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Last Updated :2024/09/07

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Master

Affiliation (Master)

  • Research Institute for Electronic Science　Photonics and Optical Science

Affiliation (Master)

  • Research Institute for Electronic Science　Photonics and Optical Science

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Profile and Settings

Affiliation

  • Hokkaido University, Research Institute for Electronic Science

Profile and Settings

  • Name (Japanese)

    Tanaka

  • Name (Kana)

    Yoshito

  • Name

    ABC-4663-2021, 201201000527362613

Affiliation

  • Hokkaido University, Research Institute for Electronic Science

Achievement

Research Interests

  • メタマテリアル   プラズモニクス   ナノ構造光制御   光駆動ナノモーター   プラズモントラッピング   光圧   金属ナノ構造   キラルナノ構造   

Research Areas

  • Nanotechnology/Materials / Nano/micro-systems
  • Nanotechnology/Materials / Optical engineering and photonics
  • Nanotechnology/Materials / Nanostructure physics
  • Nanotechnology/Materials / Nanostructure chemistry
  • Nanotechnology/Materials / Nanomaterials

Research Experience

  • 2023/04 - Today Hokkaido University Research Institute for Electronic Science
  • 2015/03 - 2023/03 The University of Tokyo Institute of Industrial Science, (Department of Fundamental Engineering)
  • 2015/12 - 2019/03 科学技術振興機構 戦略的創造研究推進事業 「さきがけ」研究員（兼任）
  • 2014/07 - 2015/03 Kwansei Gakuin University School of Science and Technology, Department of Chemistry
  • 2013/03 - 2014/06 Imperial College London
  • 2012/04 - 2014/06 Hokkaido University Research Institute for Electronic Science
  • 2008/10 - 2012/03 Hokkaido University Research Institute for Electronic Science
  • 2006/04 - 2008/09 Osaka University

Awards

  • 2021/10 日本光学会 第62回日本光学会光学論文賞
     ナノ構造光圧エンジニアリングによる光駆動ナノマシン
  • 2020 レーザー学会 第44回レーザー学会奨励賞
     プラズモニック光波制御デバイスとナノモーター応用 
    受賞者: 田中嘉人
  • 2018 文部科学大臣 若手科学者賞
     局在プラズモン制御による超解像光マニピュレーションの研究 
    受賞者: 田中嘉人
  • 2015 東京大学 生産技術研究所 生研弥生賞 最優秀
     デザインされたプラズモニックナノモーターによる運動駆動・制御 
    受賞者: 田中嘉人
  • 2010 北海道大学 電子科学研究所 第39回松本・羽鳥奨励賞
     局在プラズモン場の高空間分解イメージング法の開発 
    受賞者: 田中嘉人

Published Papers

  • Hao-Tse Su, Shao-Yuan Liu, Minoru Fujii, Hiroshi Sugimoto, Yoshito Y. Tanaka, Teruki Sugiyama
    Photochemical & Photobiological Sciences 1474-905X 2024/08/30 [Refereed][Not invited]
     
    Abstract This study investigates the promotion of sodium chlorate (NaClO3) crystallization through optical trapping, enhanced by the addition of gold nanoparticles (AuNPs) and silicon nanoparticles (SiNPs). Using a focused laser beam at the air–solution interface of a saturated NaClO3 solution with AuNPs or SiNPs, the aggregates of these particles were formed at the laser focus, the nucleation and growth of metastable NaClO3 (m-NaClO3) crystals were induced. Continued laser irradiation caused these m-NaClO3 crystals to undergo repeated cycles of growth and dissolution, eventually transitioning to a stable crystal form. Our comparative analysis showed that AuNPs, due to their significant heating due to higher photon absorption efficiency, caused more pronounced size fluctuations in m-NaClO3 crystals compared to the stable behavior observed with SiNPs. Interestingly, the maximum diameter of the m-NaClO3 crystals that appeared during the size fluctuation step was consistent, regardless of nanoparticle type, concentration, or size. The crystallization process was also promoted by using polystyrene nanoparticles, which have minimal heating and electric field enhancement, suggesting that the reduction in activation energy for nucleation at the particle surface is a key factor. These findings provide critical insights into the mechanisms of laser-induced crystallization, emphasizing the roles of plasmonic heating, particle surfaces, and optical forces. Graphical abstract
  • Shun Hashiyada, Yoshito Y. Tanaka
    Review of Scientific Instruments 95 (5) 0034-6748 2024/05/01 [Refereed][Not invited]
     
    Helical dichroism (HD), which is defined as the difference in optical absorption between chiral pairs of lights involving left-handed (LH) and right-handed (RH) optical vortices (OVs) carrying orbital angular momentum (OAM), is a promising way to characterize chiral materials. In the current major methods of OV generation using spatial light modulators (SLMs), the speed of OAM switching is typically as slow as 100 Hz, which is comparable to low-frequency noise, making precise chiral detection difficult. Here, we theoretically propose and experimentally demonstrate a rapid modulation of the LH and RH OVs at around 50 kHz. This modulation is achieved through a rapid modulation of circularly polarized lights carrying spin angular momentum (SAM), combined with a SAM–OAM conversion technique. We establish a theory not only for rapid OV modulation but also for HD measurements using the modulated OVs. We experimentally verify the theory using helical phase holograms drawn on a SLM as a pseudo-HD active sample. Our work addresses the limitations of current methods and offers a new avenue for precise HD measurements, paving the way for the development of sensitive chiral-optical spectroscopy techniques.
  • Tomoya Kimura, Yoshito Y. Tanaka, Tsutomu Shimura
    ACS PHOTONICS 2330-4022 2022/10 
    Controlling the process of second-harmonic (SH) generation at the nanoscale is important in photonic applications but remains challenging in nanophotonics. Herein, based on theoretical and experimental studies, we found that a cross-shaped single-element plasmonic nanoantenna resonating with bright and dark modes at the fundamental (excitation) and SH wavelengths, respectively, generates radially polarized ring-shaped SH radiations with axial symmetry by exciting the bright dipole mode with linearly polarized light. Mode expansion analysis of the generated SH radiation revealed that the radial polarization arises from a dark breathing mode in which coherent dipoles are radially distributed along the arms of the cross-shaped antenna. This study reveals that surface SH polarization generated by the electric near-field of the bright mode at the fundamental wavelength efficiently is coupled to the dark modes rather than the bright mode because its spatial distribution has the same symmetry as the dark modes. By engineering bright and dark modes at the fundamental and SH wavelengths, respectively, novel plasmonic nanodevices that simultaneously perform polarization control and efficient wavelength conversion are expected.
  • Jianying Hao, Xiao Lin, Soki Hirayama, Ryushi Fujimura, Yoshito Tanaka, Xiaodi Tan, Tsutomu Shimura
    Optics InfoBase Conference Papers 2022 
    In this paper, we proposed a simple complex amplitude modulated holographic data storage（HDS）system and retrieved both the amplitude and phase data by deep learning from a single-shot intensity image. The complex amplitude of signal beam is modulated by a single spatial light modulator (SLM) based on double phase hologram method, and one object intensity image is captured by the detector. Two convolutional neural networks (CNN) are used to build the relationships between the captured intensity and complex amplitude data pages, respectively. After training the neural networks, both the amplitude and phase information can be directly decoded from one intensity image.
  • Yoshito Tanaka
    Nanophotonics 10 (18) 4601 - 4609 2192-8606 2021/12/02 [Refereed]
     
    Abstract Shaping the emission pattern of second harmonic (SH) generation from plasmonic nanoparticles is important for practical applications in nonlinear nanophotonics but is rendered challenging by the complex second-order nonlinear-optical processes. Here, we theoretically and experimentally demonstrate that a pair of V- and Y-shaped gold nanoparticles directs the SH emission perpendicularly to an incident light direction. Owing to spatial overlap of two orthogonal plasmonic dipole modes at the fundamental and SH wavelengths of the individual particles, surface SH polarizations induced by the fundamental field is efficiently near-field coupled to the SH plasmon mode, resulting in dipolar SH emission from the individual particles. Moreover, the phase of this emission can be tuned simply by altering the part of the Y-particle shape, which changes the SH plasmon resonance while keeping the fundamental resonance. Our approach is a promising platform for engineering not only directional nonlinear nanoantennas but also nonlinear metamaterials.
  • An'an Wu, Yoshito Y. Tanaka, Tsutomu Shimura
    APL Photonics 6 (12) 126104 - 126104 2378-0967 2021/12/01 [Refereed]
  • Yoshito Y. Tanaka, Pablo Albella, Mohsen Rahmani, Vincenzo Giannini, Stefan A. Maier, Tsutomu Shimura
    SCIENCE ADVANCES 6 (45) 2375-2548 2020/11 [Refereed]
     
    Optical force is a powerful tool to actuate micromachines. Conventional approaches often require focusing and steering an incident laser beam, resulting in a bottleneck for the integration of the optically actuated machines. Here, we propose a linear nanomotor based on a plasmonic particle that generates, even when illuminated with a plane wave, a lateral optical force due to its directional side scattering. This force direction is determined by the orientation of the nanoparticle rather than a field gradient or propagation direction of the incident light. We demonstrate the arrangements of the particles allow controlling the lateral force distributions with the resolution beyond the diffraction limit, which can produce movements, as designed, of microobjects in which they are embedded without shaping and steering the laser beam. Our nanomotor to engineer the experienced force can open the door to a new class of micro/nanomechanical devices that can be entirely operated by light.
  • Shohei Uemura, Sanpon Vantasin, Yasutaka Kitahama, Yoshito Yannick Tanaka, Toshiaki Suzuki, Daichi Doujima, Tadaaki Kaneko, Yukihiro Ozaki
    APPLIED SPECTROSCOPY 74 (11) 1384 - 1390 0003-7028 2020/11 [Refereed]
     
    Interactions between epitaxial graphene grown on Si- and C-faces were investigated using Raman imaging and tip-enhanced Raman scattering (TERS). In the TERS spectrum, which has a spatial resolution exceeding the diffraction limit, a D band was observed not from graphene surface, but from the edges of the epitaxial graphene ribbons without a buffer layer, which interacts with SiC on the Si-face. In contrast, for a graphene micro-island on the C-face, the D band disappeared even on the edges where the C atoms were arranged in armchair configurations. The disappearance of the edge chirality via combination between the C atoms and SiC on the C-face is responsible for this phenomenon. The TERS signals from the C-face were weaker than those from the Si-face without the buffer layer. On the Si-face with a buffer layer, the graphene TERS signal was hardly observed. TERS enhancement was suppressed by interactions on the edges or by the buffer layer between the SiC and graphene on the C- or Si-face, respectively.
  • An'an Wu, Yoshito Y. Tanaka, Ryoma Fukuhara, Tsutomu Shimura
    PHYSICAL REVIEW A 102 (2) 2469-9926 2020/08 [Refereed]
     
    Spin and orbital angular momenta are separately observable properties of light. Both of them can interact with matter, causing an exchange between each other and generating an optical torque on the matter. However, thus far, we have been unable to separately discuss the transfer of the spin and orbital angular momenta or the origin of the optical torques with regard to them. Here, we consider the optical chirality density and propose an approach to analyzing the transfer of spin and orbital angular momenta separately by considering this property. Our approach is based on a continuity equation for spin angular momentum density. This equation is derived from an extended proportional relation between the chirality flux density and spin angular momentum density in sources-added space. Our findings will greatly facilitate the study of angular momentum interaction between light and matter.
  • Ichiro Tanabe, Yoshito Y. Tanaka, Koji Watari, Wataru Inami, Yoshimasa Kawata, Yukihiro Ozaki
    SCIENTIFIC REPORTS 10 (1) 2045-2322 2020/06 [Refereed]
     
    In this study, surface plasmon resonance (SPR) wavelength shifts due to molecular electronic absorptions in the far-ultraviolet (FUV,<200nm) and deep-ultraviolet (DUV,<300nm) regions were investigated by attenuated total reflectance (ATR) spectroscopy. Due to the strong absorption in the DUV region, N,N-dimethylformamide (DMF) significantly increased the SPR wavelength shift of Al film. On the other hand, no such shift enhancement was observed in the visible region for Au film because DMF does not have absorbance compared to non-absorbing materials such as water and alcohols. The enhanced SPR wavelength shift, caused by the overlap between SPR and molecular resonance wavelengths in FUV-DUV region, is expected to result in high sensitivity for resonant materials.
  • Jinpeng Liu, Le Zhang, Anan Wu, Yoshito Tanaka, Masanobu Shigaki, Tsutomu Shimura, Xiao Lin, Xiaodi Tan
    OPTICS EXPRESS 28 (5) 7139 - 7151 1094-4087 2020/03 [Refereed]
     
    In holographic data storage systems, the quality of the reconstructed data pattern is decisive and directly affects the system performance. However, noise from the optical component, electronic component and recording material deteriorates reconstruction quality. A high noise margin decoding method developed from compressed sensing technology was proposed to reduce the impact of noise in the decoding process. Compared with the conventional threshold decoding method, the proposed method is more robust to noise and more suitable for multilevel modulation. The decoding performance with five-level amplitude modulation was evaluated by both simulation and experimentation. For the combination of Gaussian noise, Rician noise and Rayleigh noise, the proposed decoding method reduces the BER of the threshold method to one-sixth with an SNR of -1 in the simulation. In the experiment, it behaves up to 8.3 times better than conventional threshold decoding. (C) 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
  • An'an Wu, Yoshito Y. Tanaka, Tsutomu Shimura
    OPTICS EXPRESS 28 (2) 2398 - 2410 1094-4087 2020/01 [Refereed]
     
    We present a numerical study of optical torque between two twisted metal nanorods due to the angular momentum of the electromagnetic field emerging from their plasmonic coupling. Our results indicate that the interaction optical torque on the nanorods can be strongly enhanced by their plasmon coupling, which highly depends on not only the gap size but also the twisted angle between the nanorods. The behaviors of the optical torque are different between two plasmon coupling modes: hybridized bonding and anti-bonding modes with different resonances. The rotations of the twisted nanorods with the bonding and anti-bonding mode excitations lead to mutually parallel and perpendicular alignments, respectively. At an incident intensity of 10 mW/mu m(2), the rotational potential depths are more than 30 times as large as the Brownian motion energy, enabling the optical alignments with angle fluctuations less than similar to +/- 10 degrees. Thus, this optical alignment of the nanoparticles with the plasmon coupling allows dynamic control of the plasmonic characteristics and functions. (C) 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
  • Ryoma Fukuhara, Yoshito Y. Tanaka, Tsutomu Shimura
    PHYSICAL REVIEW A 100 (2) 2469-9926 2019/08 [Refereed]
     
    We report that localized surface plasmon resonance allows a single-element nanostructure to induce an extrinsic angular momentum of light in its interaction with a propagating plane wave. The recoil of the angular momentum results in an optical torque on the structure along an axis perpendicular to the optical axis, and the characteristics of this transverse torque depend on the incident polarization state, including the spin direction. Our results suggest that the designed dark plasmon mode can provide a new degree of freedom for optical manipulation of nanoparticles smaller than the diffraction limit.
  • Ichiro Tanabe, Yoshito Y. Tanaka
    CHEMICAL RECORD 19 (7) 1210 - 1219 1527-8999 2019/07 [Refereed]
     
    Plasmonics in the UV region has been widely focused because of the higher energy and the abundant electronic resonances compared to the conventional visible plasmonics. Recently, we have investigated the surface plasmon resonance (SPR) properties of the Al film, aiming for the application as refractive index sensors. Utilizing the UV lights, we expect three advantages: high sensitivity, material selectivity, and surface selectivity. By using an original attenuated total reflectance spectroscopic instrument, Al-SPR angle and wavelength were investigated with changing environments on the Al film. Al film thickness and materials of prisms on which Al was evaporated were also important factors for the SPR properties. By optimizing the conditions, the Al film worked as a sensor both in air and in liquids. In addition, our established system expands the plasmonics into an even higher energy region than 200 nm, while the UV-plasmonics have been studied in the wavelength region longer than 200 nm.
  • Soki Hirayama, Ryushi Fujimura, Shinsuke Umegaki, Yoshito Y. Tanaka, Tsutomu Shimura
    PHOTONICS 6 (2) 2019/06 [Refereed]
     
    Holographic memory is currently attracting attention as a data storage system capable of achieving a data transfer rate of about 10(5)similar to 10(6) times that of an optical disc such as Blu-ray disc. In conventional holographic memory, data is generally recorded by optical writing using volume holograms. However, a volume hologram has the problem not only that it is required to have high mechanical accuracy of a system and low coefficient of thermal expansion of a recording medium, because reconstruction tolerance is extremely low, but also that duplicating time efficiency is poor because whole data cannot be recorded at once. In this paper we proposed surface holographic memory that achieved a high data transfer rate, stable readout performance, and collective duplication by expressing holograms with fine surface asperity. Furthermore, the theoretical formulas of recording and reconstruction processes in the proposed system were derived and the reconstruction characteristics of the hologram were evaluated by numerical simulation. As a result, the proposed method generated reconstructed image readout with sufficient signal for a single page recording. However, the reconstructed image had noise, which was particular to a surface holographic memory.
  • Itsuo Hanasaki, Takahiro Nemoto, Yoshito Y. Tanaka
    PHYSICAL REVIEW E 99 (2) 2470-0045 2019/02 [Refereed]
     
    It is often regarded that the dwell time (or residence time, escape time, trapping duration) of trapped Brownian particles is described by the multiplication of two separate factors, i.e., the diffusive traveling time of the trapping domain size without taking into account the trapping force, and the stochastic event of overcoming the trapping energy by thermal one instantaneously. However, we show that the ratio of dwell time to the typical traveling time for the trapping domain size depends on the shape of the force field. The shape of the trapping potential affects this ratio even if the trapping energy gap is the same and the smooth potential has a single minimum. Our finding suggests the possible application of the potential shape to realize the desired trapping characteristics.
  • Kazutaka Akiyoshi, Yoshito Y. Tanaka, Takuya Ishida, Tsutomu Shimura, Tetsu Tatsuma
    ACS APPLIED NANO MATERIALS 1 (11) 5994 - 5999 2574-0970 2018/11 [Refereed]
     
    In order to improve the performance of localized surface plasmon resonance (LSPR) sensors, lattice plasmons of plasmonic nanoparticle (NP) arrays are often used. The lattice plasmon sensors based on coupling between LSPR of each NP and diffraction of the NP array generally have a higher ability to detect local refractive index changes than conventional LSPR sensors based on random NP ensembles. However, the surface selectivity of the lattice plasmon sensors is lower than that of the LSPR sensors because the local electric field of the former extends far from the particle array surface. In this work, we develop a novel plasmonic diffractive hybrid sensor that can simultaneously determine both local and bulk refractive indices from two output signals, namely, extinction dip and peak, the wavelengths of which are differently dependent on those indices. The hybrid sensor can evaluate the refractive index changes in the immediate vicinity of the sensor surface by canceling the difference in the bulk refractive index between the sample and standard solutions.
  • Sanpon Vantasin, Yoshito Y. Tanaka, Tsutomu Shimura
    ACS PHOTONICS 5 (3) 1050 - 1057 2330-4022 2018/03 [Refereed]
     
    The unique properties of graphene plasmons show great potential for plasmonic nanodevice applications such as sensors and modulators. Graphene plasmon launching, propagation control, and ultimately launching with directional control are therefore crucial for the development of such devices. However, previous studies have used foreign objects or external influencing factors to attain directional plasmon launching on graphene, which introduce defects and add complexity to the system. This study introduces a theoretical framework for a graphene-only approach to direction-controlled plasmon launching. We use graphene nanoridges, a defect-free natural structure of graphene, as a plasmon launcher. Through proper arrangement of the nanoridges, unidirectional, bidirectional, and wavelength-sorted plasmon launching with normal illumination can be achieved.
  • Xiao Lin, Yong Huang, Tsutomu Shimura, Ryushi Fujimura, Yoshito Tanaka, Masao Endo, Hajimu Nishimoto, Jinpeng Liu, Yang Li, Ying Liu, Xiaodi Tan
    OPTICS EXPRESS 25 (25) 30905 - 30915 1094-4087 2017/12 [Refereed]
     
    Fast non-interferometric phase retrieval is a very important technique for phase-encoded holographic data storage and other phase based applications due to its advantage of easy implementation, simple system setup, and robust noise tolerance. Here we present an iterative non-interferometric phase retrieval for 4-level phase encoded holographic data storage based on an iterative Fourier transform algorithm and known portion of the encoded data, which increases the storage code rate to two-times that of an amplitude based method. Only a single image at the Fourier plane of the beam is captured for the iterative reconstruction. Since beam intensity at the Fourier plane of the reconstructed beam is more concentrated than the reconstructed beam itself, the requirement of diffractive efficiency of the recording media is reduced, which will improve the dynamic range of recording media significantly. The phase retrieval only requires 10 iterations to achieve a less than 5% phase data error rate, which is successfully demonstrated by recording and reconstructing a test image data experimentally. We believe our method will further advance the holographic data storage technique in the era of big data. (C) 2017 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
  • Ichiro Tanabe, Yoshito Y. Tanaka, Koji Watari, Taras Hanulia, Takeyoshi Goto, Wataru Inami, Yoshimasa Kawata, Yukihiro Ozaki
    CHEMISTRY LETTERS 46 (10) 1560 - 1563 0366-7022 2017/10 [Refereed]
     
    Aluminum film thickness dependence of surface plasmon resonance (SPR) in the far- and deep-ultraviolet regions (170300 nm) was revealed using an attenuated total reflectance spectrometer. In the far-ultraviolet region (< 200 nm), films of thicknesses 23 and 9 nm showed strong SPR excitations in air and 1,1,1,3,3,3-hexafluoro-2-propanol, which were reproduced by simulations based on the Fresnel equations, respectively. Therefore, Al-SPR sensors are expected to work both in air and in liquids by controlling the film thickness.
  • Ichiro Tanabe, Yoshito Y. Tanaka, Koji Watari, Taras Hanulia, Takeyoshi Goto, Wataru Inami, Yoshimasa Kawata, Yukihiro Ozaki
    SCIENTIFIC REPORTS 7 2045-2322 2017/07 [Refereed]
     
    Surface plasmon resonance (SPR) sensors detect refractive index changes on metal thin films and are frequently used in aqueous solutions as bio-and chemical-sensors. Recently, we proposed new SPR sensors using aluminum (Al) thin films that work in the far-and deep-ultraviolet (FUV-DUV, 120-300 nm) regions and investigated SPR properties by an attenuated total reflectance (ATR) based spectrometer. The FUV-DUV-SPR sensors are expected to have three advantages compared to visible-SPR sensors: higher sensitivity, material selectivity, and surface specificity. However, in this study, it was revealed that the Al thin film on a quartz prism cannot be used as the FUV-DUV-SPR sensor in water solutions. This is because its SPR wavelength shifts to the visible region owing to the presence of water. On the other hand, the SPR wavelength of the Al thin film on the sapphire prism remained in the DUV region even in water. In addition, the SPR wavelength shifted to longer wavelengths with increasing refractive index on the Al thin film. These results mean that the Al thin film on the sapphire prism can be used as the FUV-DUV-SPR sensor in solutions, which may lead to the development of novel and sophisticated SPR sensors.
  • Yoshito Y. Tanaka, Tsutomu Shimura
    NANO LETTERS 17 (5) 3165 - 3170 1530-6984 2017/05 [Refereed]
     
    Achieving high directionality of scattered light in combination with high flexibility of the direction using plasmonic nanoparticles is desirable for future optical nanocircuits and on-chip optical links. The plasmonic characteristics of nanoparticles strongly depend on their geometry. Here, we studied directional light scattering by a single-element triangular plasmonic nanoparticle. Our experimental and simulation results demonstrated that the triangular nanoparticle spatially sorted the incoming photons into three different scattering directions according to their polarization direction, including circular polarization, despite its compact overall volume of similar to lambda(3)/300. The broken mirror symmetry and rotational symmetry of the triangular nanoparticle enabled such passive tridirectional polarization routing through the constructive and destructive interference of different plasmon modes. Our findings should markedly broaden the versatility of triangular plasmonic nanodevices, extending their possible practical applications in photon couplers and sorters and chemo-/biosensors.
  • Yasutaka Kitahama, Takuji Nagahiro, Yoshito Tanaka, Tamitake Itoh, Yukihiro Ozaki
    JOURNAL OF RAMAN SPECTROSCOPY 48 (4) 570 - 577 0377-0486 2017/04 [Refereed]
     
    Silver colloidal nanoaggregates have been widely used for surface-enhanced Raman scattering (SERS) measurements. SERS-active nanoaggregates generally show monotone colors, although multicoloured-blinking emissions, which occur at the single-molecule level, have been observed from each Ag colloidal nanoaggregate with poly-L-lysine. For the same single nanoaggregates, SERS peaks of poly-L-lysine were observed at shorter wavelengths than those of the background emission band; thus, their blinking can be measured separately. The background emission is attributed to fluorescence from an Ag atomic cluster. This may be modulated by the surface plasmon enhancement of each Ag nanoaggregate and is the likely cause of multicoloured background emission. Notably, the probability distributions as a function of time for not only the dark events of SERS but also those of the background emission were reproduced via a truncated power law, indicating the same random-walk mechanism for their blinking despite the different illuminants. The power law for the dark background emission events was truncated at a shorter tail than that for the dark SERS events. This result suggests that the fluorescent Ag cluster reaches a junction of the Ag nanoaggregate covered with citrate anions more quickly than the cationic polymer chain. Copyright (c) 2017 John Wiley & Sons, Ltd.
  • Ichiro Tanabe, Yoshito Y. Tanaka, Takayuki Ryoki, Koji Watari, Takeyoshi Goto, Masakazu Kikawada, Wataru Inami, Yoshimawa Kawata, Yukihiro Ozaki
    OPTICS EXPRESS 24 (19) 21886 - 21896 1094-4087 2016/09 [Refereed]
     
    The surface plasmon resonance (SPR) of Al thin films was investigated by varying the refractive index of the environment near the films in the far-ultraviolet (FUV, 120-200 nm) and deep-ultraviolet (DUV, 200-300 nm) regions. An original FUV-DUV spectrometer that adopts an attenuated total reflectance (ATR) system was used. The measurable wavelength range was down to the 180 nm, and the environment near the Al surface could be controlled. The resultant spectra enabled the dispersion relationship of Al-SPR in the FUV and DUV regions to be obtained. In the presence of 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) on the Al film, the angle and wavelength of the SPR became larger and longer, respectively, compared to those in air. These shifts correspond well with the results of simulations performed using Fresnel equations. (C) 2016 Optical Society of America
  • Sanpon Vantasin, Wei Ji, Yoshito Tanaka, Yasutaka Kitahama, Mengfan Wang, Kanet Wongravee, Harnchana Gatemala, Sanong Ekgasit, Yukihiro Ozaki
    ANGEWANDTE CHEMIE-INTERNATIONAL EDITION 55 (29) 8391 - 8395 1433-7851 2016/07 [Refereed]
     
    3D surface-enhanced Raman scattering (SERS) imaging with highly symmetric 3D silver microparticles as a SERS substrate was developed. Although the synthesis method is purely chemical and does not involve lithography, the synthesized nanoporous silver microparticles possess a regular hexapod shape and octahedral symmetry. By using p-aminothiophenol (PATP) as a probe molecule, the 3D enhancement patterns of the particles were shown to be very regular and predictable, resembling the particle shape and exhibiting symmetry. An application to the detection of 3D inhomogeneity in a polymer blend, which relies on the predictable enhancement pattern of the substrate, is presented. 3D SERS imaging using the substrate also provides an improvement in spatial resolution along the Z axis, which is a challenge for Raman measurement in polymers, especially layered polymeric systems.
  • Yoshito Okuno, Sanpon Vantasin, In-Sang Yang, Jangyup Son, Jongill Hong, Yoshito Yannick Tanaka, Yasushi Nakata, Yukihiro Ozaki, Nobuyuki Naka
    APPLIED PHYSICS LETTERS 108 (16) 0003-6951 2016/04 
    Nanoscale integration of graphene into a circuit requires a stable performance under high current density. However, the effects of the current density that approach the electronic breakdown limit of graphene are not well understood. We explored the effects of a high current density, close to the electronic breakdown limit of 10 A/cm (similar to 3.0 x 10(8) A/cm(2)), on graphene, using tip-enhanced Raman scattering. The results showed that the high current density induces Raman bands at 1456 and 1530 cm(-1), which were assigned to edge-phonon modes originating from zigzag and armchair edges. This led us to conclude that C-C bonds are cleaved due to the high current density, leaving edge structures behind, which were detected through the observation of localized phonons. Published by AIP Publishing.
  • Sanpon Vantasin, Shohei Uemura, Yoshito Tanaka, Daichi Doujima, Tadaaki Kaneko, Yukihiro Ozaki
    ACS Symposium Series 227 - 245 0097-6156 2016/01
  • Yukihiro Ozaki, Sanpon Vantasin, Wei Ji, Yoshito Tanaka, Yasutaka Kitahama, Kanet Wongrawee, Sanong Ekgasit
    NANOIMAGING AND NANOSPECTROSCOPY IV 9925 0277-786X 2016 
    This study presents the synthesis, SERS properties in three dimensions, and an application of 3D symmetric nanoporous silver microparticles. The particles are synthesized by purely chemical process: controlled precipitation of AgCl to acquire highly symmetric AgCl microparticle, followed by in-place to convert AgCl into nanoporous silver. The particles display highly predictable SERS enhancement pattern in three dimensions, which resembles particle shape and retains symmetry. The highly regular enhancement pattern allows an application in the study of inhomogeneity in two-layer polymer system, by improving spatial resolution in Z axis.
  • Sanpon Vantasin, Toshiaki Suzuki, Yoshito Tanaka, Yasutaka Kitahama, Shohei Uemura, Doujima Daichi, Tadaaki Kaneko, Yukihiro Ozaki
    NANOIMAGING AND NANOSPECTROSCOPY IV 9925 0277-786X 2016 
    Despite often illustrated as a perfect two-dimensional sheet, real graphene sample is not always flat. Nanostructures can be occurred on graphene sheet, especially for epitaxial graphene. The nanostructures alter the electrical and mechanical properties of graphene. This is crucial for epitaxial graphene since its main potential is in the electronics and optics application. This study investigates nanostructures on epitaxial graphene by tip-enhanced Raman spectroscopy, which is a technique that can provide Raman spectra with great spatial resolution, exceeding the diffraction limit of light. The results suggest that the compressive strain on nanoridges is weaker compared to neighbor flat area, supporting the 'ridge as compressive strain relaxation' mechanism. TERS measurement of nanoridges on epitaxial graphene microisland also indicates that the 'Si vapor trapping' mechanism for ridge formation is unlikely to occur.
  • Ichiro Tanabe, Yoshito Y. Tanaka, Takayuki Roki, Koji Watari, Takeyoshi Goto, Masakazu Kikawada, Wataru Inami, Yoshimasa Kawata, Yukihiro Ozaki
    UV AND HIGHER ENERGY PHOTONICS: FROM MATERIALS TO APPLICATIONS 9926 0277-786X 2016 
    We investigated the surface plasmon resonance (SPR) of aluminum (Al) thin films with varying refractive index of the environment near the films in the far. ultraviolet (FUV, <= 200 nm) and deep. ultraviolet (DUV, <= 300 nm) regions. By using our original FUV-DUV spectrometer which adopts an attenuated total reflectance (ATR) system, the measurable wavelength range was down to the 180 nm, and the environment near the Al surface could be controlled. In addition, this spectrometer was equipped with a variable incident angle apparatus, which enabled us to measure the FUV-DUV reflectance spectra (170-450 nm) with various incident angles ranging from 45 degrees to 85 degrees. Based on the obtained spectra, the dispersion relation of Al-SPR in the FUV and DUV regions was obtained. In the presence of various liquids (HFIP, water, alcohols etc.) on the Al film, the angle and wavelength of the SPR became larger and longer, respectively, compared with those in the air (i.e., with no materials on the film). These shifts correspond well with the results of simulations performed according to the Fresnel equations, and can be used in the application of SPR sensors. FUV-DUV-SPR sensors (in particular, FUV-SPR sensors) with tunable incident light wavelength have three experimental advantages compared with conventional visible-SPR sensors, as discussed based on the Fresnel equations, i.e., higher sensitivity, more narrowly limited surface measurement, and better material selectivity.
  • Yoshito Y. Tanaka, Masaya Komatsu, Hideki Fujiwara, Keiji Sasaki
    NANO LETTERS 15 (10) 7086 - 7090 1530-6984 2015/10 [Refereed][Not invited]
     
    We demonstrated a new plasmonic nanodevice that spatially sorts photons according to their colors on the nanoscale while maintaining their nano concentration. The properties of this nanoscale color sorting based on constructive and destructive interferences between different multipolar plasmon modes are controlled by tuning the incidence angle of the incoming photons. The added ability of color sorting and its manipulation could significantly influence the development of possible photonic applications, including nanoscale spectroscopy and sensing.
  • Fang Ren, Hideaki Takashima, Yoshito Tanaka, Hideki Fujiwara, Keiji Sasaki
    OPTICS EXPRESS 23 (17) 21730 - 21740 1094-4087 2015/08 [Refereed][Not invited]
     
    A plasmonic-photonic hybrid system with efficient coupling of light from a fiber-coupled microspherical cavity to localized surface plasmon (LSP) modes of a gold-coated tip was proposed, which was composed of a fiber-coupled microspherical cavity and a pseudoisocyanine (PIC)-attached gold tip. To prove efficient excitation of LSP at the goldcoated tip, we experimentally demonstrated two-photon excited fluorescence from the PIC-attached gold-coated tip via a fiber-coupled microspherical cavity under a weak continuous wave excitation condition. This hybrid system could focus the incident light with coupling efficiency of around 64% into a nanoscale domain of the metal tip with an effective area of a 79-nm circle. (C) 2015 Optical Society of America
  • Sanpon Vantasin, Yoshito Tanaka, Shohei Uemura, Toshiaki Suzuki, Yasunori Kutsuma, Daichi Doujima, Tadaaki Kaneko, Yukihiro Ozaki
    PHYSICAL CHEMISTRY CHEMICAL PHYSICS 17 (43) 28993 - 28999 1463-9076 2015 [Refereed]
     
    bSingle-layer graphene microislands with smooth edges and no visible grain boundary were epitaxially grown on the C-face of 4H-SiC and then characterized at the nanoscale using tip-enhanced Raman spectroscopy (TERS). Although these graphene islands appear highly homogeneous in micro-Raman imaging, TERS reveals the nanoscale strain variation caused by ridge nanostructures. A G' band position shift up to 9 cm(-1) and a band broadening up to 30 cm(-1) are found in TERS spectra obtained from nanoridges, which is explained by the compressive strain relaxation mechanism. The small size and refined nature of the graphene islands help in minimizing the inhomogeneity caused by macroscale factors, and allow a comparative discussion of proposed mechanisms of nanoridge formation.
  • Yue Wang, Zhi Yu, Wei Ji, Yoshito Tanaka, Huimin Sui, Bing Zhao, Yukihiro Ozaki
    ANGEWANDTE CHEMIE-INTERNATIONAL EDITION 53 (50) 13866 - 13870 1433-7851 2014/12 
    Efficient and generic enantioselective discrimination of various chiral alcohols is achieved by using surface-enhanced Raman scattering (SERS) spectroscopy through charge-transfer (CT) contributions. The relative intensities of the peaks in the SERS spectra of a chiral selector are strongly dependent on the chirality of its surroundings. This highly distinct spectral discrepancy may be due to the tendency of chiral isomers to form intermolecular hydrogen-bonding complexes with the chiral selector in different molecular orientations, resulting in different CT states and SERS intensities of the adsorbates in the system. This study opens a new avenue leading to the development of novel enantiosensing strategies. A particular advantage of this approach is that it is label-free and does not employ any chiral reagents, including chiral light.
  • Sanpon Vantasin, Ichiro Tanabe, Yoshito Tanaka, Tamitake Itoh, Toshiaki Suzuki, Yasunori Kutsuma, Koji Ashida, Tadaaki Kaneko, Yukihiro Ozaki
    JOURNAL OF PHYSICAL CHEMISTRY C 118 (44) 25809 - 25815 1932-7447 2014/11 
    Step, ridge, and crack submicro/nanostructures of epitaxial graphene on 4H-SiC (000 (1) over bar) were characterized using tip-enhanced Raman scattering (TERS) spectroscopy. The nanostructures were created during graphene synthesis due to a difference in the thermal expansion coefficient of graphene and SiC. These structures are a distinctive property of epitaxial graphene, together with other desirable properties, such as large graphene sheet and minimal defects. The results of this study illustrate that the exceptional spatial resolution of TERS allows spectroscopic measurements of individual nanostructures, a feat which normal Raman spectroscopy is not capable of. By analyzing TERS spectra, the change of local strain on the nanoridge and decreased graphene content in the submicrometer crack were detected. Using G' band positions in the TERS spectra, the strain difference between the ridge center and flat area was calculated to be 1.6 x 10(3) and 5.8 x 10(4) for uniaxial and biaxial strain, respectively. This confirms the proposed mechanism in previous researches that nanoridges on epitaxial graphene form as a relief against compressive strain. With this study, we demonstrate that TERS is a powerful technique for the characterization of individual local nanostructures on epitaxial graphene.
  • Hideaki Takashima, Kazutaka Kitajima, Yoshito Tanaka, Hideki Fujiwara, Keiji Sasaki
    PHYSICAL REVIEW A 89 (2) 021801 - 021801 1050-2947 2014/02 [Refereed][Not invited]
     
    Toward complete coupling between propagating light (PL) and a single localized-surface-plasmon (LSP) nanostructure, we propose a tapered-fiber-coupled microspherical cavity system combining an Au-coated probe tip. This system possesses the unique characteristic of precise adjustability for the fiber-cavity coupling rate and the cavity-plasmon coupling rate, which is indispensable for achieving the critical coupling conditions. We successfully demonstrate the 93% PL coupling into the LSP antenna with an effective area of a 58 nm circle, exceeding the diffraction limit.
  • Fang Ren, Hideaki Takashima, Yoshito Tanaka, Hideki Fujiwara, Keiji Sasaki
    OPTICS EXPRESS 21 (23) 27759 - 27769 1094-4087 2013/11 
    A simple tapered fiber based photonic-plasmonic hybrid nanostructure composed of a thin tapered fiber and a pseudoisocyanine (PIC)-attached Au-coated tip was demonstrated. Using this simple hybrid nanostructure, we succeeded in observing two-photon excited fluorescence from the PIC dye molecules under a weak continuous wave excitation condition. From the results of the tip-fiber distance dependence and excitation polarization dependence, we found that using a thin tapered fiber and an Au-coated tip realized efficient coupling of the incident light (similar to 95%) and LSP excitation at the Au-coated tip, suggesting the possibility of efficiently inducing two-photon excited fluorescence from the PIC dye molecules attached on the Au-coated tip. This simple photonic-plasmonic hybrid system is one of the promising tools for single photon sources, highly efficient plasmonic sensors, and integrated nonlinear plasmonic devices. (C) 2013 Optical Society of America
  • Kyosuke Sakai, Kensuke Nomura, Yoshito Tanaka, Keiji Sasaki
    JOURNAL OF APPLIED PHYSICS 114 (2) 0021-8979 2013/07 
    We have studied two-dimensional periodically arrayed plasmonic nanogap antennas that exploit a "resonant coupling effect," efficiently collecting incident light and enhancing the nanogap field. A theoretical investigation was carried out on an infinite array of gold nanoblock dimers in a square-lattice arrangement. Under resonant conditions, the near-field intensity spectra in the gap region showed a narrow peak (full width at half maximum of 50 nm) with a field enhancement that was a factor of similar to 5 stronger than that of an isolated single-nanogap antenna. The influence of the geometrical parameters of the array (period, dimer orientation, nanogap size, and number of periods) is discussed. (C) 2013 AIP Publishing LLC.
  • Yoshito Tanaka, Shogo Kaneda, Keiji Sasaki
    NANO LETTERS 13 (5) 2146 - 2150 1530-6984 2013/05 [Refereed]
     
    We performed two-dimensional mapping of optical trapping potentials experienced by a 100 nm dielectric particle above a plasmon-resonant gold nanoblock pair with a gap of several nanometers. Our results demonstrate that the potentials have nanoscale spatial structures that reflect the near-field landscape of the nanoblock pair. When an incident polarization parallel to the pair axis is rotated by 90 degrees, a single potential well turns into multiple potential wells separated by a distance smaller than the diffraction limit; this is associated with super-resolution optical trapping. In addition, we show that the trap stiffness can be enhanced by approximately 3 orders of magnitude compared to that with conventional far-field trapping.
  • Hideki Fujiwara, Yoshito Tanaka, Keiji Sasaki
    2013 CONFERENCE ON LASERS AND ELECTRO-OPTICS PACIFIC RIM (CLEO-PR) 2013 [Refereed][Not invited]
     
    Localized fields in metal nanostructures with a nanometer-size gap were measured with high spatial resolution beyond the diffraction limit by using a scatteringtype near-field microscope.
  • Fang Ren, Hideaki Takashima, Yoshito Tanaka, Hideki Fujiwara, Keiji Sasaki
    NANOENGINEERING: FABRICATION, PROPERTIES, OPTICS, AND DEVICES X 8816 0277-786X 2013 [Refereed][Not invited]
     
    We proposed a plasmonic-WGM hybrid system composed of a tapered-fiber-coupled microsphere resonator and a PIC-attached gold tip to focus the incident light into a nanoscale domain(similar to 728.8 nm(2)) with high coupling efficiency of similar to 80.3 % and the Q factor of similar to 1.9x10(6). In order to experimentally verify the strong interaction between light and matter owing to efficient excitation of localized surface plasmon at the gold-coated tip, we demonstrated to observe two-photon excited fluorescence from PIC dye molecules attached on the gold-coated tip even under a weak CW excitation condition via a tapered-fiber-coupled microsphere resonator.
  • Naonobu Shimamoto, Yoshito Tanaka, Hideyuki Mitomo, Ryuzo Kawamura, Kuniharu Ijiro, Keiji Sasaki, Yoshihito Osada
    ADVANCED MATERIALS 24 (38) 5243 - 5248 0935-9648 2012/10
  • Yoshito Tanaka, Akio Sanada, Keiji Sasaki
    SCIENTIFIC REPORTS 2 2045-2322 2012/10 [Refereed]
     
    Arbitrary spatial distributions of the electric field of light are formed through the interference of individual wavenumber mode fields with appropriate amplitudes and phases, while the maximum wavenumber in the far field is limited by the wavelength of light. In contrast, localized surface plasmons (LSPs) possess the ability to confine photons strongly into nanometer-scale areas, exceeding the diffraction limit. In particular, gap-mode LSPs produce single-nanometer-sized, highly intense localized fields, known as hot spots. Here, we show the nanoscale spatial profiles of the LSP fields within hot spots, which exhibit complicated fine structures, rather than single peaks. The nanopatterns are created by constructive and destructive interferences of dipolar, quadrupolar, and higher-order multipolar plasmonic modes, which can be drastically altered by controlling parameters of the excitation optical system. The analysis in this study would be useful for proposing new concepts for manipulation and control of light-matter interactions in nanospaces.
  • Yoshito Tanaka, Keiji Sasaki
    APPLIED PHYSICS LETTERS 100 (2) 0003-6951 2012/01 [Refereed][Not invited]
     
    We report that a small two-dimensional array of gold nanoblock pairs separated by a nanometric gap significantly improves the performance of optical trapping compared to a single nanoblock pair. The array of 4 x 4 pairs suppresses the Brownian motion of a trapped 1 mu m diameter particle by a factor of six compared to the single pair. In addition, the array enables particle trapping for a longer period of time. These results are essential for biological applications where intense optical irradiation is a concern. (C) 2012 American Institute of Physics. [doi: 10.1063/1.3675550]
  • Syoji Ito, Yoshito Tanaka, Hiroyuki Yoshikawa, Yukihide Ishibashi, Hiroshi Miyasaka, Hiroshi Masuhara
    JOURNAL OF THE AMERICAN CHEMICAL SOCIETY 133 (37) 14472 - 14475 0002-7863 2011/09 [Refereed][Not invited]
     
    Controlling chemical reactions within a small space is a significant issue in chemistry, and methods to induce reactions within a desired position have various potential applications. Here we demonstrate localized, efficient photopolymerization by radiation pressure. We induced a one-photon UV polymerization of liquid acrylate solutions in the optical-trapping potential of a focused near-IR (NIR) laser beam, leading to the confinement of solidification to a minute space with dimensions smaller or equal to one-fifth of the wavelength of the NIR laser. Our approach can produce solidification volumes smaller than those achievable with conventional one-photon polymerization, thus enabling the production of tiny polymeric structures that are smaller than the diffraction limit of the trapping light. This is the first demonstration of a radiation pressure effect on a photochemical reaction.
  • Yoshito Tanaka, Keiji Sasaki
    OPTICS EXPRESS 19 (18) 17462 - 17468 1094-4087 2011/08 [Refereed][Not invited]
     
    We have investigated the plasmonic trapping of dielectric nanoparticles by using engineered gold nanoblock pairs with similar to 5-nm gaps. Pairs with surface-plasmon resonance peaks at the incident wavelength allow the trapping of 350-nm-diameter nanoparticles with 200 W/cm(2) laser intensities, and their plasmon resonance properties and trapping performance are drastically modified by varying the nanoblock size of similar to 20%. In addition, plasmon resonance properties of nanoblock pairs strongly depend on the direction of the linear polarization of the incident laser, which determines the trapping performance. (C) 2011 Optical Society of America
  • Yoshito Tanaka, Hiroyasu Ishiguro, Hideki Fujiwara, Yukie Yokota, Kosei Ueno, Hiroaki Misawa, Keiji Sasaki
    OPTICS EXPRESS 19 (8) 7726 - 7733 1094-4087 2011/04 [Refereed][Not invited]
     
    We perform direct local-field imaging of a plasmon-resonant gold nanoparticle pair separated by a gap of several nanometers using a scattering-type near-field optical microscope with a sharp silicon tip of atomic force microscope. The sharp tip allows the access for the nanogap and the high spatial resolution. Our results provide experimental evidence that the nanogap structure produces an optical spot with the size of a single nanometer (< 10 nm). This is not only of fundamental importance in the field of nanophotonics, but also provide significant information for the development of plasmonic devices with the nanogap structures. (C) 2011 Optical Society of America
  • Yoshito Tanaka, Keiji Sasaki
    APPLIED PHYSICS LETTERS 96 (5) 0003-6951 2010/02 [Refereed][Not invited]
     
    We present a simple method for selecting a single metal nanoparticle with desired localized surface plasmon (LSP) characteristics from particle ensembles on one surface and then transferring it to another surface. The LSP of individual nanoparticles is characterized using a microspectroscopy system. An atomic force microscope mounted on the optical microscope achieves particle capture and release with the chemically modified probe.
  • Yoshito Tanaka, Hiroyuki Yoshikawa, Tamitake Itoh, Mitsuru Ishikawa
    OPTICS EXPRESS 17 (21) 18760 - 18767 1094-4087 2009/10 [Refereed][Not invited]
     
    We report laser induced self-assembly of silver nanoparticles via plasmonic interactions. By focusing a near-infrared laser in silver nanoparticle suspension, nanoparticle assembly is formed as a result of optical trapping. The shape of Rayleigh scattering spectra of the nanoassembly strongly depends on the polarization of the laser beam. Particularly, a linearly polarized laser induces the formation of arrayed structure along the laser polarization, that shows a sharp plasmon resonance band and harnesses excellent plasmonic properties applicable for nonlinear surface enhanced spectroscopy. (C) 2009 Optical Society of America
  • Hideki Fujiwara, Yoshito Tanaka, Hiroyasu Ishiguro, Atsushi Saito, Keiji Sasaki
    APPLIED PHYSICS EXPRESS 2 (10) 102002  1882-0778 2009/10 [Refereed][Not invited]
     
    Localized optical fields in metal nanostructures were measured with high spatial resolution beyond the diffraction limit by using a scattering-type near-field microscope. The semiconductor tip of an atomic force microscope probes nanostructures illuminated with laser light under total internal reflection, so that topographic and scattered near-field images are obtained simultaneously. We successfully observed polarization-dependent plasmonic field enhancement in nanometer-size gaps between adjacent gold particles. (C) 2009 The Japan Society of Applied Physics DOI: 10.1143/APEX.2.102002
  • Yoshito Tanaka, Hiroyuki Yoshikawa, Tamitake Itoh, Mitsuru Ishikawa
    JOURNAL OF PHYSICAL CHEMISTRY C 113 (27) 11856 - 11860 1932-7447 2009/07 [Refereed][Not invited]
     
    We have investigated surface enhanced Raman scattering (SERS) stemming from pseudoisocyanine (PIC) molecules adsorbed on Ag nanoparticles by using optical trapping techniques in aqueous solution. By focusing a near-infrared (NIR) laser beam with linear polarization in addition to a visible excitation laser beam, the PIC concentration necessary to detect SERS is dramatically reduced to 10(-14) M, whereas no effect is confirmed with circular polarization. This finding suggests that optical trapping by a linearly polarized laser beam induces the formation of Ag nanoaggregates that incorporate PIC molecules at a specific nanosite where the localized electromagnetic (EM) field is strongly enhanced. The optical force exerted on these Ag nanoparticles and PIC molecules is discussed on the basis of experimental results for the laser-polarization dependence of the SERS and Rayleigh scattering spectra.
  • Yoshito Tanaka, Hiroyuki Yoshikawa, Tsuyoshi Asahi, Hiroshi Masuhara
    APPLIED PHYSICS LETTERS 91 (4) 0003-6951 2007/07 
    The authors have developed a laser microfixation technique of J aggregates from pseudoisocyanine solution onto a glass substrate. By focusing a near-infrared (NIR) laser beam into the solution near the glass substrate, conglomerate J aggregates are deposited at the focal point and fixed on the substrate. Fixed J aggregates show a narrower fluorescence band than the deposited one without laser focusing, indicating that highly ordered J aggregates are selectively confined in the laser focus due to optical trapping. Furthermore, they have demonstrated that the orientation of the transition dipole in fixed J aggregates is controlled by the polarization direction of the NIR laser beam.

MISC

Research Projects

  • ナノ構造が拓くマクロな物体の光マニピュレーション
    科学技術振興機構：創発的研究支援事業
    Date (from‐to) : 2022/04 -2029/03 
    Author : 田中嘉人
  • 日本学術振興会：科学研究費助成事業
    Date (from‐to) : 2024/04 -2027/03 
    Author : 田中 嘉人, 松本 伸之, 蓑輪 陽介
  • Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research Grant-in-Aid for Transformative Research Areas (A)
    Date (from‐to) : 2022/06 -2027/03 
    Author : 余越 伸彦, 田中 嘉人
  • 日本学術振興会：科学研究費助成事業 基盤研究(B)
    Date (from‐to) : 2019/04 -2022/03 
    Author : 田中 嘉人
     
    金属ナノ構造表面近傍の分子の状態や化学反応等に応じて自律的に運動を制御する光駆動ナノモーターの創出を目指し、分子とナノ構造の相互作用に伴う局在プラズモン共鳴変化により光圧が高感度に応答する金属ナノ構造体を研究した。特に、本年度は、シミュレーションによるナノ構造の設計を集中して行った。 長さの異なる２本のナノロッドは、局在プラズモン間の位相差により指向性側方光散乱を生じる。この反作用として、散乱方向と逆向きにナノロッドペアに面内光圧が働く。この光圧を駆動力とするナノモーターを基に設計を行った。この光圧は局在プラズモン共鳴に基づくものであるため、周辺屈折率変化に応じて力の大きさが変化する。今回、片方のナノロッドのみをSiO2で囲んだナノ構造を考えた。つまり、周辺屈折率変化に対して、SiO2で囲んだナノロッドのプラズモン共鳴は変化しない。これにより、周辺屈折率の変化に対するロッド間のプラズモン位相差の変化が増大すると期待される。設計したナノ構造の光散乱パターンを電磁場シミュレーションにより解析したところ、周辺屈折率に対する散乱の指向性が、SiO2を囲んでない場合と比較して大きく変化することが確認された。また面白いことに、SiO2で囲んだロッドのプラズモン共鳴が変化しないため、周辺屈折率を大きくしていくと位相差の逆転が生じ、指向性散乱の方向が逆方向に転ずる様子が見られた。そこで、面内光圧をMaxwell応力法によって計算したところ、周辺屈折率に対して力の大きさが変化するだけでなく、力方向が逆転することも確認できた。従って、周辺屈折率に応じて運動方向を自律的に制御するナノモーターが期待される。
  • 日本学術振興会：科学研究費助成事業 新学術領域研究(研究領域提案型)
    Date (from‐to) : 2019/04 -2021/03 
    Author : 田中 嘉人
     
    光トラップしたマイクロプラットフォーム（MPF）を用いて、ナノ粒子に働く光圧・光トルクを計測する方法の開発を引き続き行った。具体的には、MPFの作製プロセス、特に薄膜形成プロセスの真空度等を見直すことでサンプル成功率を、当初の10%から100%近くに向上させた。また、MPFの界面で働く光圧や内部多重反射による影響がナノ粒子に働く光圧の精密計測において致命的なノイズになることを突き止め、この問題を解決するためにMPFと周辺媒質の屈折率をマッチングしてMPFの光捕捉・位置計測をできるようにした。これまで用いてきたMPFのパターンマッチングによる位置測定から、MPFに埋め込んだ４つの金ナノ粒子からの散乱光の非点収差による3次元位置測定に変更することで分解能を大幅に向上させた。これにより、MPFの位置(x, y, z)と回転(θx, θy, θz)の3次元測定を行うことで、~1.5fNと~2.0fNnmの感度・分解能でナノ粒子に働く光圧(Fx, Fy, Fz)と光トルク(Tθx, Tθy, Tθz)を計測できることを明らかにした。 本手法を用いて新学術領域で活発に議論されているキラルナノ物質に働く光圧・光トルクを明らかにするため、2本の金ナノロッドから構成される3次元キラルナノ構造を10nmの空間分解能で距離を制御して作製するプロセス（電子線ビームリソグラフィの２重露光とドライエッチング）を開発した。ナノロッド間の距離とねじれ角度を最適化することで、自然界の分子の1000倍大きい1.03のg値(キラリティーの指標)を実験と理論により実現した。
  • Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research Grant-in-Aid for Challenging Research (Exploratory)
    Date (from‐to) : 2018/06 -2020/03 
    Author : Tanaka Yoshito
     
    In this study, we developed new approach for super-resolution imaging of wavevector in mid-infrared graphene plasmon via analysis of optical trapping potential for a nanoparticle. During the period, we constructed the system that is transparent in the region between visible and mid-IR and realized up-conversion measurement of infrared optical force on nanoparticles from the analysis of the potential analysis. Furthermore, we revealed that graphene-nanoridge structures allows launching and controlling their plasmons. In addition, we demonstrated, for the first time, Raman chiral activity on twisted stack graphene.
  • 日本学術振興会：科学研究費助成事業 新学術領域研究(研究領域提案型)
    Date (from‐to) : 2017/04 -2019/03 
    Author : 田中 嘉人
     
    本研究は、ナノ物質に働く光圧のポテンシャル解析における問題点（1. 短い光捕捉時間、2. 微弱なプローブ信号）を解決するために、マイクロ粒子を対象とした光捕捉ポテンシャル解析を活用することにより、ナノ物質に作用するフェムトニュートンオーダーの光圧を精密測定する手法を世界に先駆けて開発するものである。 本年度は、昨年度に開発した光駆動マイクロマシンによるナノ物質に働く光圧計測法を発展させ、さらなる力感度向上させるため、アクティブフィードバックによる光捕捉ポテンシャル制御法を開発した。具体的には、ポテンシャル深さが数kTになる低い光強度でマイクロプラットフォーム（MPF）を捕捉し、低いバネ定数（高い力検出感度）のポテンシャルを形成した。次に、CMOSカメラで撮像したMPFの位置をマルチスレッド処理によりリアルタイムに高速解析し、設定した捕捉領域から外れる場合に、電気光学素子（EOM）で光強度を上げてMPFに働く捕捉力を一時的に増大し、捕捉領域に戻すというフィードバックを行った。このようなMPFが感じる光場を高速に制御するシステムを開発し、従来法では難しかった高い力感度と長時間捕捉（高SN比）を併せ持つ捕捉ポテンシャルを実現し、原理的には10fN程度の力計測が可能であることが見積もられた。また、このフィードバックによるポテンシャル形状の制御を有限差分法でシミュレーションし、実験結果から光圧を正確に解析する方法を検討した。さらに、本手法の光圧計測における有効性を示すために、従来法では難しかったナノロッドに働く光トルクの定量計測を行った。現在のことろ、ナノロッドを中心に配置したMPFの光捕捉ポテンシャル計測に成功し、170pNnm/mradのバネ定数で30pNnm以上の分解能が得られることを明らかにした。
  • 局在プラズモン制御による光駆動ナノモーター創出
    科学技術振興機構：戦略的創造研究推進事業 「さきがけ」
    Date (from‐to) : 2015/12 -2019/03 
    Author : 田中嘉人
  • Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research (A)
    Date (from‐to) : 2011/04 -2015/03 
    Author : SASAKI Keiji, FUJIWARA Hideki, TANAKA Yoshito
     
    Utilizing the synergistic effect of the plasmonic enhancement and temporal entangled photons in plasmonic metal nanostructures, we have developed elemental technologies for the realization of highly-efficient two-photon reaction process. We performed theoretical evaluations of enhanced electric fields and found that further electric field enhancement could be expected by the periodic array of gold nano-gap structures. Then, we experimentally examined that two-photon excited fluorescence was enhanced by at least 5 order of magnitude in the periodic gold nanostructure. Furthermore, using a tapered-fiber coupled microsphere, we also developed a plasmonic-photonic hybrid system for highly-efficient plasmon excitation, which can focus an incident light to a small gold nanostructure with 100% efficiency. To experimentally verify the efficient plasmon excitation, we succeeded in the observation of SHG and two-photon excited fluorescence under a weak CW excitation (a few kW/cm2).
  • 日本学術振興会：科学研究費助成事業 特別研究員奨励費
    Date (from‐to) : 2012 -2014 
    Author : 田中 嘉人
     
    プラズモン共鳴を励振する際に生じる局所温度上昇は、微粒子のプラズモントラッピングにおける安定性を考える上で極めて重要であることがら、本年度は1. 熱伝導性の高い基板をヒートシンクとして用いる方針と、2. プラズモニックナノ構造に代わって、高屈折率を持ち誘電体の虚数が小さい半導体ナノ構造を用いる方針で課題克服に向けて取り組んで来た。1. について具体的には、熱伝導性がガラスの数十倍高いサファイアを基板として用い、これまでと同じように電子線ビームリソグラフィ/リフトオフ法によってナノ構造を作製し、SEMやAFMによって形状を、顕微分光によりプラズモン共鳴特性を、近接場顕微鏡により光局在場分布を評価し、シミュレーションで用いたモデルと極めて近い形状・特性を得る事ができた。サファイア基板上金ナノブロックペアを用いて100nm粒子のプラズモントラッピングをしたところ、ガラス基板を用いた場合と比べナノ粒子の位置揺らぎが1/5まで抑えることが可能であることを明らかにした。次に、2. についてSiやGaPのナノブロックペアを対象に可視光照射に伴う局所温度上昇を計算によって見積もったところ、Siナノ構造については金を使った場合に比べ1/100の温度上昇、GaPナノ構造については誘電率の虚部がゼロのため全く温度上昇しないということを見出した。SiやGaPナノ構造で生じる局在場が微粒子捕捉で働く放射圧をMaxwell応力法によって計算したところ、金を使った場合にくらべ1/3程度になることがわかり、金ナノ構造を使った場合より3倍以上の光照射をすれば放射圧の大きさを同程度に保ちながら、局所温度上昇の影響をかなり抑えることができる可能性を示す事ができた。
  • Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research Grant-in-Aid for Challenging Exploratory Research
    Date (from‐to) : 2011 -2012 
    Author : SASAKI Keiji, FUJIWARA Hideki, TANAKA Yoshito
     
    We have theoretically and experimentally investigated radiation pressure induced by localized surface plasmon (LSP) of metal nano-gap structures, and analyzed the strength and spatial distribution of the optical force exerted on nanometer-sized particles and molecules under thermal motions. We showed that the nanoscale spatial profiles of the LSP fields within the nano-gap exhibit complicated fine structures, rather than single peaks. The nanopatterns are formed by interferences of dipolar, quadrupolar, and higher-order multipolar plasmonic modes. In addition, we developed the measurement system for quantitatively analyzing the radiation pressure exerted on nanoparticles. The experimental results demonstrated that the force strength is enhance by 3 order of magnitude compared to that with conventional far-filed trapping. We also succeeded in observation of two-dimensional trapping potentials, which is the first demonstration of super-resolution optical trapping.
  • 文部科学省：科学研究費補助金(若手研究(B))
    Date (from‐to) : 2009 -2011 
    Author : 田中嘉人
     
    金属ナノ構造に光を照射すると、金属構造中に含まれる自由電子が光エネルギーを吸収することにより集団振動(局在表面プラズモン)を始め、その結果金属ナノ構造の表面近傍のナノ領域に極めて高い光局在場が生じる。この光局在場では極めて大きな光放射圧(電場勾配力)が分子やナノ物質に働くことが予想される。そこで、時間領域差分法(FDTD)を用いて金属ナノ構造近傍の電場分布を計算し、光放射圧が働く領域と場所毎に働く光放射圧の強さを見積もった。その結果、1.光増強度の高い構造は光の局在特性も高く一度光捕捉した分子の閉じ込めは大きい一方、光放射圧が働く領域が極めて小さくなる、2.光増強度の低い構造は光の局在性も低く、光放射圧が働く領域は広くなるが分子に働く光放射圧の大きさは小さくなることがわかった。そこで、金ナノダイマー構造のギャップ距離が小さくかつその距離を緩やかに変化する金ナノ構造を提案した。提案した金属ナノ構造を電子線ビームリソグラフィで作製するにあたり、電子線描画の条件、現像条件、金属蒸着条件、リフトオフ条件等の最適化を行った。現在の所高い再現性は未だ得られないものの2ナノメートル以下の精度で金ナノ構造のギャップを作製することに成功している。広い入射波長に対応するために、多様なサイズのナノ構造作製条件も模索している。来年度はFDTDを用いて構造の最適化を進めると共に電子線ビームリソグラフィ...
  • 日本学術振興会：科学研究費助成事業 特別研究員奨励費
    Date (from‐to) : 2006 -2008 
    Author : 田中 嘉人
     
    これまでの研究で、光圧ポテンシャルが働くレーザー集光位置において、J会合体の核形成が促進されること、核形成したJ会合体は自然析出するJ会合体に比べ分子の配向秩序が高く非線形光学材料として優れていることなどを明らかにしてきた。つまり、光圧ポテンシャルによりJ会合体の形成や分子配列が制御できることがわかってきた。次のステップとして、本年度はJ会合体にかわり銀ナノ粒子凝集体に注目した。銀ナノ粒子凝集体もJ会合体同様、ナノ粒子の配列構造によって光圧ポテンシャルエネルギーが大きく変化する。以下に本研究成果の要旨を示す。 1.レーザー集光位置で形成する銀ナノ粒子凝集体の凝集構造を、粒子間のプラズモニック相互作用によって直線偏光と円偏光で制御できることを明らかにした。 2.直線偏光したレーザー集光位置では1次元状に銀ナノ粒子が配列し、電場の偏光方向に長軸が配向することを見出した。 3.2の銀ナノ粒子凝集体は照射レーザー光波長において強いプラズモン共鳴を示し、吸着分子の表面増強ハイパーラマン散乱を測定することに成功した。 4.レーザー集光位置で形成した銀ナノ粒子凝集体は、強い電場増強サイト(いわゆるホットスポット)に分子を光捕捉できることを実証した。 本研究成果は、分子だけでなく金属ナノ粒子の配列構造も光圧ポテンシャルにより制御できることを示すものであり、光圧ポテンシャル効果の高い一般性を示すと共にその有用性を示すことにも成功した。

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