciton peak was observed near the absorption edge of MQWs. On the other hand, we confirmed a split of the exciton peak for the thin-barrier SL, suggesting the formation of mini-band. Moreover, in the step-barrier cell, the mini-band at the ground state disappears since thick GaAs interlayers isolate each quantum-well ground state and, instead, the mini-band formation of highenergy states could be observed. By estimating from the energy-level calculation, this is attributed to the mini-band formation of light-hole states. This can well explain the improvement of carrier collection efficiency (CCE) of the thinbarrier and the step-barrier cells compared with the thick-barrier cell.

DOI： 10.1117/12.2214094

Publishing type：Research paper (international conference proceedings)  

© 2016 SPIE. Insertion of InGaAs/GaAsP strain-balanced multiple quantum wells (MQWs) into i-regions of GaAs p-i-n solar cells show several advantages against GaAs bulk p-i-n solar cells. Particularly under high-concentration sunlight condition, enhancement of the open-circuit voltage with increasing concentration ratio in thin-barrier MQW cells has been reported to be more apparent than that in GaAs bulk cells. However, investigation of the MQW cell mechanisms in terms of I-V characteristics under high-concentration sunlight suffers from the increase in cell temperature and series resistance. In order to investigate the mechanism of the steep enhancement of open-circuit voltage in MQW cells under high-concentration sunlight without affected by temperature, the quasi-Fermi level splitting was evaluated by analyzing electroluminescence (EL) from a cell. Since a cell under current injection with a density Jinjhas similar excess carrier density to a cell under concentrated sunlight with an equivalent short-circuit current Jsc = Jinj, EL measurement with varied Jinj can approximately evaluate a cell performance under a variety of concentration ratio. In addition to the evaluation of quasi-Fermi level splitting, the external luminescence efficiency was also investigated with the EL measurement. The MQW cells showed higher external luminescence efficiency than the GaAs reference cells especially under high-concentration condition. The results suggest that since the MQW region can trap and confine carriers, the localized excess carriers inside the cells make radiative recombination more dominant.

DOI： 10.1117/12.2212572

Publishing type：Research paper (scientific journal)  

DOI： 10.1149/2.1191613jes

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

Electronic control of coherence in 2D arrays of photonic crystal surface emitting lasers is discussed.

Publishing type：Research paper (international conference proceedings)  

© OSA 2016. We review our recent progress in developing various InP-based devices for on-chip polarization multiplexing, modulation, and detection. We employ half-ridge InP/InGaAsP polarization converter as a building block to realize monolithic integration with active components.

DOI： 10.1364/iprsn.2016.itu2a.4

Publishing type：Research paper (international conference proceedings)  

© VDE VERLAG GMBH. Berlin. Offenbach. We propose and demonstrate novel RSOA design with segmented electrodes to enhance the modulation cancelling effect without sacrificing the bandwidth for self-seeded colorless WDM transmitter applications. Optimal driving condition is derived numerically and confirmed experimentally at 10 Gb/s.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We propose a novel design of sub-wavelength metallic-cavity InP/InGaAs laser with unidirectional output on a silicon-on-insulator waveguide. By engineering the length and shape of the integrated optical feedback stub, formed on the non-output end of the waveguide, we improve the overall Q factor while maintaining confinement factor.

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1109/JPHOTOV.2015.2470131

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

A light trapping effect in GaAs p-i-n solar cells with InGaAs/GaAsP multiple quantum wells (MQWs) in the i-layer was demonstrated by applying a light scattering texture to the rear surface of the cell. A thin-film MQW solar cell was successfully fabricated by metal organic vapor phase epitaxy (MOVPE) to grow an inverted n-i-p photovoltaic (PV) structure; this structure was then transferred to a Si support substrate to prevent optical loss due to free carrier absorption. For the light scattering texture, the use of both the wet-etched micro-hole arrayed SiO2 dielectric layer on the rear surface of the cell and the secondarily etched micro hole array on the GaAs layer was attempted. On the SiO2 layer, the micro hole array pattern was obtained by the radio frequency sputtering of the layer followed by wet etching with photolithographic patterning. On the GaAs layer, the micro-hole array pattern was obtained by direct etching through a SiO2 template. Compared with the light scattering effects of the micro-hole-arrayed SiO2 layer, the secondarily etched GaAs rear contact layer showed a significant improvement in external quantum efficiency (EQE) in the wavelength range from 855 to 1000 nm that corresponds to the photon absorption wavelength in MQWs. (C) 2015 The Japan Society of Applied Physics

DOI： 10.7567/JJAP.54.08KA13

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AIP Publishing  

In this study, we propose a carrier time-of-flight technique to evaluate the carrier transport time across a quantum structure in an active region of solar cells. By observing the time-resolved photoluminescence signal with a quantum-well probe inserted under the quantum structure at forward bias, the carrier transport time can be efficiently determined at room temperature. The

DOI： 10.1063/1.4927612

Publishing type：Research paper (international conference proceedings)  

© 2015 The Japan Institute of Electronics Packaging. Effect of ion species for fast atom beam (FAB) irradiation of surface activated bonding (SAB) of GaAs wafers was investigated by current-voltage (I-V) measurements and transmission electron microscopy (TEM) observations. Ne,Ar,Kr and Xe gases were employed for FAB source. We confirm that it is possible to reduce the interfacial damage and improve the conductivity of GaAs/GaAs bonded interface by changing the ion species for FAB irradiation.

DOI： 10.1109/ICEP-IAAC.2015.7111062

Publishing type：Research paper (scientific journal)  

© 2011-2012 IEEE. Carrier transport inside InGaAs/GaAs/GaAsP multiple quantum well (MQW) solar cells was discussed under high-concentrated sunlight illumination up to 338 suns. Current-voltage (I-V) characteristic curves for a GaAs reference cell and MQW cells with GaAsP barrier thickness of 2, 4, and 6 nm were investigated under dark and high-concentration illumination. Carrier collection efficiency (CCE) was estimated by net photocurrent, which is the difference between illuminated current and dark current density at each bias voltage normalized by the value at the saturated point. For the 2-nm barrier, CCE exhibited almost no degradation compared with the GaAs reference cell. On the other hand, CCE for the 6-nm barrier degraded with forward biases as the sunlight concentration ratio increased. The degradation in CCE under a high-concentration ratio is shown to be the result of carrier accumulation in quantum wells. Thin barriers seemed to eliminate such accumulation with the help of the carrier tunneling effect through the barriers.

DOI： 10.1109/JPHOTOV.2015.2407159

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

A compact, novel photonic crystal cavity aimed at applications with strict area limitations is presented. Optimization shows that the gentle confinement method previously used for line-defect cavities can be applied to more limited geometries. It also shows that it is paramount to consider the boundary region to minimize in-plane losses. The investigation show that a near optimum boundary thickness can be found by considering the boundary region as a Fabry-Perot resonator. This optimization strategy is shown to be deterministic in terms of resonance wavelength. For an optimized air-clad, silicon cavity, finite-difference time-domain simulations give Q-values as high as 75,000 which is comparable to other photonic crystal cavities of similar size. (c) 2015 The Japan Society of Applied Physics

DOI： 10.7567/JJAP.54.042001

Publishing type：Research paper (scientific journal)  

© 2015 AIP Publishing LLC. InGaAs/GaAsP superlattice (SL) is a promising narrow-gap material for III-V multi-junction solar cells on Ge. In metal-organic vapor phase epitaxy (MOVPE) of SL on vicinal substrates, the component layers tend to be undulated due to step bunching occurring at high temperature. In this paper, the effects of growth temperature and thickness modulation of the SL-region on the photovoltaic performance were investigated. Lowering the growth temperature successfully enabled epitaxy of an extremely uniform SL, from which a clear step-like absorption spectrum including sharp exciton peaks was obtained due to layer-by-layer deposition of the individual layers. Larger layer undulation at higher temperature led to poorer in-plane coverage of the InGaAs region, resulting in the reduction of both light absorption and short circuit current. The open circuit voltage, on the other hand, was higher for the cells grown at higher temperature owing to suppressed dark current as a result of reduced crystal defects. Moreover, the lateral thickness variation of the GaAsP barriers in the undulated SL allowed efficient tunnel transport through the thinner part of the barrier, and improved the carrier collection and the fill factor. By optimizing the growth temperature for SL on vicinal substrates, an N-on-P cell including 100-period SL with a bandgap of 1.21eV achieved 1.11 times higher efficiency than a GaAs reference cell with 36% current enhancement as middle cell performance.

DOI： 10.1063/1.4917535

Publishing type：Research paper (scientific journal)  

© 2011-2012 IEEE. Insertion of multiple quantum wells (MQWs) into the i-region of GaAs p-i-n solar cells can shift the effective bandgap to the optimal value for single-junction solar cells under high sunlight concentration. The quantum efficiency corresponding to the absorption by MQWs must be sufficiently high for achieving high-efficiency single-junction MQW solar cells. Here, we report light-trapping thin-film MQW solar cells for increasing photoabsorption in MQWs. In order to suppress the free-carrier absorption loss and enhance the light trapping, only the active layers including MQWs were processed to a cell by flip-chip bonding and dissolution of a conductive GaAs substrate. The periodic grooves formed on the back side of the cell scattered photons in the subbandgap range and trapped light inside the cell. For absorption loss reduction in a back contact metal, a dielectric interlayer was introduced between the metal and GaAs. The light-trapping structure resulted in a fivefold increase in effective optical path length compared with the physical thickness of MQWs. External quantum efficiency at wavelengths longer than the GaAs edge exceeded 50% with only 20-period MQWs. As a result, we achieved thin-film light-trapping MQW solar cells with 20% conversion efficiency.

DOI： 10.1109/JPHOTOV.2015.2392941

Publishing type：Research paper (scientific journal)  

DOI： 10.1021/jp509043h

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Wiley  

DOI： 10.1002/pip.2585

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We propose a novel practical design of silicon rib-waveguide electro-absorption modulator using two indium-tin oxide layers with optimized carrier densities. Efficient and low loss modulation is numerically demonstrated for a 1-mu m-long modulator.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We design active integrated polarization controller using half-ridge-waveguide polarization converter and InGaAlAs/InAlAs multiple-quantum-well-based phase shifters. From the numerical simulation and preliminary measurement, we demonstrate the feasibility of polarization switching to arbitrary state with less than 16.2-mW DC power consumption using a 5.6-mm-long device.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

Solar hydrogen production is one of the most required technologies for the realization of sustainable energy systems which is independent of fossil fuel. The solar-to-hydrogen (STH) energy conversion efficiency for photocatalysts, which have been studied quite intensively for decades, remains much lower than 10%. The combination of photovoltaic and electrochemical cells, on the other hand, can easily achieve the STH efficiency over 10%. We here present the field demonstration of a system combining a concentrator photovoltaic (CPV) module and polymer electrolyzers, which led us to the STH efficiency of 17.1 %. A method will be presented to optimize the series/parallel connection between CPV and electrolyzers for maximizing the STH, which predicts 28% STH when 35%-efficient CPV module is used.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

A strain-compensated InGaAs/GaAsP superlattice (SL) was successfully integrated into the GaAs middle cell of a lattice-matched InGaP/GaAs/Ge triple junction (3J) solar cell. Wafer shuttle technology was employed for proof of concept because the growth of the SL was optimized for a different metalorganic vapor phase epitaxy (MOVPE) reactor from the one for conventional 3J cells, but an entire structure can be grown in a single reactor in the future. The SL extended the absorption edge of the middle cell from 875 nm of GaAs to 1150 nm. Consequently, the photo current generated in the SL middle cells was enhanced by approximately 30% compared to a normal GaAs middle cell, improving current matching among subcells. The performance of SL cell, especially photo-current, was affected by the structure of the SL middle cell and the regrowth condition of the InGaP top cell. The conversion efficiency reached approximately 37% under 100-sun. The results encourage the implementation of InGaAs/GaAsP SL for high efficiency multi junction solar cells (MJSCs) with reduced total layer thickness.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：SPIE-INT SOC OPTICAL ENGINEERING  

Multi-Quantum well solar cells (MQWSC) have been shown to present several advantages, among which are low dark currents and tunable bandgaps. They are especially suited for implementation in multi-junction cells, and are highly promising for absorbers in Hot Carrier Solar Cells (HCSC). Such applications require high concentration ratio, which arises the issue of collection efficiency. Whereas it is usually considered that collection in MQW is very close to unity at one sun, it has been shown to not be the case under high concentration at the maximum power point. We propose in this work to take advantage of the luminescence spectral variation to investigate the depth collection efficiency. In order to validate the model, a series of strain compensated InGaAs/GaAsP MQW solar cells with intentional variation of the MQW doping concentration are grown. This has the effect of switching the space charge region position and width as well as the electric field intensity. Recording the luminescence spectra at various illumination intensities and applied voltages, we show that the in-depth quasi-Fermi level splitting and thus collection properties can be probed. Other measurements (EQE, luminescence intensity variation) are shown to be consistent with these results. Regarding their use as HCSC, the luminescence of MQW solar cells has been mainly used so far for investigating the quasi-Fermi level splitting and the temperature. Our results improve our understanding by adding information on carrier transport.

DOI： 10.1117/12.2081303

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

Compact strictly non-blocking 8x8 silicon photonic switch based on optical phased array is designed and fabricated. Broadband (1530-1560 nm) switching with on-chip loss of 9.8 dB and response time below 25 mu s is demonstrated experimentally.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

Modulation-cancellation property of InGaAlAs/InP MQW reflective semiconductor optical amplifier (RSOA) and its temperature dependence are investigated for uncooled colorless WDM transmitter applications. Using the optimized RSOA length, we demonstrate self-seeded 2.5-Gb/s transmission over 25-km SSMF at 25 degrees C and 50 degrees C.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Institute of Electrical and Electronics Engineers Inc.  

Quantum dots (QDs) photonic devices based on III-V compound semiconductor are very attractive optoelectronic due to their thermal independence, low power consumption, and high-speed modulation. We have developed a defect-free, ultimate top-down nanofabrication process for sub-20-nm-diameter GaAs quantum nanodisks (NDs) by employing a bio template and neutral beam etching, and produced 8-nm-thick GaAs NDs embedded in an AlGaAs barrier regrown by the metal organic vapour phase epitaxy (MOVPE). We successfully fabricated a light emitting diode structure and observed the light emission from the quantum energy levels of NDs.

DOI： 10.1109/ISLC.2014.199

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEICE-INST ELECTRONICS INFORMATION COMMUNICATIONS ENG  

Integrated InP polarization converters based on half-ridge structure are studied numerically. We demonstrate that the fabrication tolerance of the half-ridge structure can be extended significantly by introducing a slope at the ridge side and optimizing the thickness of the residual InGaAsP layer. High polarization conversion over 90% is achieved with the broad range of the waveguide width from 705 to 915 nm, corresponding to a factor-of-two or larger improvement in the fabrication tolerance compared with that of the conventional polarization converters. Finally we present a simple fabrication procedure of this newly proposed structure, where the thickness of the residual InGaAsP layer is controlled precisely by using a thin etch-stop layer.

DOI： 10.1587/transele.E97.C.731

Other Link： http://dblp.uni-trier.de/db/journals/ieicet/ieicet97c.html#journals/ieicet/ZaitsuTN14

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Cambridge University Press  

The n-type GaN has stability problem of the surface anodic corrosion during the photoelectrochemical reaction for H2 generation. The photoelectrochemical surface stabilities of n-type GaN dependent on the electrolytes were investigated. The flatband potential in HCl obtained from Mott-Schottky plot shifted 0.1 V to positive direction compared with that in H2SO4. The variation of saturated photocurrent of 1 to 3 cycles in H2SO4 was much larger than that of HCl, NaOH and KOH. The surface morphologies also changed by the electrolytes. These results show the absorbed materials on the GaN electrode surface during the photoelectrochemical reactions were changed by the electrolyte and affected the surface reactions.

DOI： 10.1557/opl.2014.172

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Optical Society of America (OSA)  

We propose novel 1.55-μm capsule-shaped metallic-cavity lasers with curved facets to reduce plasmonic losses. Significant reduction of threshold current from 291 μA to 60 μA is demonstrated with effective modal volume of 0.45 μm3. © 2014 OSA.

DOI： 10.1364/cleo_at.2014.jw2a.90

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Materials Research Society  

Electrochemical reduction of CO&lt
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2&lt
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into useful organics combined with photovoltaics is thought to be one of the promising ways to effectively store and transport the solar energy. In most of the previous researches, CO&lt
inf&gt
2&lt
/inf&gt
bubbling in different solutions were used as the electrolyte. However, the effects of the electrolyte and the CO&lt
inf&gt
2&lt
/inf&gt
bubbling are not clear. Therefore, in this research, the effects of different electrolyte, CO&lt
inf&gt
2&lt
/inf&gt
bubbling, concentration of the electrolyte and temperature on the electrochemical reduction of CO&lt
inf&gt
2&lt
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on a Cu working electrode were studied. The results showed that the form of the carbon source in the electrolyte, such as HCO&lt
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3&lt
/inf&gt
-, CO&lt
inf&gt
3&lt
/inf&gt
&lt
sup&gt
2-&lt
/sup&gt
and H&lt
inf&gt
2&lt
/inf&gt
CO&lt
inf&gt
3&lt
/inf&gt
, had a strong effect on this reaction, which was controlled by the pH of the electrolyte. Furthermore, high concentration of the HCO&lt
inf&gt
3&lt
/inf&gt
- and elevated temperature can strongly improve the reaction current density.

DOI： 10.1557/opl.2014.404

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

The monolithic integrated series-connected InGaP/GaAs dual junction (2J) cells with bypass diodes were demonstrated in order to achieve the ultra high concentrator application. The current-voltage (I-V) characteristics for 10 series-connected 2J cell under the dark condition showed each sub-cell was successfully interconnected each other. Epitaxially grown 2J solar cell on the semi-insulating (SI) GaAs substrate was used for electrical isolation. However, the significant deterioration of the fill factor (FF) in light I-V was apparent due to the current leakage caused by intrinsic carrier excitation in SI-GaAs. Finally, improved cell performance was achieved by applying shading layer for preventing illumination to the SI-GaAs layer.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE COMPUTER SOC  

Quantum dots (QDs) photonic devices based on III-V compound semiconductor are very attractive optoelectronic due to their thermal independence, low power consumption, and high-speed modulation. We have developed a defect-free, ultimate top-down nanofabrication process for sub-20-nm-diameter GaAs quantum nanodisks (NDs) by employing a biotemplate and neutral beam etching, and produced 8-nm-thick GaAs NDs embedded in an A1GaAs barrier regrown by the metalorganic vapor phase epitaxy (MOVPE). We successfully fabricated a light emitting diode structure and observed the light emission from the quantum energy levels of NDs.

DOI： 10.1109/ISLC.2014.45

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Materials Research Society  

Water-splitting by using electric power produced by solar cells is promising system to produce hydrogen without fossil fuels. Oxygen evolving catalyst is, however, major problem to prevent using this system widely because precious materials are used in the catalyst. Considering from the photosynthesis II of plants, the compound of Ca-Mn-0 is one of the candidates for the oxygen evolving catalyst. In this study, the synthesis condition and the oxygen evolving electrocatalytic activity of CaMn&lt
inf&gt
2&lt
/inf&gt
O&lt
inf&gt
4&lt
/inf&gt
•xH&lt
inf&gt
2&lt
/inf&gt
O are investigated. The overpotential at 0.1 mA/cm&lt
sup&gt
2&lt
/sup&gt
was 0.28 V when using the electrode of carbon paste and CaMn&lt
inf&gt
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/inf&gt
O&lt
inf&gt
4&lt
/inf&gt
•H&lt
inf&gt
2&lt
/inf&gt
O with the weight ratio of 3:1.

DOI： 10.1557/opl.2014.406

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEICE-INST ELECTRONICS INFORMATION COMMUNICATIONS ENG  

Optical buffering has been one of the major technical challenges in realizing optical packet switching routers and interconnects. We demonstrate a compact optical buffer module, comprising an InP 1 x 8 phased-array switch and a silica-based delay line circuit. The integrated delay line circuit is fabricated on the silica-based planar-lightwave circuit (PLC) platform, and has the ladder architecture for reducing the size. In addition, variable optical couplers are integrated to achieve effective power equalization. Tunable and uniform buffering of up to 21 ns is obtained with 3-ns temporal resolution.

DOI： 10.1587/transele.E96.C.738

Other Link： http://dblp.uni-trier.de/db/journals/ieicet/ieicet96c.html#journals/ieicet/KwackOHMZTN13

Language：English   Publishing type：Research paper (scientific journal)   Publisher：OPTICAL SOC AMER  

Highly efficient, low-loss, and compact InP/InGaAsP polarization converter based on a half-ridge waveguide structure is fabricated and demonstrated experimentally. The device is fabricated by a simple self-aligned process and integrated with a ridge InP waveguide. Using a 150-mu m-long device, we obtain the mode conversion of more than 96% and the on-chip loss of less than 1.0 dB over the broad wavelength range from 1510 to 1575 nm. The experimental results are explained quantitatively using the full-vector eigenmode calculation, which also reveals large fabrication tolerance of the demonstrated device. (C) 2013 Optical Society of America

DOI： 10.1364/OE.21.006910

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：AMER INST PHYSICS  

Novel band engineering material of a super-lattice (SL) consists of strain-balanced InGaAs and GaAsP was applied to GaAs p-i-n single junction solar cells. The SL solar cell sample with 60 periods of 5.4-nm-thick In0.13Ga0.87As well and 3.1-nm-thick GaAs0.75P0.25 barrier showed extended quantum efficiency cutoff from 870 nm to 930 nm, resulted in substantial increment of short circuit current density Delta JSC= 2.1 mA/cm(2). Comparing current-voltage (I-V) characteristics under the different illumination condition from 1SUN to 200 SUN for the bulk GaAs cell and the SL cell, the fraction of Delta JSC was constant in this range. And the increasing dependency of open circuit voltage (VOC) of SL cell on concentration ratio showed steeper tendency than the bulk GaAs cell. Finally the SL cell achieved larger conversion efficiency (eta) than the bulk GaAs p-i-n junction solar cell under &gt; 100 SUN.

DOI： 10.1063/1.4822200

Publishing type：Research paper (scientific journal)  

DOI： 10.1063/1.4848084

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEICE-INST ELECTRONICS INFORMATION COMMUNICATIONS ENG  

We demonstrate a simple and efficient optical coupler for vertical coupling between optical fibers and InP-based waveguides using a slant-etched mirror. The angle of the etched mirror can be controlled by using an aluminum jig with a beveled surface inserted under the substrate during RIE (reactive ion etching) of InP/InGaAsP. The off-chip coupler is fabricated simultaneously with a high-mesa waveguide with only one etching process. Coupling loss of 7.3 dB between the tapered single-mode fibers is obtained within the wavelength of 1530-1570 nm.

DOI： 10.1587/elex.10.20130116

Publishing type：Research paper (international conference proceedings)  

DOI： 10.1063/1.4848077

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：AMER INST PHYSICS  

The concentrator photovoltaic (CPV) system provides excellent cost performance and conversion efficiency by increasing the concentration ratio. The problem is that concentration ratio is limited by short-circuit current density (J(sc)) due to cell resistance loss. In order to achieve much larger concentration ratio, the monolithically integrated series-connected GaAs photovoltaic (PV) cells were fabricated. By dividing a cell into sub-cells on a chip and connecting them in series, the cell provides smaller short-circuit current (I-sc) and larger open-circuit voltage (V-oc). This approach can reduce joule energy loss inside a cell without decreasing electrical power output and allow much larger concentration ratio. In our design, 10 series-connected sub-cells, with bypass diodes in parallel with each sub-cell, were integrated monolithically on semi-insulating GaAs. When some sub-cells in the array were shaded, the bypass diodes prevented the shaded sub-cells from breakdown and reduced fluctuation of power output. The detection area of a unit cell was 1.73 mm(2) and an entire detection area occupied over 68% of the whole chip area. The arrayed 10 cells exhibited V-oc of 9.0 V under illumination (AM 1.5G). The series-connected cell achieved maximum efficiency at higher concentration ratio than non-series-connected cell.

DOI： 10.1063/1.4822191

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

A quantum-well (QW) solar cell including InGaAs wells is a promising candidate for the purpose of current matching in InGaP/GaAs/Ge tandem solar cells by extending the edge of quantum efficiency to longer wavelengths. Even though QWs increase short-circuit current by the extended effective band edge, they tend to obstruct carrier transport and degrade the efficiency of a cell. Therefore, a superlattice (SL) structure has been proposed to prevent the recombination of carriers inside of the wells and, more importantly, to enable carriers to tunnel to a neighboring well, leading to an efficient carrier transportation in such a photovoltaic device. In this paper, a SL solar cell was implemented with a strain-balancing technique. It exhibited excellent performance: Enhanced photocurrent (3.0 mA/cm(2)) with minimized drop (0.03 V) in open-circuit voltage. Behind these achievements, substantial contribution of tunneling transport has been confirmed for the SL cell by external quantum efficiency measurement at 77 K.

DOI： 10.1109/JPHOTOV.2012.2187044

Publishing type：Research paper (scientific journal)  

DOI： 10.1364/OE.20.008718

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

A multiple stepped quantum wells (MSQWs) solar cell, in which GaAs stepped-potential layers are sandwiched between strain-balanced InGaAs wells and GaAsP barriers, has been proposed, and improvements in short-circuit current and fill factor have been demonstrated. We have studied carrier recombination dynamics and carrier escape kinetics from the wells, comparing the MSQW with the normal multiple quantum well (MQW). Carrier recombination rate was examined by time-resolved photoluminescence (PL) and a longer carrier lifetime was observed for MSQWs than for MQWs. Carrier escape from the wells was also investigated in terms of temperature-dependent PL. Smaller thermal activation energy was observed for MSQWs than for MQWs. Carrier radiative recombination loss was investigated by bias-dependent PL, and it was found to be smaller for the MSQW than for the normal MQW. Such advantages of the MSQW allowed us to stack a sufficient number of quantum wells to increase short-circuit current without degrading fill factor. For normal MQW solar cells, degradation in fill factor is unavoidable.

DOI： 10.1109/JPHOTOV.2012.2185684

Language：English   Publishing type：Research paper (international conference proceedings)  

The primary targets of our project are to drastically improve the photovoltaic conversion efficiency and to develop new energy storage and delivery technologies. Our approach to obtain an efficiency over 40% starts from the improvement of III-V multi-junction solar cells by introducing a novel material for each cell realizing an ideal combination of bandgaps and lattice-matching. Further improvement incorporates quantum structures such as stacked quantum wells and quantum dots, which allow higher degree of freedom in the design of the bandgap and the lattice strain. Highly controlled arrangement of either quantum dots or quantum wells permits the coupling of the wavefunctions, and thus forms intermediate bands in the bandgap of a host material, which allows multiple photon absorption theoretically leading to a conversion efficiency exceeding 50%. In addition to such improvements, microfabrication technology for the integrated high-efficiency cells and the development of novel material systems that realizes high efficiency and low cost at the same time are investigated. © Royal Swedish Academy of Sciences 2012.

DOI： 10.1007/s13280-012-0267-4

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEICE-INST ELECTRONICS INFORMATION COMMUNICATIONS ENG  

A novel type of optically clocked all-optical flip-flop based on a coupled-mode distributed Braggw reflector laser diode is proposed. The device operates as a bistable laser, where the two lasing modes at different wavelength are switched all-optically by injecting a clock pulse together with a set/reset signal. We employ an analytical model based on the two-mode coupled rate equations to verify the basic operation of the device numerically. Optically clocked flip-flop operation is obtained with a set/reset power of 0.60 mW and clock power of 1.8 mW. The device features simple structure, small footprint, and synchronized all-optical flip-flop operation, which should be attractive in the future digital photonic integrated circuits.

DOI： 10.1587/transele.E95.C.218

Other Link： http://dblp.uni-trier.de/db/journals/ieicet/ieicet95c.html#journals/ieicet/ZaitsuHTN12

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEICE-INST ELECTRONICS INFORMATION COMMUNICATIONS ENG  

We propose a novel 1 x 3 planar optical switch using aspheric lenses and carrier-induced tunable prisms on InP. An input light beam is collimated by the aspheric lenses in a slab waveguide. The tunable prism, whose refractive indices are tuned by the carrier plasma effect, deflect the collimated light beam and guide it to the output ports. The switching operations of the I x 3 optical switch that consists of five lenses and eight prisms with a footprint of 500 x 3500 mu m are performed by three-dimensional beam propagation methods:A static switching operation with a 5-dB insertion loss and a 13-dB extinction ratio is obtained with 70-mA current injection for each prism. This device has a simple structure and low power consumption and may be useful for optical packet switching systems.

DOI： 10.1587/transele.E95.C.213

Other Link： http://dblp.uni-trier.de/db/journals/ieicet/ieicet95c.html#journals/ieicet/CheZHN12

Language：English   Publishing type：Research paper (scientific journal)  

Insertion of quantum wells (QWs) extends the absorption edge to a longer wavelength than the value of a p-i-n cell without the QWs, which is preferable for the improved current matching of a InGaP/GaAs/Ge multijunction cell. The QWs, however, reduce the open-circuit voltage (Voc) and degrade the fill factor
the latter is significant for a large number of QWs that are mandatory for sufficient light absorption. As a structure to minimize these drawbacks, a QW superlattice, a strain-balanced In0.13Ga 0.86As (4.7 nm)/GaAs0.57P0.43 (3.1 nm) stack, was implemented by metalorganic vapour-phase epitaxy. It brought about an enhancement in short-circuit current density (3.0 mA cm-2) with a minimal drop in Voc(0.03 V) compared with a p-i-n cell without the superlattice. The collection efficiency of photocarriers from the wells to an external circuit was evaluated: the efficiency was above 0.95 for the superlattice, while it was below 0.8 at a large forward bias for a conventional QW cell with thicker barriers. With the fast electron-hole separation in the superlattice owing to tunnelling transport, the superlattice cell exhibited a steeper increase in Voc as a function of the sunlight concentration ratio than the conventional QW cell: at the concentration ratio of 50, the value of Voc for the superlattice cell was almost equivalent to the value of the GaAs p-i-n cell without QWs. As a possible mechanism behind such an enhancement in Voc, photocurrent generation by two-step photon absorption was observed, using the electron ground state of the superlattice as an intermediate state. © 2013 IOP Publishing Ltd.

DOI： 10.1088/0022-3727/46/2/024001

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

A between strain-balanced InGaAs wells and GaAsP barriers, has been proposed and improvements in sub-GaAs-bandgap QE have been demonstrated. We have studied the effect of indium diffusion on the optical properties in quantum well solar cell by measuring the absorptance and photoluminescence spectra. Theoretical calculations indicate that the transition energy level and the absorption magnitude are sensitive to indium diffusion. The MSQW structure with GaAs step layer is a feasible way to suppress indium diffusion and enhance the absorption. (C) 2012 American Institute of Physics. [doi:10.1063/1.3681785]

DOI： 10.1063/1.3681785

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：SPIE-INT SOC OPTICAL ENGINEERING  

A quantum-well suparlattice cell, in which In0.13Ga0.86As (4.7 nm) / GaAs0.57P0.43 (3.1 nm) strain-balanced quantum wells are inserted in the intrinsic region of a GaAs pin cell, has been implemented by metalorganic vapor-phase epitaxy (MOVPE) and has exhibited an enhanced short-circuit current density, with an increment of 3.0 mA/cm(2) and a minimal drop in open-circuit voltage (0.03 V) compared to a pin cell without the superlattice. The collection efficiency of photocarriers, which are generated in a cell upon the irradiation of monochromatic light, to an external circuit has been evaluated for both the superlattice cell and a conventional quantum-well cell with thicker wells and barriers. This carrier collection efficiency is was above 0.95 for the superlattice cell, regardless of a wavelength and an external bias, while the value for the quantum-well cell degraded to be below 0.8 at a large forward bias, which evidenced superior carrier transport with the help of tunneling through the thin barriers. With such a fast electron-hole separation in the superlattice, photo-current generation by two-step photon absorption has been observed, using the electron ground state of the superlattice as an intermediate band.

DOI： 10.1117/12.911078

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：WILEY-V C H VERLAG GMBH  

Aiming at monolithic in-plane integration of light emitters with multiple wavelengths in a designed manner, InGaN/GaN multiple quantum wells (MQWs) on the semipolar planes of GaN hexagonal pyramids were grown in selective growth windows surrounded by the masks with varied widths (40-240 mu m). Growth rate of the pyramid was modulated by a factor of 3 with an increase in the mask width. Its tendency was in accordance with the accumulation of a precursor in the vicinity of the selective area, which was numerically estimated assuming vapourphase diffusion and surface incorporation of the precursor. Peak wavelength modulation in both cathode luminescence and electro luminescence from the MQWs were observed, and both peaks exhibited similar dependencies on the mask width, which was related to an increase in the thickness of InGaN wells. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

DOI： 10.1002/pssc.201100443

Publishing type：Research paper (scientific journal)  

DOI： 10.1021/jp2088324

Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN SOC APPLIED PHYSICS  

In order to develop a high-quality material that is lattice-matched to GaAs for III-V tandem solar cells, we introduce In0.16Ga0.84As/GaAs/GaAs0.79P0.21 strain-compensated multiple stepped quantum well (SC-MSQW). To maximize the contribution of the quantum well to the spectral response, we inserted a GaAs step layer between well and barrier. This leads to a stronger photoabsorption by the wells and enhanced carrier transport in the stacked wells. A short-circuit current density of 19.29 mA/cm(2) was obtained under AM1.5G illumination, which is approximately 14% higher than that of a conventional quantum well solar cell. (C) 2011 The Japan Society of Applied Physics

DOI： 10.1143/APEX.4.122301

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

Precise strain compensation for lattice-mismatched quantum wells is crucial for obtaining high performance devices such as quantum well solar cells. High-accuracy in situ curvature monitoring is a more efficient tool to adjust growth conditions for perfect strain balancing, and we have achieved curvature measurement during growth of InGaAs/GaAsP multiple quantum wells by metal organic vapor phase epitaxy. We have also developed the curvature calculation model taking into account of thermal expansion and lattice relaxation effects based on Stoney's equation. The measured periodical curvature behavior corresponds to the growth of compressive InGaAs well layers and tensile GaAsP barrier layers and fits perfectly with a theoretical curve assuming the structural parameters (thicknesses and atomic contents) obtained by x-ray diffraction analysis, confirming correctness of the developed calculation method. Considering the proper thermal expansion coefficients for InGaAs and GaAsP, we have obtained much accurate fitting results for measured curvature. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3663309]

DOI： 10.1063/1.3663309

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

We have demonstrated sub-10-nm extremely thin body (ETB) InGaAs-on-insulator (InGaAs-OI) nMOSFETs on Si wafers with Al2O3 ultrathin buried oxide (UTBOX) layers fabricated by direct wafer bonding process. We have fabricated the ETB InGaAs-OI nMOSFETs with channel thicknesses of 9 and 3.5 nm. The 9-nm-thick ETB InGaAs-OI nMOSFETs with a doping concentration (N-D) of 10(19) cm(-3) exhibit a peak electron mobility of 912 cm(2)/V . s and a mobility enhancement factor of 1.7 times against the Si nMOSFET at a surface carrier density (N-s) of 3 x 10(12) cm(-2). In addition, it has been found that, owing to Al2O3 UTBOX layers, the double-gate operation improves the cutoff properties. As a result, the highest on-current to the lowest off-current (I-on/I-off) ratio of approximately 10(7) has been obtained in the 3.5-nm-thick ETB InGaAs-OI nMOSFETs. These results indicate that the high-mobility III-V nMOSFETs can be realized even in sub-10-nm-thick channels.

DOI： 10.1109/LED.2011.2158568

Other Link： http://orcid.org/0000-0001-6802-3346

Language：English   Publishing type：Research paper (scientific journal)   Publisher：WILEY-BLACKWELL  

We have achieved continuous wavelength modulation from InGaN quantum wells on semi-polar {11-22} planes with selective-area growth of hexagonal pyramids with varied widths of masks surrounding them. The thickness of InGaN wells on the surface of the pyramids was tailored by the mask width, which led to wavelength modulation. The use of semi-polar planes reduced quantum confined stark effect, allowing a wider range of wavelength modulation: from 446 to 500 nm as measured by cathode luminescence. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

DOI： 10.1002/pssa.201000942

Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN SOC APPLIED PHYSICS  

We demonstrated the formation of ultrathin-body (UTB) III-V-semiconductor-on-insulator (III-V-OI) n-channel metal-oxide-semiconductor field-effect transistors (nMOSFETs) on Si by low-damage direct wafer bonding, where channel thickness was been reduced to 7 nm. However, we found a significant reduction of the effective mobility of the InGaAs-OI nMOSFETs upon thinning InGaAs channels without surface passivation layers. Instead, we demonstrated the high electron mobility of 1004 cm(2).V(-1).s(-1) in a 9-nm-thick InGaAs-OI nMOSFET with an InP surface passivation layer. This finding indicates that the proper surface passivation is important in realizing the high-performance UTB III-V-OI nMOSFETs. (C) 2011 The Japan Society of Applied Physics

DOI： 10.1143/APEX.4.054202

Other Link： http://orcid.org/0000-0001-6802-3346

Publishing type：Research paper (scientific journal)  

DOI： 10.1117/1.3574143

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

Optical buffering has been one of the major technical challenges in realizing optical packet switching (OPS) routers. While fiber-delay-line-based (FDL) buffers are the most practical and realistic solution to offer useful amount of capacity, the bulkiness of long FDLs and optical switches has been the main obstacle to practical implementation. This paper demonstrates a compact optical buffer with up to 750-ns capacity and 50-ns temporal resolution by using an InP integrated 1 X 16 optical phased-array switch and compact FDL module based on thin-cladding highly nonlinear fiber (HNLF). Owing to the high mode confinement inside HNLF, 15 fibers with the total length of 1.2 km are coiled onto a single bobbin with a coin-sized footprint without increasing the propagation loss. At the interface between the InP switch and FDLs, a pitch-converting silica planar-lightwave circuit chip is employed to achieve 16-port simultaneous uniform interconnection. Using the developed module, variable optical buffering experiment is demonstrated, where the packet intervals are expanded from 20 to 70 ns successfully.

DOI： 10.1109/JLT.2010.2102338

Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

Strain-balanced growth of highly mismatched quantum wells is of crucial importance for obtaining high performance devices such as quantum-well solar cells and quantum cascade lasers. In situ curvature measurement successfully captured strain accumulation in lattice-mismatched InGaAs/GaAsP multiple quantum wells that were grown by metal-organic vapor phase epitaxy. Average strain in the layers was detectable using the slope of curvature versus layer thickness. High-sensitivity measurement made it possible to detect strain accumulation and release within a single layer of InGaAs and GaAsP, respectively, by looking at the see-saw-like oscillation of curvature, which is an indication of successful strain balancing in a period of well/barrier. In situ curvature monitoring makes it easier and more efficient to adjust growth conditions for perfect strain balancing, as compared with conventional repetition of growth and X-ray diffraction measurement. (c) 2010 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jcrysgro.2010.09.070

Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN SOC APPLIED PHYSICS  

The gas phase doping of arsenic (As) into (100), (110), and (111) germanium (Ge) substrates at 500 to 700 degrees C using a metal-organic vapor phase epitaxy (MOVPE) system with tertiarybutylarsine (TBAs) as the As source was investigated for the n-type source/drain formation of Ge metaloxide-semiconductor field effect transistors (MOSFETs). The surface concentration of As analyzed by secondary ion mass spectroscopy (SIMS) was approximately 1 x 10(19) cm(-3) and the diffusion constant was one order of magnitude smaller than that of As induced by ion implantation at 700 degrees C. The diffusion constant was linearly dependent on the As concentration, and the activation energy and pre-exponential factor of the As diffusion constant were found to be 1.9 eV and 6.2 x 10(-3) cm(2)/s, respectively. The electron carrier concentration profiles were also evaluated by spreading resistance profiling (SRP) to confirm dopant activation. (c) 2011 The Japan Society of Applied Physics

DOI： 10.1143/JJAP.50.010105

Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

The upper limit of GaAs growth rate was explored for the metal-organic vapor phase epitaxy (MOVPE) of a GaAs pin photovoltaic cell, in order to increase the throughput and to reduce material wastage during MOVPE. The growth rate of the undoped GaAs (i-layer) alone was increased from 0.55 to 3.3 mu m/h at a constant partial pressure of tertiarybutylarsine (TBAs), and the V/III ratio was reduced from 15 to 2.2 accordingly. The conversion efficiency of the pin cell with a 1-mu m i-GaAs layer was almost independent of growth rate and reached 20%, although too thick an i-layer of 3 mu m led to degraded efficiency. The crystal quality of that i-GaAs layer was investigated with Hall effect measurement and secondary ion mass spectroscopy (SIMS). With increase in growth rate, the hole concentration increased from 1 x 10(16) to 8 x 10(16) cm(-3) due to increased carbon impurity concentration. We have estimated the upper limit of carbon impurity that does not degrade conversion efficiency of a GaAs pin cell with a 1-mu m-thick i-layer: 1 x 10(17) cm(-3), corresponding to a growth rate of 6 mu m/h. (C) 2010 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jcrysgro.2010.09.069

Publishing type：Research paper (international conference proceedings)  

A strain-balanced InGaAs/GaAsP multiple quantum-wells (MQWs) structure was grown with metal organic vapor phase epitaxy (MOVPE) on GaAs substrate towards enhanced current gain by extending adsortion range. In our previous investigation, it has been verified that growth of such a hetero-structure can be diagnosed by periodical oscillation of transient reflectance spectra: an excess amount of strian accumulation (SA) would derive lattice relaxation in MQWs and degrades surface mophology, resulting in a decay in surface reflectance. In this report, we applied a series of detailed in situ mornitoring of transient reflectance spectra (RS). According to a combined analysis of transient RS in a course of p-i-n MQWs solar cell growth and spectral response measurement, a comprehensive understanding can be offered to explain SA process through the whole p-i-n hetero-layer, and its influnce on solar cell properties. © VDE VERLAG GMBH.

Publishing type：Research paper (international conference proceedings)  

We have successfully demonstrated the CMOS integration of InGaAs nMOSFETs and Ge pMOSFETs with self-align Ni-InGaAs and Ni-Ge metal source/drain (S/D) on a Ge substrate, by using direct wafer bonding (DWB), for the first time. Ni-based metal S/D allows us to fabricate high performance nMOSFETs and pMOSFETs simultaneously at the single-step S/D formation process. The fabricated InGaAs nMOSFET and Ge pMOSFET have exhibited the high electron and hole mobilities of 1800 and 260 cm2/Vs and the mobility enhancement against Si of 3.5x and 2.3x, respectively. © 2011 JSAP (Japan Society of Applied Physi.

Publishing type：Research paper (international conference proceedings)  

Atomic layer deposited Al 2O 3 activated direct wafer bonding has been developed to form III-V on Insulator wafer on Si for III-V CMOS photonics. The surface energy of the bonded interface was improved to be 620 mJ/m 2 without any plasma process. The propagation loss of an InGaAsP photonic wire waveguide was also improved to 1.7 dB/rom. © VDE VERLAG GMBH.

Publishing type：Research paper (international conference proceedings)  

The plasma assisted direct bonding of an InP-based III-V active layer on highly doped silicon micro rib for producing direct-current-pumped Fabry-Perot lasing is presented. Electrical pumping from silicon micro rib to InGaAsP MQWs to generate Fabry-Perot lasing was successfully demonstrated at a threshold current of 55mA at 12 °C. The semiconductive and optical properties of the heterojunction between silicon micro rib and InGaAsP multiple quantum wells (MQWs) under direct current injection were measured and discussed. © VDE VERLAG GMBH.

Publishing type：Research paper (scientific journal)  

Other Link： http://dblp.uni-trier.de/db/journals/ieicet/ieicet94c.html#journals/ieicet/Nakano11

Language：English   Publishing type：Research paper (international conference proceedings)  

Photoelectrochemical properties of nitride semiconductors are paid attention due to their possibilities of water splitting by visible light absorption. However, the photocurrent density of InxGa 1-xN, which absorbs visible light, is usually lower than that of GaN, which has larger band-gap and absorbing only UV light. The reasons of this are thought to be the band-edge position at the semiconductor-electrolyte interface and the crystal quality. The conduction band-edge decreases with increasing of indium composition and across the hydrogen generation energy at around the indium composition of 0.2. This means that the hydrogen generation ability decreases with increasing of indium composition. Low crystal quality is obtained because the lower growth temperature of InxGa1-xN than that of GaN to achieve the indium incorporation. In order to improve the photocurrent density, band-edge energy control and quantum tunneling effect are tried using the structure of thin GaN layer on InxGa1-xN here. The effect for the photocurrent densities is also discussed. © 2012 Materials Research Society.

DOI： 10.1557/opl.2012.769

Language：English   Publishing type：Research paper (international conference proceedings)  

Effects of multiple quantum well (MQW) position and p-type background doping in InGaAs / GaAsP MQW solar cells have been investigated using a semiconductor device simulator, APSYS, focusing on short circuit current I sc. With a high p-type background doping, degradation of I sc as MQW approaches p-region occurs due to recombination in the p-region. Carriers generated in the n-region are found out to be less sensitive to background doping than those in the p-region. Background doping also has indesired influences on carrier escape from wells, a critical issue for design of quantum well solar cells. © 2011 IEEE.

DOI： 10.1109/PVSC.2011.6186482

Language：English   Publishing type：Research paper (international conference proceedings)  

A new fabrication method for graded refractive index anti-reflection structure was proposed. Using self-assembled nano-scale silica particles as an etching mask on the first dielectric layer, an arrayed conic-shaped structure was fabricated. Deposition of second material on that textured dielectric film completed the dual-layer anti-reflection structure. We applied ZnS as the first layer and SiO 2 as the second layer. This fabricated 3-dimensional anti-reflection structure (ARS) demonstrated lower averaged reflectivity than a conventional planar dual-layer anti-reflection coating, the thickness of which had been optimized for a GaAs single-junction PV. The averaged reflectance on the fabricated ARS was 2.58% in the wavelength of 350-850nm demonstrated on the Si substrate. Finally, the ARS was applied to a GaAs single-junction PV, leading to enhanced external quantum efficiency (EQE) which is consistent with the reflectance spectrum. © 2011 IEEE.

DOI： 10.1109/PVSC.2011.6186020

Publishing type：Research paper (international conference proceedings)  

A multiple stepped quantum wells (MSQWs) solar cell, in which GaAs stepped-potential layers are sandwiched between strain-balanced InGaAs wells and GaAsP barriers, has been proposed. The bias-dependent photoluminescence has been studied to estimate the radiative recombination velocity, in comparison between the MSQW and the normal multiple quantum well (MQW) structures. Thermalized energy distribution from the wells was examined in terms of temperaturedependent PL and higher average carrier temperature was observed for MSQWs than for MQWs. Such enhanced carrier escape in the case of MSQWs allowed us to obtain increased short-circuit current density 25 mA/cm 2 which is approximately 14% higher than that of a normal MQWs solar cell. © VDE VERLAG GMBH.

Publishing type：Research paper (international conference proceedings)  

Optically clocked all-optical flip-flop based on coupled DBR laser diode is proposed and demonstrated numerically. The device features simple and compact structure and enables synchronized flip-flop operation without asynchronous waveform distortion. © 2011 National Sun Yat-Sen Univ.

Publishing type：Research paper (international conference proceedings)  

A silicon/III-V material active layer heterointegrated vertical PIN waveguide photodiode was developed and realized by silicon MEMS process and plasma assisted direct bonding. The InGaAsP Multiple Quantum Well (MQW) was integrated onto a SOI platform with highly doped micro rib by O2/Ar plasma assisted bonding which greatly improves the electrical property between the heterointerface. The photon-electron conversion of the photodiode was successfully demonstrated. The waveguide photodiode shows around 50% quantum efficiency at 1550nm and TE polarization. © 2011 IEEE.

DOI： 10.1109/TRANSDUCERS.2011.5969752

Publishing type：Research paper (international conference proceedings)  

We have developed the calculation technique using wafer's curvature and X-ray diffraction (XRD) pattern to analyze the structure of InGaAs/GaAsP multiple quantum wells (MQWs) grown by metalorganic vapor phase epitaxy (MOVPE). Instead of using difficult and time consuming XRD reciprocal space mapping (RSM), the curvature can effectively supplement the uncertain information of MQWs obtained from XRD 2θ-ωscan. We have proposed an alternative method to determine the physical parameters of MQWs structure. © VDE VERLAG GMBH.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We present a new method of tuning transmission colors in the visible wavelength range through a stacked pair of sub-wavelength wire-grid polarizers made of EB-patterned 100-nm thick aluminum on glass substrate.

Language：English   Publishing type：Research paper (international conference proceedings)  

This paper presents a new principle of color changing pixel by controlling the boundary condition for the surface plasmon polariton resonance on a metallic micro/nano mechanical device. A nanometric wire-grid polarizer was developed by the electron-beam patterning of 100 nm thick aluminum on glass
a pair of such polarizers was stacked with a sub-micron gap, and the separation or the in-plane rotation alignment angle was mechanically controlled to tune the coloration of the transmission light in the visible wavelength range. FDTD simulation indicated that the polarization-dependent spectrum had an effect to choose the transmitting peak wavelengths from the white backlight. © 2011 IEEE.

DOI： 10.1109/TRANSDUCERS.2011.5969733

Language：English   Publishing type：Research paper (international conference proceedings)  

Dark current reduction of Ge photodetectors has been investigated by GeO2 passivation and gas-phase doped junction. Ultralow junction and surface leakages indicate the dark current of &lt
1 nA is achievable in the waveguide geometry. © 2011 IEEE.

DOI： 10.1109/GROUP4.2011.6053707

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

A nonreciprocal polarization converter compatible with InP photonic integrated circuits is demonstrated. The device consists of an asymmetric InGaAsP waveguide combined with a ferrimagnetic Ce:YIG layer. It makes use of the direction dependence of the propagation of light in the waveguide. A trial device was made using orientation-dependent etching of InGaAsP and wafer-bonding of YIG to InGaAsP. It showed a nonreciprocal TE-TM conversion efficiency of 38% at 1.55 mu m wavelength.

DOI： 10.1109/JQE.2010.2058093

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

A broadband optical 2R regenerator based on a single distributed feedback laser is demonstrated for nonreturn to zero signals at a bitrate of 10 Gb/s. A semi-analytical approach for the influence of hysteresis on the transfer function of a 2R regenerator is shown.

DOI： 10.1109/JSTQE.2009.2037022

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

1 x 8 and 1 x 16 optical packet switching (OPS) node subsystems are demonstrated. The OPS node consists of a 1 x N monolithically integrated InP optical phased-array switch, an electronic switch controller, and a label extractor/eraser that utilizes cascaded reflective fiber Bragg gratings. In-band parallel labels are employed to encode the address information. 160-Gb/s optical time-domain-multiplexed packets are dynamically switched to 8 and 16 outputs with power penalties of 0.5 and 0.7 dB, respectively.

DOI： 10.1109/LPT.2010.2046161

Publishing type：Research paper (scientific journal)  

DOI： 10.1063/1.3374447

Other Link： http://orcid.org/0000-0003-0964-2694

Language：English   Publishing type：Research paper (scientific journal)  

The possibility of growing a GaAs single-junction solar cell structure with high growth rate and low partial pressure of tertiary-butylarsine (TBAs) has been explored in order to minimize the cost and material consumption in metal-organic vapor phase epitaxy (MOVPE). In situ surface reflectance anisotropy measurements suggested that we can grow reasonable quality GaAs layers at a growth rate of 3.3 μm/h and a V/III ratio of 2.5. Dark current-voltage characteristics of the pn junction that was grown at this condition yielded an ideality factor of 1.3, which was equivalent to the value obtained with a lower growth rate and higher V/III ratio, suggesting a possibility that we can grow GaAs with reasonable crystal quality at a high material efficiency. However, there seemed to exist spots of accumulated defects with a spacing of hundreds of micrometers at such a growth condition with a high material efficiency. If they are situated in the vicinity of the pn junction, the efficiency of the PV cell was significantly degraded, which suggests the necessity of tuning the growth conditions according to the impact of a layer on the electrical properties of a PV cell. © 2009 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jcrysgro.2009.11.066

Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

A strain-balanced InGaAs/GaAsP multiple quantum-wells (MQWs) structure was grown by metal organic vapor phase epitaxy (MOVPE) on GaAs substrate, aiming at a middle cell that improves current matching in a tandem solar cell. In order to detect the instant of strain relaxation in the course of MOVPE, which deteriorates crystal quality significantly, we employed in situ optical surface reflectivity measurement. When strain balancing was incomplete, surface reflectivity dropped during the growth of MQWs, indicating lattice relaxation. Such drop in surface reflectivity occurred at a smaller number of stacked quantum wells when the absolute value of an average strain per well/barrier pair was larger. The accumulated strain, i.e., the product between the average strain and the total thickness at the moment of reflectivity dropped, was roughly constant for all the MQWs, indicating a possibility that we can use this value as the measure to predict the maximum number of MQWs for a given value of the average strain. The reflectance anisotropy (RA) was also monitored in the course of the growth. The value of RA showed linear periodic behavior before the lattice relaxation, corresponding to the well/barrier stacks, suggesting that anisotropy of surface atoms reflects accumulated strain of the growth surface. (C) 2009 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jcrysgro.2009.11.063

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

The static and dynamic characteristics of a monolithically integrated InP 1 x 16 optical phased-array switch are presented. The device demonstrates static switching with an average extinction ratio of 18.6 dB, on-chip loss below 7 dB, and wavelength dependence of less than 0.8 dB in the entire C-band. A 40-Gb/s nonreturn-to-zero signal is transmitted through the switch with a power penalty below 0.4 dB. Using a programmable electronic circuit, dynamic switching to all 16 outputs is achieved with response times less than 11 ns.

DOI： 10.1109/LPT.2009.2036859

Publishing type：Research paper (international conference proceedings)  

We perform and demonstrate synchronous all-optical flip-flop operations with optical clock injection using Mach-Zehnder interferometer bistable laser diode. Saturable absorbers play the key role which allow broadband wavelength tuning of the clock wavelength. © OSA/IPR/PS 2010.

Language：English   Publishing type：Research paper (international conference proceedings)  

We have already proposed a simulation model for a 90-deg-tilted MEMS mirror, but in some reasons, numerical simulation methods for several angle counter electrode devices are not established. In order to solve these problems, we propose a cascade drive voltage signal technique and demonstrate a 51.5 deg tilted angle counter electrode and assure the effectiveness of the equivalence circuit. ©2010 IEEE.

DOI： 10.1109/OMEMS.2010.5672161

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We design a hybrid optical add/drop node combining Multi-Wavelength Optical Packet and Optical Switching for Metro-Area Ring Networks. The feasibility of hybrid switching has been confirmed at a 120 Gb/s network testbed.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We demonstrate a 1x16 optical packet switch sub-system for 160 Gbps RZ-OOK and 12 x 10 Gbps multi-wavelength DPSK packets. We show error-free operation with maximum penalties of 0.7 dB for 160 Gbps RZ-OOK and 0.6 dB for multi-wavelength DPSK packets. (c) 2010 Optical Society of America

Language：English   Publishing type：Research paper (international conference proceedings)  

A nano-gap Pirani gauge for integration with bulk MEMS sensors and actuators was fabricated by cutting-edge high aspect ratio bulk micromachining. The 150nm-wide, 5000nm-dep vertical trench enables wide-range pressure measurement
the device showed sensitity in all the tested range from 6.3 to 101.3kPa. ±0.27% of power variation around 35 mW was measured for ±1 kPa of pressure change from atmospheric pressure. ©2010 IEEE.

DOI： 10.1109/ICMTS.2010.5466871

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

The direct current pumping from highly doped silicon microwire to InP-based III-V active layer for spontaneous light emission was realized by air ambient plasma-assisted direct bonding. The semi-conductive properties of the hetero-integration and the effects of plasma-assisted bonding process on InGaAsP multiple quantum well (MQW) were measured and discussed. The electrical pumping from silicon microwire to InGaAsP MQW material for spontaneous light emission was successfully demonstrated afterwards.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

Using newly developed PLZT-based OBS core node, low-loss Ether-frame transmission with application-layer connectivity is demonstrated over 92-km 2-hop field testbed. Recent progresses in developing OPS/OCS hybrid router are also reviewed. (c) 2010 Optical Society of America

Language：English   Publishing type：Research paper (international conference proceedings)  

We have demonstrated extremely-thin-body (ETB) (3.5 and 9 nm) InGaAs-on-insulator (InGaAs-OI) MOSFETs on Si substrates with Al 2O3 ultrathin buried oxide (UTBOX) layers fabricated by direct wafer bonding (DWB). We have found that the ETB highly-doped InGaAs-OI n-channel MOSFETs without p-n junction can perform a normal MOSFET operation under front- and back-gate configuration and the double-gate operation can provide excellent on-current/off-current (Ion/Ioff) properties of ∼107 and the improved S factor even for InGaAs-OI MOSFETs with ND of 1×1019 cm-3. ©2010 IEEE.

DOI： 10.1109/IEDM.2010.5703286

Publishing type：Research paper (scientific journal)  

DOI： 10.1109/vlsit.2010.5556241

Publishing type：Research paper (scientific journal)  

DOI： 10.1149/1.3483529

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

For the purpose of getting maximum short-circuit current for a InGaAs/GsAsP strain-compensated multiple-quantum-well (MQW) solar cell with a minimum drop in both open-circuit voltage and fill factor, we have optimized the position of the quantum wells in the i-region of a pin host structure. Numerical and experimental analysis revealed the best position, where the MQWs divide the i-region at a ratio of 1:3 for p-side and n-side, respectively. The physics behind this finding is that (1) MQWs should not touch the edge of either p- or n-region in order to avoid accumulation of majority carrier that deteriorates carrier transport at a short-circuit condition, and (2) MQWs should not be around the center of the i-region to avoid carrier recombination at a larger forward bias condition.

Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

A superlattice solar cell with InGaAs well and GaAsP barrier was realized for the first time. Increase in short-circuit current by 2 mA/cm(2) was achieved with 60 stacks of the 3.7-nm-thick wells as compared with a GaAs p-i-n reference cell. No degradation of spectral response for the wavelength range of GaAs absorption suggested efficient carrier transport across the wells due to tunneling through the thin barriers. A breakthrough for this achievement was strain management with monolayer-thin GaAs layer at the InGaAs/GaAsP interface, where huge lattice mismatch seems to have induced localized defects and have functioned as recombination centers.

Publishing type：Research paper (international conference proceedings)  

We demonstrate the first all-optical 2R regeneration of 25 Gbit/s NRZ data based on hysteresis in a DFB laser. The scheme results in BER improvement, exhibits low power consumption and is effective after fiber transmission. © 2010 Optical society of America.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

Novel type of wavelength-insensitive monolithically integrated InP 1xN switch is developed using optical phased-array architecture. Low-polarization-dependent 1x8 switch, 1x16 switch, and application to 320-Gbps WDM optical packet switching are demonstrated. (C) 2010 Optical Society of America

Language：English   Publishing type：Research paper (international conference proceedings)  

Semiconductor integrated photonic devices and circuits on InP for high-speed and large scale optical switching, optical buffer memories, and for digital photonic processing are reviewed, as well as PLC-based optical lead frames for multi-port coupling. ©2010 IEEE.

DOI： 10.1109/ACP.2010.5682493

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEICE-INST ELECTRONICS INFORMATION & COMMUNICATION ENGINEERS  

Broadband and high-speed monolithically integrated InP 1x16 switches are developed using optical phased-array structure. Application to 160-Gbps packet switching and large-capacity variable optical buffering are demonstrated.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

A photonic integrated circuit capable of 1x100 switching is reported. The switch, incorporating more than 250 photonic components along with numerous passive waveguides and bends, demonstrates power penalty &lt; 1 dB at 10 Gbps and extinction ratio &gt; 50 dB.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We demonstrate large-capacity high-resolution optical buffer, comprising 1x16 InP switch and ultra-compact delay lines based on thin-cladding highly nonlinear fibers. Silica-PLC-based pitch converter is employed to realize uniform coupling from all 16 switch ports simultaneously. (C) 2010 Optical Society of America

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We demonstrate a 1x16 optical packet switch sub-system for 160 Gbps RZ-OOK and 12 x 10 Gbps multi-wavelength DPSK packets. We show error-free operation with maximum penalties of 0.7 dB for 160 Gbps RZ-OOK and 0.6 dB for multi-wavelength DPSK packets.

Language：English   Publishing type：Research paper (international conference proceedings)  

Monolithically integrated InP 1×16 switch with broadband and low-noise operation is developed by using optical phased-array scheme. Potential applications to 160-Gbps packet switching and large-capacity variable optical buffering are demonstrated successfully.

DOI： 10.1109/COIN.2010.5546470

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We propose and experimentally demonstrate a novel structure for monolithically integrating a TM-mode waveguide optical isolator with active InP devices. A semiconductor ring laser with integrated isolator is fabricated and demonstrated for the first time.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

An anti-reflection coating (ARC) with sub-micrometer size of structure has brought significant advantage in solar cell efficiency. Owing to the diffraction and light trapping effect caused by a sub-wavelength size of structure, the reduced reflectance covering a wide-wavelength can be expected at a surface of the photovoltaic (PV) cell. We proposed and tried a nano-scale structure fabrication by small-size particles combined with conventional 2 layers ARC on GaAs PV cells. The small-size spherical particles show well aligned self-assembly on the substrate when the solution containing micro-spheres were dried with a proper sheer force. Using the pure water solution of colloidal SiO2 spheres and an ordinal spin-coating technique, a well aligned mono-particle layer has been fabricated on the 2 layer ARC (TiO2 and SiO2) on a GaAs PV cell. In this research, we tried 3 samples with different size of particles (400 nm, 200 nm and 100 nm in diameter).
As the evaluation of PV cell, I-V characteristics under the AM1.5G illumination and external quantum efficiency (EQE) spectra were measured. For each sample, the I-V and EQE were obtained from the same sample: before and after application of particle coating. Measured relative efficiency enhancement of the additional mono-layer coating was +7.1, +5.7 and -0.3 in % for particle sizes of 100, 200 and 400 nm, respectively. As well as enhancement in light incorporation to the inside of a PV cell, there seem to be scattering loss by the particles on the cell. In the range of tested particle size, the smaller particle was preferable to avoid scattering loss and provides a larger efficiency enhancement to GaAs PV cells.

Language：English   Publishing type：Research paper (international conference proceedings)  

In order to develop a high quality material which is both lattice-matched to GaAs and with extended absorption edge below 1000 nm for improved efficiency of III-V tandem solar cells, we have introduced In0.16Ga 0.84As/GaAs/GaAs0.79P0.21 strain-compensated multiple stepped quantum wells (SC-MSOWs) solar cell. To maximize the contribution of quantum well to spectral response, we inserted a GaAs step layer between the well and the barrier, leading to stronger photo-absorption of the wells and enhanced carrier transport in the stacked wells. Combined with strain compensation between highly-strained wells and barriers, the SC-MSOWs allow us sufficient number of quantum-well stacks to obtain significant spectral response below the bandgap of GaAs. A short circuit current density 25 mA/cm2 was obtained under AM1.5G illumination, which is approximately 14% higher than that of a conventional MOWs solar cell. © 2010 IEEE.

DOI： 10.1109/PVSC.2010.5616839

Language：English   Publishing type：Research paper (international conference proceedings)  

DOI： 10.1109/OMEMS.2010.5672215

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

Extremely compact low-loss 4 channel arrayed waveguide grating with 147x92 mu m(2) device size was demonstrated using ultrahigh index contrast InGaAsP photonic wire waveguides on Si, compatible for the monolithic integration of high-performance III-V CMOS transistors. (C) 2010 Optical Society of America

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We fabricate and demonstrate all-optical packet switching using a single photonic integrated circuit (PIC), which is constructed from monolithically integrated all-optical flip-flop and switch. Both 10 and 40-Gb/s signal can be transmitted through the PIC. (C) 2010 Optical Society of America

Language：English   Publishing type：Research paper (scientific journal)   Publisher：OPTICAL SOC AMER  

Spot-size converters for an all-optical switch utilizing the intersubband transition in GaN/AlN multiple quantum wells are studied with the purpose of reducing operation power by improving the coupling efficiency between the input fiber and the switch. With a stair-like spot-size converter, the absorption saturation of 5 dB is achieved with a pulse energy of 25 pJ. The switch is integrated with a SiN/AlN waveguide and spot-size converters, and the structure provides the possibility of an integration of the switch with other functional devices. To further improve the coupling loss between the waveguide and the switch, triangular-shaped converters are investigated, demonstrating losses as low as 2 dB/facet. (C) 2009 Optical Society of America

DOI： 10.1364/OE.17.023247

Publishing type：Research paper (scientific journal)  

DOI： 10.1143/APEX.2.122201

Language：English   Publishing type：Research paper (international conference proceedings)  

Publishing type：Research paper (scientific journal)  

DOI： 10.1143/APEX.2.124501

Publishing type：Research paper (scientific journal)  

DOI： 10.1109/JLT.2009.2015776

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

A nonreciprocal polarization converter compatible with InP-based photonic integrated circuits is proposed. The device consists of an asymmetric InGaAsP waveguide combined with a ferrimagnetic cerium-substituted yttrium iron garnet layer. It makes use of the nonreciprocal conversion of the polarization state in the waveguide. A nonreciprocal TE-TM conversion efficiency of 93% at 1.55 mu m wavelength can be obtained with a device length of 0.27 mm.

DOI： 10.1109/JQE.2009.2013123

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

All-optical flip-flops (AOFFs) have recently received increased attention as elements for all-optical packet-switched networks. In this letter, we use a single off-the-shelf distributed-feedback laser as AOFF to switch 40-Gb/s packets with a guard time as low as 150 ps.

DOI： 10.1109/LPT.2009.2016671

Publishing type：Research paper (scientific journal)  

DOI： 10.2320/matertrans.MC200830

Language：English   Publishing type：Research paper (scientific journal)   Publisher：OPTICAL SOC AMER  

A monolithically integrated InP/InGaAsP 1x5 optical phased-array switch is demonstrated for broadband wavelength-division multiplexed (WDM) optical packet switching (OPS) application. Using the wide optical bandwidth of the switch, we achieve error-free forwarding of 320-Gbps (40-Gbps x 8 channel) WDM signal with less than 1.3-dB penalty. Since the switch consists of only phase-modulating section, it is free from nonlinear signal distortion and inter-channel crosstalks, allowing large dynamic range of input power. The application to WDM-OPS testbed is demonstrated successfully, where 320-Gbps WDM payloads are routed synchronously to the optical label. (C) 2009 Optical Society of America

DOI： 10.1364/OE.17.009454

Publishing type：Research paper (scientific journal)  

DOI： 10.1143/APEX.2.011101

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

We report 92-km 2-hop field trial of an optical burst-switching (OBS) network testbed, consisting of a (Pb,La)(Zr,Ti)O-3 (PLZT) space switching matrix and a contention-resolving wavelength converter. Owing to the low polarization-dependent loss of the PLZT switch, we achieve stable forwarding of 40/10-Gb/s mixed-bit-rate signals with wavelength conversion. We also perform Layer-2 evaluation at a line rate of 40 Gb/s by using specially developed Ether-frame media converters. Loss-free transport of encapsulated Ether-frames is demonstrated successfully over the 2-hop OBS field testbed.

DOI： 10.1109/LPT.2008.2008198

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

In this paper, we design a novel hybrid optical network architecture combining both MW-OPS and OCS, and experimentally demonstrate its feasibility in a 280-Gbps network testbed.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We fabricate and experimentally demonstrate all-optical flip-flop switching based on novel Mach-Zehnder interferometer bistable laser structure. The MZI-BLD successfully acts as set-reset flip-flop with continuous light injection.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

In this paper, we demonstrate 200Gb/s Multi-Wavelength Optical Packet Switching with an ultra-fast optical switch. The feasibility has been confirmed by the experimental results.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We summarize our recent results on field trial of optical burst switching node prototype developed under NEDO-funded Photonic Network Project. Stable switching and transmission of 40/10-Gbps; mixed-bitrate signals are achieved over 92-km 2-hop field testbed.

Publishing type：Research paper (international conference proceedings)  

Fast all-optical switching of 40 Gb/s packets is demonstrated using a single off-theshelf DFB laser as all-optical flip-flop, thus offering a fast (150 ps) and economically viable method to implement a transparent network node. © 2008 IEEE.

DOI： 10.1364/ofc.2009.omu4

Publishing type：Research paper (scientific journal)  

Other Link： http://dblp.uni-trier.de/db/journals/ieicet/ieicet92c.html#journals/ieicet/Nakano09

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

We demonstrate a wavelength-tunable all-optical flip-flop (AOFF) based on a multimode-interference (MMI) bistable laser diode (BLD) with distributed Bragg reflectors (DBRs). The wavelength tuning can be achieved by the band-filling effect and the free-carrier plasma effect with carrier injections to the DBRs. The tuning range of 3.1 nm will enable flexible and reconfigurable design of future photonic integrated circuits (PICs). Since the DBR-MMI-BLD does not require cleaved facets for its operation, multiple AOFFs can be monolithically integrated on a single PIC. In addition, the device has a large operable bandwidth of wider than 34 nm, even when we tune the lasing wavelength. A dynamic flip-flop operation is also demonstrated with rising and falling times of faster than 280 and 228 ps, respectively. No waveform distortion is observed, even when we tune the lasing wavelength.

DOI： 10.1109/JPHOT.2009.2025670

Publishing type：Research paper (scientific journal)  

Other Link： http://orcid.org/0000-0001-6802-3346

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：VDE VERLAG GMBH  

We present the first fully integrated high-speed 1x16 InP optical switch based on arrayed phase modulators, demonstrating average extinction ratio of 17.0 dB, estimated on-chip loss of &lt;8 dB, and reconfiguration time of &lt;6 ns.

Language：English   Publishing type：Research paper (international conference proceedings)  

This paper presents a novel Hybrid Optical neT-work ARchitecUre concept named HOTARU which combines multi-wavelength optical packet switching (MW-OPS) and optical circuit switching (OCS) in WDM networks
herein, we also experimentally demonstrate its feasibility in a 400 Gbps network testbed. ©2009 IEEE.

DOI： 10.1109/APCC.2009.5375570

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

In this paper, we study on the input power dynamic range of SOA switches for MW-OPS. Power penalty are presented with approximately 15dB dynamic range for 16 wavelengths at an optical gain of 5dB.

Language：English   Publishing type：Research paper (international conference proceedings)  

Integrated InP/InGaAsP phased-array 1×16 optical switch is fabricated and characterized for broadband WDM optical packet switching. Wavelength-insensitive operation covering the C-band and penalty-free transmission of 40-Gbps signal are demonstrated. ©2009 IEEE.

DOI： 10.1109/PS.2009.5307760

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We report on the switching times and energies of a DFB all-optical flip-flop and their dependence on device and operating parameters. Numerical as well as experimental results are discussed.

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

An InP/InGaAsP integrated 1 x 8 optical phased-array switch is designed, fabricated, and characterized for high-throughput broadband wavelength-division multiplexed (WDM) optical packet switching application. With a single stage of phase shifters utilizing carrier-induced refractive index change, switching to the eight output ports is demonstrated successfully. The polarization-dependent loss is 2.2 dB in the worst case, and the wavelength-dependent loss is less than 2.5 dB across the entire C-band (1520-1580 nm) with the average extinction ratio of 17.7 dB. The measured reconfiguration time is shorter than 6 ns. Low-penalty transmission of a 40-Gb/s non-return-to-zero signal is also demonstrated for the first time. These features make the integrated phased-array switch an attractive candidate to be deployed in the future high-capacity optical packet switching networks.

DOI： 10.1109/JPHOT.2009.2025971

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：ELECTROCHEMICAL SOC INC  

The growth mechanism and rate constants of GaN metal-organic vapor-phase epitaxy (MOVPE) have been unclear in spite of the necessity for large-scale production of this material. In this work, the major precursor of GaN has been deduced and its surface reaction rate constant has been obtained through the analysis of multi-scale growth-rate profiles both in the reactor-scale and in the micrometer-scale profiles that were obtained by selective-area growth. Usually, it is difficult to explore surface reaction kinetics, especially for the MOVPE of GaN, because of mass-transferlimited kinetics. This multi-scale analysis, however, has clarified that a single precursor, the gas-phase reaction product between (CH3)(3)Ga and NH3, leads to the growth of GaN with a surface reaction probability of approximately 0.4 at 1400 K which is a typical growth temperature. A lumped reaction model of GaN MOVPE was proposed that led to reasonable agreement between the simulated growth-rate profile in the reactor-scale and the corresponding measured profile.

DOI： 10.1149/1.3207632

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

An InP/InGaAsP 1x8 optical phased-array switch is designed, fabricated, and demonstrated for the first time. The device demonstrates successful operation in the entire C-band (1520-1580 nm) for both the TE- and TM-polarization states. (C) 2009 Optical Society of America

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We propose and investigate a novel type of waveguide polarization converter, which is particularly suited for monolithic integration in InP photonic integrated circuits. Efficient mode conversion with 0.4-dB loss and 16.6-dB extinction is demonstrated numerically.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

The hetero-integration of InP/InGaAs on high doped silicon micro wires for Si hybrid laser using plasma assisted direct bonding was carried out. Bonding assisted pattern was used for increasing the bonding force of the silicon wire to InGaAs/InP bulk. The I-V characteristics of the highly doped silicon micro wire to InP/InGaAs bulk are measured and compared to the Si/InP bulk to bulk results. The improvement of semiconductor parameters of the hetero-integration by introducing long time annealing was also shown and discussed for its ability to realize the direct electrical pumping from Si wire to compound semiconductor active layer for silicon hybrid laser.

DOI： 10.1109/ICIPRM.2009.5012490

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

Defect-free n-type InP source/drain for III-V nMOSFETs was successfully formed by epitaxial regrowth at 610 degrees C. The impurity concentration of 1 x 10(19) cm(-3) with the S/D junction steepness of around 10 nm/dec. was obtained.

DOI： 10.1109/ICIPRM.2009.5012454

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We demonstrate all-optical flip-flop operation of Mach-Zehnder histable lasers with falling time Of less than 68 ps. The device can be controlled with continuous 58-nm wavelength range of input light.

DOI： 10.1109/LEOS.2009.5343124

Publishing type：Research paper (scientific journal)  

DOI： 10.1109/LPT.2008.2004350

Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN SOCIETY APPLIED PHYSICS  

The surface kinetic information of metalorganic vapor phase epitaxy (MOVPE) is difficult to obtain, because growth rate is normally limited by diffusional mass-transfer rate. In this study, by using a selective area growth (SAG) technique. the surface kinetics has been successfully clarified for InP and InAs growth. The temperature dependence of surface reaction rate constant (k(s)) was examined, and it revealed that for both compounds k(s) continuously increases with activation energies of 20.1 kJ/mol for InP and 15.0 kJ/mol for InAs. The sticking probability of indium species, converted from k(s), was in the range of 0.54-0.79. This is two or three times that of gallium species during GaAs MOVPE. For indium-related binary Compounds, the k(s) of InP is always larger than that of InAs. This kinetic information suggests that group V elements have a significant effect on the k(s), of Ill-V binary compounds. These preliminary results show that indium species have quite different reactivities in phosphorus and arsenic sites, which could be fundamental for the kinetic analysis of ternary and quaternary compounds, such as InAsP and InGaAsP. [DOI: 10.1143/JJAP.47.7788]

DOI： 10.1143/JJAP.47.7788

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEICE-INST ELECTRONICS INFORMATION COMMUNICATIONS ENG  

Simulation and fabrication results on back-illuminated 4-channel photodiode (PD) array with a self-aligned micro ball lens are described. The channel pitch and diameter of each photosensitive area are 250 mu m and 40 mu m, respectively. Measured photocurrent is 1.92 times larger than that without a lens. Alignment tolerance between the single mode fiber (SMF) optical axis and the photodiode is improved from 21.2 mu m to 42.7 mu m. Moreover, the separation tolerance between the fiber and the lens is 210.5 mu m. These large tolerances agree with simulation results, demonstrating that the device configuration is suitable for receivers for multi-channel inter-connection. Frequency response and inter-channel cross talk are also discussed.

DOI： 10.1093/ietele/e91-c.9.1472

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEICE-INST ELECTRONICS INFORMATION COMMUNICATIONS ENG  

We derive explicit design rules of optical phased-array planar 1 x N switch to achieve small insertion loss, large extinction ratio, and non-dispersive waveguide structure required for wavelength-independent broadband operation. The minimum number of phase shifters and slab lengths were studied as a function of N and the optimized array shape was derived to minimize the optical path length. We show that the length depends nonlinearly on N, and may become significantly shorter compared with that of a tree-structured switch under certain conditions.

DOI： 10.1587/elex.5.603

Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN SOCIETY APPLIED PHYSICS  

The selective area metal-organic vapor phase epitaxy (SA-MOVPE) of InN was realized and its kinetics was investigated. InN deposition selectivity between the mask and the crystal surface was found to strongly depend on the growth temperature, and good selectivity was obtained above 600 degrees C. Under this growth condition, the thickness profile of InN between the masks exhibited a catenary shape, which resulted from the vapor phase diffusion. This fact shows that vapor phase diffusion is the dominant supply mechanism in InN SA-MOVPE as well as for the growth of GaN and InGaAsP. These kinetic analysis will contribute to the development of monolithically integrated AlInGaN-based devices. (C) 2008 The Japan Society of Applied Physics.

DOI： 10.1143/APEX.1.071102

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

Reactor-scale simulation of metalorganic vapor phase epitaxy (MOVPE) by computational fluid dynamics is now widely attempted, but it is still difficult to predict group V composition precisely for InGaAsP. In this study, desorption of As and P from the surface of GaAs, InAs, GaP, InP, InGaAs and InGaP was measured around growth temperatures using in situ reflectance anisotropy (RA) spectroscopy. Time transient curves of RA were measured when group V supply was turned on/off and the rate constants of the desorption of As and P were measured assuming first-order kinetics. These data were incorporated into the simulation to obtain the composition of InGaAsP. The simulated distribution of group V composition showed better agreement with experimental data than the results using previous kinetic data. (c) 2008 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jcrysgro.2008.03.005

Publishing type：Research paper (scientific journal)  

DOI： 10.1109/LPT.2008.924290

Language：English   Publishing type：Research paper (international conference proceedings)  

The static performance of active multimode interference (MMI) couplers is analyzed in the presence of two different optical signals using a beam propagation method (BPM)-based simulation. To examine the feasibility of a novel all-optical switch based on active MMI
the simulation is mainly used to evaluate the amount of phase change resulting in the output of one optical signal by varying the intensity of the other at different injected carrier density. The simulation code is based on the finite difference (FD)-BPM and is including the distribution of photon density, carrier density, and refractive index. © 2008 Springer Science+Business Media, LLC.

DOI： 10.1007/s11082-008-9215-8

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

Fabrication of abrupt InGaP/GaAs heterointerfaces has been difficult using metal organic vapor phase epitaxy (MOVPE) due to the exchange of P and As during the fabrication steps. An optimized gas-switching sequence to fabricate heterointerface of InGaP on GaAs layer by MOVPE was previously developed in which the unstable top surface layer of GaAs is stabilized and the exchange of P and As between InGaP and GaAs layers is suppressed. In this study, the effect of this optimized gas-switching sequence was quantitatively evaluated by using scanning transmission electron microscopy (STEM). Changes in atomic composition from GaAs to InGaP at the interface at the atomic layer level were revealed by using Z-contrast method in STEM. Quantitative evaluation using the Z-contrast method confirmed that the abruptness of the GaAs/InGaP interface was improved by this optimized gas-switching sequence. (c) 2008 American Institute of Physics.

DOI： 10.1063/1.2884694

Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN SOC APPLIED PHYSICS  

InGaP layers grown by metal-organic vapor phase epitaxy (MOVPE) often have an In-rich region within several nanometers from the top of the film, which cannot be explained by normal surface segregation theory. This surface segregation deteriorates the interface abruptness of the InGaP/GaAs system, which is attractive for high-performance devices. This segregation in lattice-matched InGaP/GaAs systems can be explained by considering a "subsurface", where unstable surface layers play an important role in controlling the composition of grown epi-layers. This subsurface is formed at the beginning of crystal growth, and its thickness is in a steady state under normal continuous epitaxial growth. In this study, the existence of a subsurface during the MOVPE growth of InP and InGaP is experimentally confirmed through kinetic analysis involving a flow modulation method. The kinetic parameters, such as the adsorption and desorption rate constants of In and Ga species, and the crystallization rate from the subsurface are estimated from experimental data and are then used to explain the tendency of In surface segregation in InGaP-MOVPE.

DOI： 10.1143/JJAP.47.1473

Language：English   Publishing type：Research paper (scientific journal)  

A 1.5-μm nonreciprocal-loss waveguide optical isolator having improved transverse-magnetic-mode (TM-mode) isolation ratio was developed. The device consisted of an In GaAIAs/InP semiconductor optical amplifier waveguide covered with a ferromagnetic epitaxial MnSb layer. Because of the high Curie temperature (Tc = 314°C) and strong magneto-optical effect of MnSb, the nonreciprocal propagation of 11-12dB/mm has been obtained at least up to 70°C. © 2008 The Japan Society of Applied Physics.

DOI： 10.1143/APEX.1.022002

Publishing type：Research paper (scientific journal)  

DOI： 10.1109/LPT.2007.912970

Language：English   Publishing type：Research paper (international conference proceedings)  

An AlN-based intersubband optical switch was investigated. Narrow absorption spectrum and low insertion loss were obtained. Absorption saturation by 10 dB was achieved at a wavelength of 1.42 μm with an energy of 50 pJ.

DOI： 10.1109/OECCACOFT.2008.4610534

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

A monolithically integrated 1x5 InP/InGaAsP optical switch based on optical phased array is fabricated and characterized. Wide operating bandwidth covering entire C-band (1520-1580 nm) and nanoseconds dynamic response are demonstrated for the first time.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

A novel etching and oxidation method utilizing space effect of dry etching for three dimensional silicon structure is presented. Testing devices of SOI symmetric waveguide with ultra thick SiO2 cladding and silicon waveguide structure integrated with 3D taper spot size converter are fabricated using this method

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

Cross gain modulation between two different optical signals, inserted through separate ports to an active MMI, has been experimentally measured. The observed trend of modulation reduction by increase of modulated signal power matches previous simulation results. An associated XPM is expected and could be optimized to enable using the active MMI in all-optical switching.

Language：English   Publishing type：Research paper (international conference proceedings)  

We propose and design a beam-deflecting 1x8 InGaAsP/InP optical switch with carrier-induced tunable prism and aspheric lenses. High switching efficiency of -1.9 dB at 1.93-deg deflection angle is obtained. ©2008 IEEE.

DOI： 10.1109/INOW.2008.4634518

Language：English   Publishing type：Research paper (international conference proceedings)  

Optical burst switching using fast, bit-rate transparent switches is presented. Random contention is handled without using optical memory by wavelength conversion. We demonstrate error-free 10 and 40Gb/s burst contention resolution in a 5×5 node prototype. © 2007 Optical Society of America.

DOI： 10.1109/OFC.2008.4528633

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We demonstrate polarization-insensitive all-optical flip-flop based on multimode-interference bistable laser diode. Dynamic switching is observed without waveform distortion even when the input polarization state is scrambled.

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We experimentally demonstrate QoS-aware optical burst-switching router prototype with a PLZT switch matrix and shared wavelength converter. Using the pre-emptive dropping scheme, we confirm successful service differentiation of high-priority burst signals.

Publishing type：Research paper (scientific journal)  

DOI： 10.1002/pssc.200779309

Language：English   Publishing type：Research paper (international conference proceedings)  

A review is given on InP-based integrated optical devices for robust and efficient photonic networking. The devices include an all-optical flip-flop for digital processing and memory functions in the network, and a phased-array optical matrix switch for fast and scalable switching of optical burst/packets. ©2008 IEEE.

DOI： 10.1109/INOW.2008.4634454

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

A nonreciprocal polarization converter compatible with InP-based photonic integrated circuits is proposed. The device consists of an asymmetric InGaAsP waveguide combined with a ferrimagnetic Ce:YIG layer. A nonreciprocal TE-TM mode conversion efficiency of 93% can be obtained with a device length of 0.27 mm.

Language：English   Publishing type：Research paper (international conference proceedings)  

We propose and design a novel type of beam-deflecting 1x8 InGaAsP/InP optical switch with carrier-induced tunable prism and aspheric lenses. High switching efficiency of -1.9 dB is obtained numerically. A preliminary measurement supports the validity of simulation, indicating the feasibility of the device. ©2008 IEEE.

DOI： 10.1109/LEOS.2008.4688690

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

Multimode interference bistable laser diode all-optical flip-flop has been considered to be a clever candidate to achieve all-optical memory devices. In this paper, basic results of polarization-insensitive operation of all-optical flip-flop are reported. Polarization-insensitive operation of static flip-flop was demonstrated with injection power level higher than +3 dBm.

Language：English   Publishing type：Research paper (international conference proceedings)  

The optical properties of InGaAs/InAlAs five-layer asymmetric coupled quantum well (FACQW) grown by MOVPE are investigated. The FACQW phase modulator is fabricated and its large phase shift is successfully observed.

DOI： 10.1109/OECCACOFT.2008.4610533

Language：English   Publishing type：Research paper (international conference proceedings)  

TM-mode waveguide optical isolators consisting of semiconductor waveguides with ferromagnetic MnX(X=As, Sb) layers were developed. The device with a MnSb layer had an isolation ratio of 12 dB/mm in the wavelength range 1530-1555 nm. © 2008 Optical Society of America.

DOI： 10.1109/CLEO.2008.4551354

Language：English   Publishing type：Research paper (international conference proceedings)  

We observed a propagation of surface plasmon polaritons (SPPs) along bent Au-wires on a thin SiO2-coat-InP substrate with the bending radius R from 20 to 1000 μm and evaluated the bending-part attenuation coefficients, i.e., the bending losses of propagating SPPs along them with R from 20 to 400 μm by measuring the transmittance of SPPs. We discussed about the SPP propagation and the applicability to an optical electronic device and circuit of SPPs. © 2008 SPIE.

DOI： 10.1117/12.816457

Language：English   Publishing type：Research paper (international conference proceedings)  

Selective-area growth (SAG) of InGaN/GaN multiple quantum wells (MQWs) for multiple-wavelength LED is fabricated and characterized. As a result, the emission peak shifts to longer wavelengths and the wider mask leads to longer wavelength. ©2008 IEEE.

DOI： 10.1109/INOW.2008.4634493

Language：English   Publishing type：Research paper (international conference proceedings)  

An AlN-based intersubband optical switch was fabricated with spot-size converter utilizing Si3N4 as a cladding layer. Intersubband absorption was observed at 1.48 μm. Absorption saturation by 5 dB was achieved with energy of 25 pJ. ©2008 IEEE.

DOI： 10.1109/LEOS.2008.4688759

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

Scalability of an optical-phased-array 1xN switch is studied analytically. Compared with a conventional tree-structured switch, we show that the phased-array switch has superior scalability in terms of device size under wide range of practical conditions.

Language：English   Publishing type：Research paper (international conference proceedings)  

We investigate wavelength tunability of all-optical flip-flop using distributed Bragg reflectors. Single-mode lasing and flip-flop switching is obtained with more than 24.7-dB extinction ratio and 21.5-dB SMSR at 3.1-nm tuning range. ©2008 IEEE.

DOI： 10.1109/LEOS.2008.4688554

Language：English   Publishing type：Research paper (scientific journal)   Publisher：INST PURE APPLIED PHYSICS  

Thickness profiles of GaN grown by selective area metal-organic vapor phase epitaxy (SA-MOVPE) were successfully reproduced by a vapor phase diffusion model that employs only one parameter-effective diffusion length D/k(s). The value of D/ks of Ga-containing precursors changes from 10 to 50 mu m under growth temperature of 1000-1250 degrees C and reactor total pressure of 100 mbar. It was confirmed that, in the wide-stripe SA-MOVPE of GaN, the vapor phase diffusion of Ga-containing precursors govern the profile of growth rate. Numerical simulation using the vapor phase diffusion model is of great help for the design and control of thickness profiles in the SA-MOVPE of GaN.

DOI： 10.1143/JJAP.46.L1045

Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN SOC APPLIED PHYSICS  

We measured intersubband (ISB) absorption in high-mesa waveguides consisting of AIN cladding layers and AIN/GaN multiple quantum wells (MQW). Inductively coupled plasma (ICP) dry etching for waveguide fabrication was optimized to reduce propagation loss by achieving vertical and smooth sidewalls. We changed the ICP/RF power and introduced intervals during the etching process in order to overcome the induction time of the etching rate and the instability of plasma observed from long etching experiments. The ISB absorption greater than 20 dB was clearly observed at around 1.5 pm. The insertion loss of an 800-mu m-long waveguide was 5-1OdB including the coupling loss between a fiber and the end of the waveguide. With this low-loss waveguide, we observed a saturation of 10dB in the ISB absorption with 100pJ probe light introduced to the waveguide. We thus demonstrated the possibility of achieving all-optical switching devices with low loss using the ISB transition of AIN-based materials.

DOI： 10.1143/JJAP.46.6639

Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN SOC APPLIED PHYSICS  

High quality AlN layer could be grown at 800 degrees C by metal organic vapor phase epitaxy (MOVPE) by pulse injection (PI) method in which trimethylaluminium (TMAl) and NH3 are alternately supplied. Crystal quality of AlN layer grown by PI method (PI-AlN) was comparable with AlN layer grown by conventional MOVPE with continuous sources flow sequence (continuous-AlN, C-AlN) at 1240 degrees C showing similar (0002) symmetric and (1012) skew symmetric full width at half maximum (FWHM) values by high resolution X-ray diffraction (HRXRD). Especially, it is noticeable that best crystal quality was observed when the growth rate was controlled to I monolayer/cycle. Moreover, PI method was proved to be effective in improving surface roughness of AlN layer. The root-mean-square (RMS) roughness of PI-AlN layer was 0.9 nm, which is three times smaller than C-AlN layer grown at same temperature with PI-AlN layer.

DOI： 10.1143/JJAP.46.L927

Language：English   Publishing type：Research paper (scientific journal)   Publisher：OSA - The Optical Society  

We developed a 1.5-μm band TM-mode waveguide optical isolator that makes use of the nonreciprocalloss phenomenon. The device was designed to operate in a single mode and consists of an InGaAlAs/InP ridge-waveguide optical amplifier covered with a ferromagnetic MnAs layer. The combination of the optical waveguide and the magnetized ferromagnetic metal layer produces a magneto-optic effect called the nonreciprocal-loss phenomenon-a phenomenon in which the propagation loss of light is larger in backward propagation than it is in forward propagation. We propose the guiding design principle for the structure of the device and determine the optimized structure with the aid of electromagnetic simulation using the finite-difference method. On the basis of the results, we fabricated a prototype device and evaluated its operation. The device showed an isolation ratio of 7.2 dB/mm at a wavelength from 1.53 to 1.55 μm. Our waveguide isolator can be monolithically integrated with other waveguide-based optical devices on an InP substrate. © 2007 Optical Society of America.

DOI： 10.1364/AO.46.005784

Publishing type：Research paper (scientific journal)   Publisher：The Optical Society  

DOI： 10.1364/ao.46.005784

Language：English   Publishing type：Research paper (scientific journal)   Publisher：OPTICAL SOC AMER  

We proposed a novel InP based photonic wire waveguide with an InAlAs oxide cladding. The InGaAsP/InAlAs-oxide structure in the vertical direction provides an ultrahigh index contrast waveguide, and it allows a bend radius of a few mu m with no vertical leakage loss. The InP photonic wire waveguide with a 500x300-nm rectangular channel core (refractive index n similar to 3.36) and an InAlAs oxide cladding (n similar to 2.4) was numerically analyzed using the three-dimensional time-domain beam propagation method (3D TD-BPM). We predicted that the U-bend waveguide with a 3-mu m bend radius can be realized with the propagation loss of &lt; 0.5 dB. (c) 2007 Optical Society of America

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEICE-INST ELECTRONICS INFORMATION COMMUNICATIONS ENG  

Monolithically integrated four-channel distributed feedback (DFB) laser array has been fabricated by metal organic vapor phase epitaxy (MOVPE) selective area growth for 1.55 mu m coarse-wavelength division multiplexing (CWDM) systems. Wide-stripe MOVPE selective area growth and electron-beam lithography are used to obtain wide CWDM channel spacing of 20 nm. Compared to hybrid integration of discrete lasers, monolithic integration of laser array on a single substrate greatly simplifies device alignment and packaging process.

DOI： 10.1093/ietele/e90-c.5.1111

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEICE-INST ELECTRONICS INFORMATION COMMUNICATIONS ENG  

An 8ch, 400GHz monolithically integrated WDM channel selector featuring an array of quantum well semiconductor optical amplifiers (SOA) and arrayed waveguidegrating demultiplexer is presented. Reduction of fabrication complexity was achieved by using a single step selective area MOVPE to realize the different bandgap profiles for the SOA array and passive region. The selective growth mask dimensions were optimized by simulation. Dry-etching with short bending radii of 20 mu m resulted in compact device size of 7 mm x 2.5 mm. Static channel selection with high ON-OFF ratio of &gt; 40 dB was achieved.

DOI： 10.1093/ietele/e90-c.5.1124

Publishing type：Research paper (scientific journal)  

DOI： 10.1109/LPT.2007.895443

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEICE-INST ELECTRONICS INFORMATION COMMUNICATIONS ENG  

In this paper we modeled and analyzed the ridge type InGaAlAs/InP semiconductor laser with lateral current confinement structure, and optimized the design for the ridge wave guide with the current confinement. We proposed and fabricated the ridge type InGaAlAs/InP laser with a cost effective selective undercut etching method and demonstrated the improvement of the ridge laser performance. This paper provides a solution to solve the cost/yield issue for conventional BH (buried hetero-structure) type laser and performance issue for conventional ridge type laser.

DOI： 10.1093/ietele/e90-c.5.1105

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEICE-INST ELECTRONICS INFORMATION COMMUNICATIONS ENG  

DOI： 10.1093/ietele/e90-c.5.1035

Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN SOC APPLIED PHYSICS  

We report the first successful realization of an AIN-waveguide-based intersubband transition device prepared using both metalorganic vapor phase epitaxy (MOVPE) and molecular beam epitaxy (MBE) techniques to grow A1N-cladding GaN/AIN quantum wells. An intersubband absorption wavelength of as short as 1.3 mu m is confirmed with a new measurement method using an ultrawide-spectrum light source. With this method, the intersubband absorption spectra of an AIN high-mesa waveguide can be directly observed for the first time. The intersubband absorption saturation measurements also show a good device characteristic with a saturation of 7 dB for a transverse magnetic (TM) polarized input pulse energy of 200 pJ.

DOI： 10.1143/JJAP.46.L352