Wataru Nomura

Kyushu University, Hukuoka, Fukuoka, Japan

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Publications (62)109.63 Total impact

  • W. Nomura · T. Yatsui · T. Kawazoe · N. Tate · M. Ohtsu
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    ABSTRACT: Dressed-photon–phonon (DPP) etching is a non-contact flattening technology that realizes ultra-flat surfaces and has been reported to achieve an arithmetic mean surface roughness, Ra, on the order of 0.1 nm in various materials, such as fused silica, plastic films, and GaN crystal. In this study, we successfully flattened the surface of a crystallized glass substrate in several seconds using laser light with a higher power density than that used in previous studies. The target substrate had an initial appearance similar to frosted glass, with an Ra of 92.5 nm. We performed DPP etching under a Cl2 atmosphere using a CW laser with a wavelength of 532 nm, a power of 8 W, and a spot diameter of 0.2 mm. After 1 s of processing, we obtained a flat surface with an Ra of 5.00 nm. This surface roughness equaled or surpassed that of a substrate flattened by conventional chemical mechanical polishing, with an Ra of 5.77 nm. Through the detailed analysis of atomic force microscopic images, we found the DPP etching resulted in the smaller standard deviation of the height difference than CMP in the smaller lateral size than 50 nm.
    No preview · Article · Sep 2015 · Applied Physics A
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    Naoya Tate · Tadashi Kawazoe · Wataru Nomura · Motoichi Ohtsu
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    ABSTRACT: Giant polarization rotation in a ZnO single crystal was experimentally demonstrated based on a novel phenomenon occurring at the nanometric scale. The ZnO crystal was doped with N(+) and N(2+) ions serving as p-type dopants. By applying an in-plane current using a unique arrangement of electrodes on the device, current-induced polarization rotation of the incident light was observed. From the results of experimental demonstrations and discussions, it was verified that this novel behavior originates from a specific distribution of dopants and the corresponding light-matter interactions in a nanometric space, which are allowed by the existence of such a dopant distribution.
    Preview · Article · Aug 2015 · Scientific Reports
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    ABSTRACT: We propose a novel method for observing and utilizing nanometrically fluctuating signals due to optical near-field interactions between a probe and target in nearfield optical microscopy. Based on a hierarchical structure of the interactions, it is possible to obtain signals that represent two-dimensional spatial patterns without requiring any scanning process. Such signals reveal individual features of each target, and these features, when appropriately extracted and defined, can be used in security applications-an approach that we call nanophotonic security. As an experimental demonstration, output signals due to interactions between a SiO2 probe and Al nanorods were observed by using near-field optical microscopy at a single readout point, and these signals were quantitatively evaluated using an algorithm that we developed for extracting and defining features that can be used for security applications.
    No preview · Article · Jul 2015 · Applied Physics A
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    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.
    Preview · Article · Feb 2015 · Advances in Optical Technologies
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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.
    Full-text · Article · Oct 2014 · Journal of Applied Physics
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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.
    Full-text · Article · Aug 2014 · Beilstein Journal of Nanotechnology
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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.
    No preview · Article · Jul 2014 · Applied Physics Letters
  • Naoya Tate · Wataru Nomura · Tadashi Kawazoe · Motoichi Ohtsu
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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.
    No preview · Article · Apr 2014 · Optics Express
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    ABSTRACT: Solution searching devices that operate on the basis of controlling the spatiotemporal dynamics of excitation transfer via dressed photon interactions between quantum dots have been proposed. Long-range excitation transfer based on dressed photon interactions between randomly distributed quantum dots is considered to be effective in realizing such devices. Here, we successfully controlled the spatiotemporal dynamics of excitation transfer using a Y-junction structure consisting of randomly dispersed CdSe/ZnS core-shell quantum dots. This Y-junction structure has two "output ends" and one "tap end." By exciting one output end with control light, we observed increased excitation transfer to the other output end via a state-filling effect. Conversely, we observed reduced excitation transfer to the output ends by irradiating the tap end with control light, due to excitation of defect levels in the tap end. These results show the possibility of controlling the optical excitation transfer dynamics between multiple quantum dots.
    No preview · Article · Apr 2014 · Advances in Optical Technologies
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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.
    No preview · Article · Mar 2014 · Applied Physics A
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    Full-text · Article · Jan 2014
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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.
    Full-text · Article · Dec 2013 · Beilstein Journal of Nanotechnology
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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.
    Full-text · Article · Feb 2013 · Applied Physics Letters
  • Wataru Nomura · Takashi Yatsui · Motoichi Ohtsu
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    ABSTRACT: In this chapter, optical near-field interactions and energy transfer between spherical quantum dots are reviewed. The energy transfer was confirmed by time-resolved spectroscopy in both CdSe and ZnO quantum dots. Furthermore, structural dependency of quantum dots was theoretically and experimentally analyzed with respect to the basic properties of optical signal transfer using optical near-field interactions. The destination selectivity in the optical near-field signal transfer system was also evaluated.
    No preview · Chapter · Jan 2013
  • Takashi Yatsui · Wataru Nomura · Gyu-Chul Yi · Motoichi Ohtsu
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    ABSTRACT: In this chapter, we review the optical near-field phenomena and their applications to realize the nanophotonic device. To realize the nanometer-scale controllability in size and position, we demonstrate the feasibility of nanometer-scale chemical vapor deposition using optical near-field techniques (see Sect. 15.2). In which, the probe-less fabrication method for mass production is also demonstrated. To confirm the promising optical properties of individual ZnO for realizing nanophotonic devices, we performed the near-field evaluation of the ZnO quantum structure (see Sect. 15.3). To drive the nanophotonic device with external conventional diffraction-limited photonic device, the far-/near-field conversion device is required. Section 15.4 reviews nanometer-scale waveguide to be used as such a conversion device of the nanophotonic ICs.
    No preview · Chapter · Jan 2013
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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.
    No preview · Article · Nov 2012 · Journal of Physics D Applied Physics
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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.
    Full-text · Article · Nov 2012 · Applied Physics Letters
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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.
    Full-text · Article · Sep 2012 · Physical Review B
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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.
    Full-text · Article · May 2012 · Applied Physics B
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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.
    Full-text · Article · Apr 2012 · Applied Physics Letters

Publication Stats

400 Citations
109.63 Total Impact Points

Institutions

  • 2015
    • Kyushu University
      Hukuoka, Fukuoka, Japan
  • 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
      • Graduate School of Interdisciplinary Science and Engineering
      Edo, Tōkyō, Japan