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

DOI： 10.1021/acsnano.2c11753

Authorship：Last author   Publishing type：Research paper (international conference proceedings)   Publisher：SPIE  

Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Physical Society  

DOI： https://doi.org/10.1103/PhysRevApplied.18.054041

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

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

Authorship：Last author   Publishing type：Research paper (scientific journal)   Publisher：OPTICA PUBLISHING GROUOP  

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

Optical trapping of dielectric and metal particles yields different types of "optically evolving assembly" at air/solution and glass/solution interfaces. However, all these structures have in common that the trapping laser is scattered and propagated through the assembly, expanding from the focus up to a few tens of micrometers. In the present work, we fabricate a single submillimeter linear assembly of polystyrene microparticles starting from the surface of a concentrated lysozyme D2O solution. Such assembly has a three-dimensional linear structure composed of a single microparticle aggregate without folding and bending. Indeed, it is prepared along the lysozyme assembly, which is also generated by optical trapping. The cooperative trapping of the microparticle and lysozyme did not arrange as a homogeneously distributed assembly. Instead, a unique anomalously long assembly of microparticles and a densely, widely, and deeply expanded lysozyme layer were simultaneously prepared. Their morphology was reconstructed by shifting the imaging plane immediately after switching off the trapping laser. Independently, the lysozyme assembly was also confirmed by fluorescence imaging and Raman scattering spectroscopy. Thus, we consider that the described cooperative "optically evolved assembling" has a large potential to fabricate hybrid materials with applications in different fields such as colloid science, protein chemistry, and soft matter.

DOI： 10.1021/acs.jpcc.1c05796

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

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

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

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

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

DOI： 10.1002/tcr.202100005

Other Link： https://onlinelibrary.wiley.com/doi/full-xml/10.1002/tcr.202100005

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

DOI： 10.1021/acs.jpcc.0c07735

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

We demonstrated the optical trapping-induced formation of a single large disc-like assembly (∼50 μm in diameter) of polystyrene (PS) nanoparticles (NPs) (100 nm in diameter) at a solution surface. Different from the conventional trapping behavior in solution, the assembly grows from the focus to the outside along the surface and contains needle structures expanding radially in all directions. Upon switching off the trapping laser, the assembly disperses and needle structures disappear, while the highly concentrated domain of the NPs is left for a while. The single assembly is quickly restored by switching on the laser again, where the needle structures are also reproduced but in a different way. When a single 10 μm PS microparticle (MP) is trapped in the NP solution, a single disc-like assembly containing needle structures is similarly prepared outside the MP. Based on backscattering imaging and tracking analyses of the MP at the solution surface, it is proposed that scattering and propagation of the trapping laser from the central part of the NP assembly or the MP lead to this new phenomenon.

DOI： 10.1021/acs.langmuir.0c02349

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

Laser trapping at an interface is a unique platform for aligning and assembling nanomaterials outside the focal spot. In our previous studies, Au nanoparticles form a dynamically evolved assembly outside the focus, leading to the formation of an antenna-like structure with their fluctuating swarms. Herein, we unravel the role of surface plasmon resonance on the swarming phenomena by tuning the trapping laser wavelength concerning the dipole mode for Au nanoparticles of different sizes. We clearly show that the swarm is formed when the laser wavelength is near to the resonance peak of the dipole mode together with an increase in the swarming area. The interpretation is well supported by the scattering spectra and the spatial light scattering profiles from single nanoparticle simulations. These findings indicate that whether the first trapped particle is resonant with trapping laser or not essentially determines the evolution of the swarming.

DOI： 10.1364/OE.401158

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

Multifocal plane microscopy allows for capturing images at different focal planes simultaneously. Using a proprietary prism which splits the emitted light into paths of different lengths, images at 8 different focal depths were obtained, covering a volume of 50x50x4 mu m(3). The position of single emitters was retrieved using a phasor-based approach across the different imaging planes, with better than 10 nm precision in the axial direction. We validated the accuracy of this approach by tracking fluorescent beads in 3D to calculate water viscosity. The fast acquisition rate (>100 fps) also enabled us to follow the capturing of 0.2 mu m fluorescent beads into an optical trap. (C) 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

DOI： 10.1364/OE.401557

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

In colloidal solution, nanoparticles can be optically trapped by a tightly focused laser beam, and they are assembled in a focal spot whose diameter is typically about one micrometer. We herein report that a large submillimeter sized assembly of polystyrene microparticles with necklace-like patterns are prepared by laser trapping at a solution surface. The light propagation outside the focal spot is directly confirmed by 1064 nm backscattering images, and finite difference time domain simulation well supports the idea that an optical potential is expanded outside the focal spot based on light propagation through whispering gallery mode. This demonstration opens a new method for fabrication of a millimeter-order huge assembly by a single tightly focused laser beam.

DOI： 10.1021/acs.jpclett.0c01602

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

Optical trapping of gold nanoparticles (Au NPs) at a glass/solution interface initially generates a periodically aligned structure of a few NPs along the direction perpendicular to a linearly polarized laser. When the number of NPs was increased, this alignment was expanded to the outside of the irradiated focus, forming a single large assembly where the Au NPs dynamically fluctuated like a swarming of bees. The morphology was dumbbell-shape, consisting of two swarms at both sides of the focus, and its size reached about 10 mu m. This optically evolved assembling and swarming was studied in poly(N-isopropylacrylamide) (PNIPAM) solution, where liquid-liquid phase separation (LLPS) was induced by photothermal heating of the trapped Au NPs forming a microdroplet of highly concentrated PNIPAM. Dynamic coupling of the NPs assembling and swarming with the droplet formation of PNIPAM leads to cooperative optical evolution, through which the assembly was embedded in the single microdroplet.

DOI： 10.1021/acs.jpcc.0c02777

Publishing type：Research paper (international conference proceedings)  

© 2020 SPIE. Polystyrene and silica nanoparticles are gathered and form a single disk-like assembly at glass/solution interface upon optical trapping with a focused laser and its size evolves much larger than the focal volume. In addition, linearly aligned aggregates of nanoparticles are prepared at specific edge sites of the assembly, which looks like horns. Transmission spectral and diffraction pattern measurements were carried out, confirming a correlation between the central arrangement of the nanoparticles and the horn formation. This dynamics and mechanism characteristic of optical trapping at interface is discussed from the viewpoint of optical propagation of the trapping laser. The assembly formation started at the focus where trapping laser light is scattered, and the trapping laser propagated trough the prepared assembly expanding its size.

DOI： 10.1117/12.2573527

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

Femtosecond (fs) laser trapping of dielectric nanoparticles displays a unique trapping and directional ejection phenomenon, which is never observed in continuous-wave (cw) laser trapping. We studied the fs laser trapping and considered its dynamics and mechanism in terms of gathering, association, and ejection of nanoparticles. By tuning solution viscosity through adding ethylene glycol, the trapping behavior by fs and cw lasers are examined and compared with each other. The viscous solution slows down the diffusional and rotational movement of nanoparticles and increases the chance of nanoparticle assembly in the trapping site, which eventually leads to the novel ejection. We took advantage of fluorescence measurement to confirm the formation of the nanoparticle assembly during fs laser trapping. A red emission irregularly appeared in the fluorescence supports the transient assembly formation in the molecular level.

DOI： 10.1021/acs.jpcc.9b04471

Publishing type：Research paper (international conference proceedings)  

© Optics & Photonics International Congress 2019. Assembling and ejection dynamics of polystyrene microparticles of 1 micrometer is revealed by applying CW laser trapping at solution surface. Initially a concentric circle-like assembly is rapidly prepared, giving a size much larger than the focus. Assembling evolves, showing some fluctuation in its size, which is accompanied with ejecting of some microparticles. The concentric-like structure is transformed into a hexagonal packing structure, then the ejection is much suppressed. Microparticles are ejected in a linearly aligned manner and its speed is slowed down with time and the distance. Assembling and ejection dynamics is considered in terms of light scattering of the trapping laser.

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

Laser trapping has been utilized as tweezers to three-dimensionally trap nanoscale objects and has provided significant impacts in nanoscience and nanotechnology. The objects are immobilized at the position where the tightly focused laser beam is irradiated. Here, we report the swarming of gold nanoparticles in which component nanoparticles dynamically interact with each other, keeping their long interparticle distance around the trapping laser focus at a glass/solution interface. A pair of swarms are directionally extended outside the focal spot perpendicular to the linear polarization like a radiation pattern of dipole scattering, while a doughnut-shaped swarm is prepared by circularly polarized trapping laser. The light field is expanded as scattered light through trapped nanoparticles; this modified light field further traps the nanoparticles, and scattering and trapping cooperatively develop. Due to these nonlinear dynamic processes, the dynamically fluctuating swarms are evolved up to tens of micrometers. This finding will open the way to create various swarms of nanoscale objects that interact and bind through the scattered light depending on the properties of the laser beam and the nanomaterials.

DOI： 10.1021/acs.nanolett.8b02519

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

We demonstrate resonance optical trapping of individual dye-doped polystyrene particles with blue- and red-detuned lasers whose energy are higher and lower compared to electronic transition of the dye molecules, respectively. Through the measurement on how long individual particles are trapped at the focus, we here show that immobilization time of dye-doped particles becomes longer than that of bare ones. We directly confirm that the immobilization time of dye-doped particles trapped by the blue-detuned laser becomes longer than that by the red-detuned one. These findings are well interpreted by our previous theoretical proposal based on nonlinear optical response under intense laser field. It is discussed that the present result is an important step toward efficient and selective manipulation of molecules, quantum dots, nanoparticles, and various nanomaterials based on their quantum mechanical properties.

DOI： 10.1364/OE.25.004655

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

We investigate the optical manipulation of nanoparticles with the resonant nonlinear optical response. Efficient trapping of nanoparticles observed in experiments under the resonance condition is elucidated by considering optical nonlinearity. Also, we propose the flexible optical manipulations of nanoparticles that have gain by optical pumping. The pulling force and the rotational switching are demonstrated, where the stimulated emission from nanoparticles with inverted population is considered. These results show that utilizing nonlinear optical effect will greatly enhance the degrees of freedom to manipulate nanoparticles.

DOI： 10.1117/12.2276757

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

Assembling dynamics of polystyrene nanoparticles by optical trapping is studied with utilizing transmission/reflection microscopy and reflection microspectroscopy. A single nanoparticle assembly with periodic structure is formed upon the focused laser irradiation at solution surface layer and continuously grows up to a steady state within few minutes. By controlling nanoparticle and salt concentrations in the colloidal solution, the assembling behavior is obviously changed. In the high concentration of nanoparticles, the assembly formation exhibits fast growth, gives large saturation size, and leads to dense packing structure. In the presence of salt, one assembly with the elongated aggregates was generated from the focal spot and 1064 nm trapping light was scattered outwardly with directions, while a small circular assembly and symmetrical expansion of the 1064 nm light were found without salt. The present nanoparticle assembling in optical trapping is driven through multiple scattering in gathered nanoparticles and directional scattering along the elongated aggregates derived from optical association of nanoparticles, which dynamic phenomenon is called optically evolved assembling. Repetitive trapping and release processes of nanoparticles between the assembly and the surrounding solution always proceed, and the steady state at the circular assembly formed by laser trapping is determined under optical and chemical equilibrium.

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

We report optical trapping and assembling of colloidal particles at a glass/solution interface with a tightly focused laser beam of high intensity. It is generally believed that the particles are gathered only in an irradiated area where optical force is exerted on the particles by laser beam. Here we demonstrate that, the propagation of trapping laser from the focus to the outside of the formed assembly leads to expansion of the assembly much larger than the irradiated area with sticking out rows of linearly aligned particles like horns. The shape of the assembly, its structure, and the number of horns can be controlled by laser polarization. Optical trapping study utilizing the light propagation will open a new avenue for assembling and crystallizing quantum dots, metal nanoparticles, molecular clusters, proteins, and DNA.

DOI： 10.1021/acs.nanolett.6b00123

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Physical Society of Japan (JPS)  

DOI： 10.11316/jpsgaiyo.71.1.0_1631

Publishing type：Research paper (international conference proceedings)  

© 2016 SPIE. We conduct the optical trapping and assembling of polystyrene particles at the glass/solution interface by utilizing tightly focused 1064 nm laser of high power. Previously we reported that this leads to form the assembly sticking out horns consisting of single row of aligned particles through light propagation. Here, we demonstrate the laser power dependence of this phenomenon. With increasing the laser power, the particles are started to distribute around the focal spot and form the assembly larger than focal spot. The shape of the assembly becomes ellipse-like and the color at the central part of the assembly in transmission images is changed. This indicates that the assembly structure is changed, and trapping laser is started to propagate through the adjoining particles leading to horn formation. Strong laser power is necessary to elongate the horns and to align them straightly. We expect that this study will offer a novel experimental approach for assembling and crystallization of nanoparticles and molecules exclusively by optical trapping.

DOI： 10.1117/12.2235272

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

Optical manipulation is a technique to control the mechanical motion of small objects by using electromagnetic radiation force. Optical tweezers are the most popular tool to trap and move microparticles suspended in a medium. Recent interest has been shifting to manipulating nano-objects considerably smaller than the wavelength of light. Since the radiation force exerted on nano-objects is extremely small, an innovative method is necessary to make this concept feasible. Utilizing the resonant optical response of the objects to electronic transitions is one of the promising ways to approach nanoscale optical manipulation, and several advances in this direction have been made recently. Despite experimental studies on resonance optical tweezers showing favorable results, conventional theories have been unable to explain the results though demonstrations of resonant manipulations for traveling and standing waves have shown favorable results. In the present article, we provide a perspective view of resonance optical manipulation based on nonlinear optical response that we have recently proposed. This idea coherently elucidates recently reported puzzling phenomena appearing in studies concerning resonance optical tweezers that contradict the conventional understanding of resonance optical trapping. Further, this concept opens up the possibility to develop potentially powerful manipulation techniques because the nonlinear optical response involves processes with considerably greater degrees of freedom than those of the linear optical response. As examples, we propose a method for trapping single organic molecules that is more effective than ever before, selectively pulling the molecules with a particular transition energy, and our proposed method allows for high-spatial-resolution optical manipulation beyond the diffraction limit.

DOI： 10.1039/c3cp51969d

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

We theoretically propose two-color laser manipulation with greatly improved efficiency to mechanically manipulate single organic molecules. The present method is based on the nonlinear resonant laser manipulation proposed in a recent study. In the first part, we describe a method to trap single organic molecules that can be more effective than ever before utilizing two-color beams. In the second part, we demonstrate the possibility to selectively "pull" single organic molecules with a particular type of electronic-level scheme by using single-side illumination of traveling light.

DOI： 10.1140/epjb/e2013-30620-8

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

We propose nonlinear resonant laser manipulation, a technique that drastically enhances the number of degrees of freedom when manipulating nano-objects. Considering the high laser intensity required to trap single molecules, we calculate the radiation force exerted on a molecule in a focused laser beam by solving the density matrix equations using the nonperturbative method. The results coherently elucidate certain recently reported puzzling phenomena that contradict the conventional understanding of laser trapping. Further, we demonstrate unconventional forms of laser manipulations using "stimulated recoil force" and "subwavelength laser manipulation."

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

We theoretically investigate how the resonant radiation force (RF) affects the laser trapping of a single molecule by comparing the RF exerted on the dye-doped molecule and that on the non-doped one. In this investigtion, we evaluate the inter-particle radiation force (IRF) in the focal spot of the incident focused beam. The result indicates the possibility of the increase of molecular density through the IRF even in the linear response regime. We also investigate the nonlinear optical effect that greatly changes the characteristics of the RF. This result shows that the nonlinear optical effect cannot be neglected if the incident laser intensity is strong enough to trap a single molecule, and that the nonlinearity makes the resonant trapping more advantageous. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

DOI： 10.1002/pssc.201000680

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

We theoretically propose laser manipulation utilizing the resonant nonlinear optical response. We calculated the radiation force exerted on a single molecule in a focused laser beam by solving density matrix equations using the non-perturbative method, because the high laser power is necessary for single molecular trapping. As the result, we coherently elecidate certain recently reported puzzling phenomena that contradict the conventional understanding of laser manipulation. Further, we demonstrate unconventional forms of laser manipulation which drastically enhances the number of degrees of freedom to manipulate nano-sized objects. For example, we can "pull" objects by using traveling wave that usually "pushes" them along the direction of traveling wave.

DOI： 10.1117/12.892452

Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

Presentation type：Oral presentation (general)  

Presentation type：Oral presentation (general)  

Presentation type：Oral presentation (general)  

Presentation type：Oral presentation (general)  

Presentation type：Oral presentation (general)  

Presentation type：Oral presentation (general)  

Presentation type：Oral presentation (invited, special)