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Search Results: 1 - 10 of 50739 matches for " Y. Arakawa "
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A Highly Tunable Virtual Topology Controller
Y. Sinan Hanay,Shinichi Arakawa,Masayuki Murata
Computer Science , 2015,
Abstract: Much research in the last two decades has focused on Virtual Topology Reconfiguration (VTR) problem. However, most of the proposed methods either has low controllability, or the analysis of a control parameter is limited to empirical analysis. In this paper, we present a highly tunable Virtual Topology (VT) controller. First, we analyze the controllability of two previously proposed VTR algorithms: a heuristic method and a neural networks based method. Then we present insights on how to transform these VTR methods to their tunable versions. To benefit from the controllability, an optimality analysis of the control parameter is needed. In the second part of the paper, through a probabilistic analysis we find an optimal parameter for the neural network based method. We validated our analysis through simulations. We propose this highly tunable method as a new VTR algorithm.
Development of Charging Infrastructure and Subsidies for Promoting Electric Vehicles  [PDF]
Kiyoshi Arakawa
Theoretical Economics Letters (TEL) , 2018, DOI: 10.4236/tel.2018.811131
Abstract: This paper clarifies an effect of policy on the development of charging infrastructure and subsidies for promoting electric vehicles (EVs). Due to the long-term function of charging infrastructure, this paper constructs a multiple-period model to analyze policy effects on the diffusion of EVs from a long-term perspective. This paper shows that charging infrastructure and subsidy encourage innovation on increasing the battery capacity of EVs. Because intertemporal cost allocation can be executed in the development of a charging infrastructure with a subsidy, a policy with a long-term perspective can enhance the charging infrastructure and promote EVs effectively.
Optical diffraction spectroscopy of excitons in uniaxially-strained GaN films
Y. Toda,S. Adachi,Y. Abe,K. Hoshino,Y. Arakawa
Physics , 2004, DOI: 10.1103/PhysRevB.71.195315
Abstract: The degenerate four-wave mixing spectroscopy of uniaxially strained GaN layers is demonstrated using colinearly polarized laser pulses. The nonlinear response of FWM signal on exciton oscillator strength enhances the sensitivity for polarized exciton, allowing for mapping out the in-plane anisotropy of the strain field. The observed high-contrast spectral polarization clearly shows fine structure splittings of excitons, which are also confirmed in the change of quantum beating periods of time.
Impact of electron-phonon interactions on quantum-dot cavity quantum electrodynamics
Y. Ota,S. Iwamoto,N. Kumagai,Y. Arakawa
Physics , 2009,
Abstract: Semiconductor quantum dots (QDs) in photonic nanocavities provide monolithic, robust platforms for both quantum information processing and cavity quantum electrodynamics (QED). An inherent feature of such solid-state cavity QED systems is the presence of electron-phonon interactions, which distinguishes these systems from conventional atomic cavity QED. Understanding the effects of electron-phonon interactions on these systems is indispensable for controlling and exploiting the rich physics that they exhibit. Here we investigate the effects of electron-phonon interactions on a QD-based cavity QED system. When the QD and the cavity are off-resonance, we observe phonon-assisted cavity mode emission that strongly depends on the temperature and cavity-detuning. When they are on-resonance, we observe an asymmetric vacuum Rabi doublet, the splitting of which narrows with increasing temperature. These experimental observations can be well reproduced using a cavity QED model that includes electron-acoustic-phonon interactions. Our work provides significant insight into the important but hitherto poorly understood mechanism of non-resonant QD-cavity coupling and into the physics of various cavity QED systems utilizing emitters coupled to phonons, such as nitrogen-vacancy centres in diamond and colloidal nanocrystals.
Laser oscillation in a strongly coupled single quantum dot-nanocavity system
M. Nomura,N. Kumagai,S. Iwamoto,Y. Ota,Y. Arakawa
Physics , 2009, DOI: 10.1038/nphys1518
Abstract: Strong coupling of photons and materials in semiconductor nanocavity systems has been investigated because of its potentials in quantum information processing and related applications, and has been testbeds for cavity quantum electrodynamics (QED). Interesting phenomena such as coherent exchange of a single quantum between a single quantum dot and an optical cavity, called vacuum Rabi oscillation, and highly efficient cavity QED lasers have been reported thus far. The coexistence of vacuum Rabi oscillation and laser oscillation appears to be contradictory in nature, because the fragile reversible process may not survive in laser oscillation. However, recently, it has been theoretically predicted that the strong-coupling effect could be sustained in laser oscillation in properly designed semiconductor systems. Nevertheless, the experimental realization of this phenomenon has remained difficult since the first demonstration of the strong-coupling, because an extremely high cavity quality factor and strong light-matter coupling are both required for this purpose. Here, we demonstrate the onset of laser oscillation in the strong-coupling regime in a single quantum dot (SQD)-cavity system. A high-quality semiconductor optical nanocavity and strong SQD-field coupling enabled to the onset of lasing while maintaining the fragile coherent exchange of quanta between the SQD and the cavity. In addition to the interesting physical features, this device is seen as a prototype of an ultimate solid state light source with an SQD gain, which operates at ultra-low power, with expected applications in future nanophotonic integrated systems and monolithic quantum information devices.
Spontaneous two photon emission from a single quantum dot
Y. Ota,S. Iwamoto,N. Kumagai,Y. Arakawa
Physics , 2011, DOI: 10.1103/PhysRevLett.107.233602
Abstract: Spontaneous two photon emission from a solid-state single quantum emitter is observed. We investigated photoluminescence from the neutral biexciton in a single semiconductor quantum dot coupled with a high Q photonic crystal nanocavity. When the cavity is resonant to the half energy of the biexciton, the strong vacuum field in the cavity inspires the biexciton to simultaneously emit two photons into the mode, resulting in clear emission enhancement of the mode. Meanwhile, suppression was observed of other single photon emission from the biexciton, as the two photon emission process becomes faster than the others at the resonance.
A single-electron probe for buried optically active quantum dot
T. Nakaoka,K. Watanabe,N. Kumagai,Y. Arakawa
AIP Advances , 2012, DOI: 10.1063/1.4738368
Abstract: We present a simple method that enables both single electron transport through a self-assembled quantum dot and photon emission from the dot. The quantum dot buried in a semiconductor matrix is electrically connected with nanogap electrodes through tunneling junctions formed by a localized diffusion of the nanogap electrode metals. Coulomb blockade stability diagrams for the optically-active dot are clearly resolved at 4.2 K. The position of the quantum dot energy levels with respect to the contact Fermi level is controlled by the kind of metal atoms diffused from the nanogap electrodes.
Diode-Laser Induced Fluorescence Spectroscopy of an Optically Thick Plasma in Combination with Laser Absorption Spectroscopy
S. Nomura,T. Kaneko,G. Ito,K. Komurasaki,Y. Arakawa
Journal of Spectroscopy , 2013, DOI: 10.1155/2013/198420
Abstract: Distortion of laser-induced fluorescence profiles attributable to optical absorption and saturation broadening was corrected in combination with laser absorption spectroscopy in argon plasma flow. At high probe-laser intensity, saturated absorption profiles were measured to correct probe-laser absorption. At low laser intensity, nonsaturated absorption profiles were measured to correct fluorescence reabsorption. Saturation broadening at the measurement point was corrected using a ratio of saturated to non-saturated broadening. Observed LIF broadening and corresponding translational temperature without correction were, respectively, ?GHz and ?K and corrected broadening and temperature were, respectively, ?GHz and ?K. Although this correction is applicable only at the center of symmetry, the deduced temperature agreed well with that obtained by LAS with Abel inversion. 1. Introduction Diode laser-induced fluorescence (DLIF) has a feature of high wavelength resolution on the order of picometers, which makes it useful to obtain translational temperature by measuring Doppler broadening of an atomic line of gases [1, 2] to the same degree as diode laser absorption spectroscopy (DLAS) [3, 4]. The advantage of DLIF over DLAS, which is a line-of-sight measurement, is the possibility of point measurements. However, in optically thick plasma, absorption of the excitation laser and reabsorption (or self-absorption) of fluorescence can broaden the fluorescence profile. The temperature deduced from observed fluorescence tends to be overestimated. Hertz-corrected DLIF profiles in flame consider the laser absorption effect by solving a nonsymmetric 1D distribution of the depleting laser using an iterative procedure [5], although fluorescence reabsorption was not corrected. Although intense lasers with substantial fluorescence that can achieve a high signal-to-noise ratio are preferred, intense lasers are known to cause additional broadening, termed as saturation broadening or power broadening [6]. Moreover, their saturation regime is usually avoided. Instead, the linear region is used in LIF temperature measurements [7]. In contrast, in low-pressure plasma, the saturation intensity is low and the fluorescence profile is easily saturated. The saturation effect in LIF was investigated in earlier studies [8–10], in which the laser spectral width was comparable to Doppler broadening, and in which the spectral wing of laser was able to induce substantial fluorescence, causing additional broadening. In DLIF, however, the laser spectral width is about 1?MHz and three others
Localized guided-mode and cavity-mode double resonance in photonic crystal nanocavities
X. Liu,T. Shimada,R. Miura,S. Iwamoto,Y. Arakawa,Y. K. Kato
Physics , 2014, DOI: 10.1103/PhysRevApplied.3.014006
Abstract: We investigate the use of guided modes bound to defects in photonic crystals for achieving double resonances. Photoluminescence enhancement by more than three orders of magnitude has been observed when the excitation and emission wavelengths are simultaneously in resonance with the localized guided mode and cavity mode, respectively. We find that the localized guided modes are relatively insensitive to the size of the defect for one of the polarizations, allowing for flexible control over the wavelength combinations. This double resonance technique is expected to enable enhancement of photoluminescence and nonlinear wavelength conversion efficiencies in a wide variety of systems.
Giant optical rotation in a three-dimensional semiconductor chiral photonic crystal
S. Takahashi,A. Tandaechanurat,R. Igusa,Y. Ota,J. Tatebayashi,S. Iwamoto,Y. Arakawa
Physics , 2013, DOI: 10.1364/OE.21.029905
Abstract: Optical rotation is experimentally demonstrated in a semiconductor-based three-dimensional chiral photonic crystal (PhC) at a telecommunication wavelength. We design a rotationally-stacked woodpile PhC structure, where neighboring layers are rotated by 45 degrees and four layers construct a single helical unit. The mirror-asymmetric PhC made from GaAs with sub-micron periodicity is fabricated by a micro-manipulation technique. The linearly polarized light incident on the structure undergoes optical rotation during transmission. The obtained results show good agreement with numerical simulations. The measurement demonstrates the largest optical rotation angle as large as 23 degrees at 1300 nm wavelength for a single helical unit.
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