Wataru Nomura

The University of Tokyo, Tōkyō, Japan

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Publications (56)97.86 Total impact

  • Takashi Yatsui, Wataru Nomura, Motoichi Ohtsu
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    ABSTRACT: We compared dressed-photon-phonon (DPP) etching to conventional photochemical etching and, using a numerical analysis of topographic images of the resultant etched polymethyl methacrylate (PMMA) substrate, we determined that the DPP etching resulted in the selective etching of smaller scale structures in comparison with the conventional photochemical etching. We investigated the wavelength dependence of the PMMA substrate etching using an O2 gas. As the dissociation energy of O2 is 5.12 eV, we applied a continuous-wave (CW) He-Cd laser ( nm, 3.81 eV) for the DPP etching and a 5th-harmonic Nd:YAG laser ( nm, 5.82 eV) for the conventional photochemical etching. From the obtained atomic force microscope images, we confirmed a reduction in surface roughness, , in both cases. However, based on calculations involving the standard deviation of the height difference function, we confirmed that the conventional photochemical etching method etched the larger scale structures only, while the DPP etching process selectively etched the smaller scale features.
    Advances in Optical Technologies 01/2015; 2015:1-5. DOI:10.1155/2015/701802
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    ABSTRACT: Optical near-field interactions between nanostructured matters, such as quantum dots, result in unidirectional optical excitation transfer when energy dissipation is induced. This results in versatile spatiotemporal dynamics of the optical excitation, which can be controlled by engineering the dissipation processes and exploited to realize intelligent capabilities such as solution searching and decision making. Here, we experimentally demonstrate the ability to solve a decision making problem on the basis of optical excitation transfer via near-field interactions by using colloidal quantum dots of different sizes, formed on a geometry-controlled substrate. We characterize the energy transfer behavior due to multiple control light patterns and experimentally demonstrate the ability to solve the multi-armed bandit problem. Our work makes a decisive step towards the practical design of nanophotonic systems capable of efficient decision making, one of the most important intellectual attributes of the human brain.
    Journal of Applied Physics 10/2014; 116(15):154303. DOI:10.1063/1.4898570 · 2.19 Impact Factor
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    ABSTRACT: Near-infrared Er photoluminescence (PL) is markedly modified using a plasmonic In2O3: Sn nanoparticle (NP) sheet. Modeling and optical measurements reveal the presence of different electric fields (E-field) in the NP sheet. The local E-field excited at the interface between the NP sheet and Er-emitting layer of ZnO contributes significantly towards the spectral modifications of Er PL due to matching with the photon energy of Er PL. We also determine the critical temperature for Er PL modifications, which is related to the energy transfer efficiency between Er transition dipoles in ZnO and the plasmon modes on the NP sheet. (C) 2014 AIP Publishing LLC.
    Applied Physics Letters 07/2014; 105(4):041903. DOI:10.1063/1.4892004 · 3.52 Impact Factor
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    ABSTRACT: The concept of nanophotonic droplets, which are individual spherical polymer structures containing accurately coupled heterogeneous quantum dots, has been previously demonstrated. Such combinations are theoretically promising for their ability to induce novel optical functions. In this paper, we focus on the implementation of wavelength conversion as one of the fundamental optical functions of nanophotonic droplets. A novel mechanism involved in the formation of nanophotonic droplets and results of experimental verification of wavelength conversion using formed nanophotonic droplets are described. By a quantitative comparison with a corresponding sample consisting of randomly dispersed quantum dots, the effectiveness of proposal was successfully demonstrated.
    Optics Express 04/2014; 22(9). DOI:10.1364/OE.22.010262 · 3.53 Impact Factor
  • Advances in Optical Technologies 04/2014; 2014:569684. DOI:10.1155/2014/569684
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    ABSTRACT: The emission intensity from the dipole-forbidden state in a GaAs quantum-ring was increased via close proximity with an aperture fiber probe to induce a near-field interaction between the probe apex and the quantum-ring. As a result, a significant decrease was observed in the decay time of the emission from a dipole-forbidden energy state.
    Applied Physics A 03/2014; 115(1). DOI:10.1007/s00339-013-7905-y · 1.69 Impact Factor
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    01/2014; 2:499-502. DOI:10.15248/proc.2.499
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    ABSTRACT: The laser-induced damage threshold (LIDT) is widely used as an index for evaluating an optical component's resistance to laser light. However, a degradation in the performance of an optical component is also caused by continuous irradiation with laser light having an energy density below the LIDT. Therefore, here we focused on the degradation in performance of an optical component caused by continuous irradiation with femtosecond laser light having a low energy density, i.e., laser-induced degradation. We performed an in situ observation and analysis of an increase in scattering light intensity in fused silica substrates. In experiments conducted using a pulsed laser with a wavelength of 800 nm, a pulse width of 160 fs and pulse repetition rate of 1 kHz, we found that the scattered light intensity increased starting from a specific accumulated fluence, namely, that the laser-induced degradation had a threshold. We evaluated the threshold fluence F t as 6.27 J/cm(2) and 9.21 J/cm(2) for the fused silica substrates with surface roughnesses of 0.20 nm and 0.13 nm in R a value, respectively, showing that the threshold decreased as the surface roughness increased. In addition, we found that the reflected light spectrum changed as degradation proceeded. We analyzed the details of the degradation by measuring instantaneous reflectance changes with a pump-probe method; we observed an increase in the generation probability of photogenerated carriers in a degraded silica substrate and a damaged silica substrate and observed a Raman signal originating from a specific molecular structure of silica. From these findings, we concluded that compositional changes in the molecular structure occurred during degradation due to femtosecond laser irradiation having an energy density below the LIDT.
    Beilstein Journal of Nanotechnology 01/2014; 5:1334-40. DOI:10.3762/bjnano.5.146 · 2.33 Impact Factor
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    ABSTRACT: Dressed-photon–phonon (DPP) etching is a disruptive technology in planarizing material surfaces because it completely eliminates mechanical contact processes. However, adequate metrics for evaluating the surface roughness and the underlying physical mechanisms are still not well understood. Here, we propose a two-dimensional hierarchical surface roughness measure, inspired by the Allan variance, that represents the effectiveness of DPP etching while conserving the original two-dimensional surface topology. Also, we build a simple physical model of DPP etching that agrees well with the experimental observations, which clearly shows the involvement of the intrinsic hierarchical properties of dressed photons, or optical near-fields, in the surface processing.
    Applied Physics Letters 02/2013; 102(7):071603. DOI:10.1063/1.4793233 · 3.52 Impact Factor
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    ABSTRACT: Ultraflat surface substrates are required to achieve an optimal performance of future optical, electronic, or optoelectronic devices for various applications, because such surfaces reduce the scattering loss of photons, electrons, or both at the surfaces and interfaces. In this paper, we review recent progress toward the realization of ultraflat materials surfaces. First, we review the development of surface-flattening techniques. Second, we briefly review the dressed photon-phonon (DPP), a nanometric quasiparticle that describes the coupled state of a photon, an electron, and a multimode-coherent phonon. Then, we review several recent developments based on DPP-photochemical etching and desorption processes, which have resulted in angstrom-scale flat surfaces. To confirm that the superior flatness of these surfaces that originated from the DPP process, we also review a simplified mathematical model that describes the scale-dependent effects of optical near-fields. Finally, we present the future outlook for these technologies.
    Beilstein Journal of Nanotechnology 01/2013; 4:875-885. DOI:10.3762/bjnano.4.99 · 2.33 Impact Factor
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    ABSTRACT: We investigate the effects of laser irradiation on the self-assembly of MnAs nanoparticles during solid-phase decomposition in a GaAs matrix. It is found that laser irradiation suppresses the growth of MnAs nanoparticles from small to large size, and that the median diameter D1 in the size distribution of small MnAs nanoparticles depends on the incident photon energy E following D1 ∼ E−1/5. We explain this behavior by the desorption of Mn atoms on the MnAs nanoparticle surface due to resonant optical absorption, in which incident photons excite intersubband electronic transitions between the quantized energy levels in the MnAs nanoparticles.
    Applied Physics Letters 11/2012; 101(19):193102. DOI:10.1063/1.4765355 · 3.52 Impact Factor
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    ABSTRACT: We obtained an atomically flat diamond surface following dressed photon–phonon (DPP) etching using 3.81 eV light and O2 gas. We obtained a surface roughness (Ra) of 0.154 nm for Ib-type (1 1 1) diamond and 0.096 nm for Ib-type (1 0 0) diamond. To evaluate the surface roughness, we grouped the surface into bins of width l and introduced the standard deviation of the height difference function for a given separation l, which allowed us to determine the height variation of the surface. Based on the calculation of standard deviation, the conventional adiabatic photochemical reaction did not remove the small surface features, while DPP etching decreased the surface roughness for all length scales.
    Journal of Physics D Applied Physics 11/2012; 45(47):475302. DOI:10.1088/0022-3727/45/47/475302 · 2.52 Impact Factor
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    ABSTRACT: Nature-inspired devices and architectures are attracting considerable attention for various purposes, including developing novel computing based on spatiotemporal dynamics, exploiting stochastic processes for computing, and reducing energy dissipation. This paper demonstrates that the optical energy transfer between quantum nanostructures mediated by optical near-field interactions occurring at scales far below the wavelength of light could be utilized for solving constraint satisfaction problems (CSPs). The optical energy transfer from smaller quantum dots to larger ones, which is a quantum stochastic process, depends on the existence of resonant energy levels between the quantum dots or a state-filling effect occurring at the larger quantum dots. Such a spatiotemporal mechanism yields different evolutions of energy transfer patterns in multi-quantum-dot systems. We numerically demonstrate that optical energy transfer processes can be used to solve a CSP. The work described in this paper is a first step in showing the applicability and potential of nanometer-scale optical near-field processes toward solving computationally demanding problems.
    Physical Review B 09/2012; 86(12):125407. DOI:10.1103/PhysRevB.86.125407 · 3.66 Impact Factor
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    ABSTRACT: This article reports the direct observation of long-distance optical excitation transfer based on resonant optical near-field interactions in randomly distributed quantum dots (QDs). We fabricated optical excitation transfer paths based on randomly distributed QDs by using CdSe/ZnS core–shell QDs and succeeded for the first time in obtaining output signals resulting from a unidirectional optical excitation transfer length of 2.4 μm. Furthermore, we demonstrate that the optical excitation transfer occurs via the resonant excited levels of the QDs with a comparative experiment using non-resonant QDs. This excitation-transfer mechanism allows for intersecting, non-interacting nano-optical wires.
    Applied Physics B 05/2012; 107(2). DOI:10.1007/s00340-012-5009-6 · 1.63 Impact Factor
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    ABSTRACT: Recently, light-assisted nanofabrication have been introduced, such as the synthesis of quantum dots using photo-induced desorption that yields reduced size fluctuations, or metal sputtering under light illumination resulting in self-organized, nanoparticle chains. The physical mechanisms have originally been attributed to material desorption or plasmon resonance effects. However, significant stochastic phenomena are also present that have not been explained yet. We introduce stochastic models taking account of the light-assisted processes that reproduce phenomenological characteristics consistent with the experimental observations.
    Applied Physics Letters 04/2012; 100(19). DOI:10.1063/1.4711808 · 3.52 Impact Factor
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    ABSTRACT: We demonstrate optical excitation transfer in a mixture composed of quantum dots of two different sizes (larger and smaller) networked via optical near-field interactions. For the optical near-field interaction network based on a density matrix formalism, we introduce an optimal mixture that agrees with experimental results. Based on these findings, we theoretically examine the topology-dependent efficiency of optical excitation transfer, which clearly exhibits autonomous, energy-efficient networking behavior occurring at the nanometer scale. We discuss what we can learn from this optical excitation transfer and its implications for information and communications applications.
    Nano Communication Networks 12/2011; 2(4):189-195. DOI:10.1016/j.nancom.2011.07.002
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    ABSTRACT: We studied photoluminescence (PL) and energy-transfer dynamics in a hybrid structure comprising a Cd(0.08)Zn(0.92)O quantum well (QW) and an Ag nanostructure. The observed PL quenching was dependent on the electronic states in the QW. Quenching occurred at low temperature where excited carriers recombined radiatively because of excitonic localization, which disappeared with increasing temperature due to delocalization of excitons. Furthermore, nanostructured Ag surfaces produced local surface plasmon (LSP) absorption that was resonant with the PL peak energy of the QW emission. These results indicate that the recombination energy of excitons transfers nonradiatively to induce LSP excitation, which was revealed using time-resolved PL measurements.
    Optics Letters 10/2011; 36(19):3735-7. DOI:10.1364/OL.36.003735 · 3.18 Impact Factor
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    ABSTRACT: We experimentally demonstrated the basic concept of modulatable optical near-field interactions by utilizing energy transfer between closely positioned resonant CdSe/ZnS quantum dot (QD) pairs dispersed on a flexible substrate. Modulation by physical flexion of the substrate changes the distances between quantum dots to control the magnitude of the coupling strength. The modulation capability was qualitatively confirmed as a change of the emission spectrum. We defined two kinds of modulatability for quantitative evaluation of the capability, and an evident difference was revealed between resonant and non-resonant QDs.
    Optics Express 09/2011; 19(19):18260-71. DOI:10.1364/OE.19.018260 · 3.53 Impact Factor
  • Takashi Yatsui, Wataru Nomura, Motoichi Ohtsu
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    ABSTRACT: We report that nanophotonic polishing of a silica substrate using a phonon-assisted photochemical reaction drastically reduced the average surface roughness for application to hard-disk and optical-disk processing.
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    ABSTRACT: We examine autonomous optical excitation transfer in mixtures of different-sized quantum dots networked via optical near-fields at the nanometer scale. We theoretically and experimentally demonstrate optical excitation transfer via the network of optical near-field interactions among quantum dots. The topology-dependent efficiency of excitation transfer is also investigated. The results of our analysis of autonomous and energy-efficient light-matter interactions at the nanoscale, called nanophotonics, will provide useful insights into the design of robust and energy-efficient information and communications systems and networks.
    Computer Communications Workshops (INFOCOM WKSHPS), 2011 IEEE Conference on; 05/2011

Publication Stats

283 Citations
97.86 Total Impact Points

Institutions

  • 2005–2013
    • The University of Tokyo
      • • Department of Electrical Engineering and Information Systems
      • • Department of Medical Engineering
      Tōkyō, Japan
  • 2006
    • Japan Science and Technology Agency (JST)
      Edo, Tōkyō, Japan
  • 2003
    • Tokyo Institute of Technology
      Edo, Tōkyō, Japan