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  2. Ema Kazuhiro

Ema Kazuhiro

  (江馬 一弘)

Profile Information

Affiliation
Professor, Graduate School of Science and Technology, Master's (Doctoral) Program in Science and Technology, Sophia University
Degree
博士（工学）(東京大学)
Contact information
k-emasophia.ac.jp
Researcher number
40194021
J-GLOBAL ID
200901045168026490
researchmap Member ID
1000167951

1983-1986 Mechanical properties of optical glass fibers
1986-1991 Pulse-shape control of ultrafast optical pulses
1991-2002 Pulse propagation properties in excitonic resonant region
1991-present Nonlinear optics at exciton resonance in semiconductors
1994-present Generation and control of coherent phonons in semiconductors
1995-2001 Optical properties and nonlinear optics in polymers
1996-present Optical properties in organic-inorganic hybrid materials
2004-present Optical properties of photo-catalysis materials　　
2006-present Optical properties of semiconductor nanocolumns
2006-2010 Optical properties and nonlinear optics in a quantum-dot ensemble
2006-present Anderson localization of light and random lasing
2007-present Optical properties of nitride semiconductors

I study optical properties in semiconductors and organic materials by means of time-resolved and/or nonlinear spectroscopy with ultrafast optical pulses. Recently I mainly study optical properties of semiconductor nanostructures. For a carrier in education, I have been teaching lectures concerning optics and optical physics for more than ten years, such as “Electromagnetics II”, “Optical physics”, “Quantum optics”, and “Optical properties in solids”. Also I have been teaching a general educational lecture named “Physics in daily life” for more than ten years.

(Subject of research)
Ultrafast Optical Signal Processing
Excitonic Nonlinear Optics
Optical properties of inorganic-organic materials
Optical properties of Photocatalytic materials
Optical properties of semiconductor nanostructure
Light loclization and random lasing

Research Interests

 13

Research Areas

 4

Research History

 13
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Education

 3

Committee Memberships

 17
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Awards

 1

Papers

 209
  • Takemasa Miura, Yuko Takeoka, Mikio Sato, Hideyuki Kunugita, Kazuhiro Ema
    PHYSICA STATUS SOLIDI C: CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 8, NO 9, 8(9) 2752-2755, 2011  
    Optical properties in a single crystal of the organic-inorganic hybrid perovskite quantum-well material (C4H9NH3)(2)(CH3NH3)Pb2Br7 have been investigated. This material has double-layer sheets of corner-sharing [PbBr6](4-) octahedrons constituting the inorganic well. The estimated longitudinal-transverse (LT) splitting, exciton exchange energy, and biexciton binding energy are about 60 meV, 17 meV and 26 meV respectively. We can also study well-width dependence in angstroms by comparing with (C4H9NH3)(2)PbBr4 which has single-layer sheets comprising the inorganic well. This is the first time the well-width dependence of excitonic properties in hybrid organic-inorganic quantum materials is investigated. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
  • Masaru Sakai, Yuta Inose, Tomi Ohtsuki, Kazuhiro Ema, Akihiko Kikuchi, Katsumi Kishino
    2011 Conference on Lasers and Electro-Optics Europe and 12th European Quantum Electronics Conference, CLEO EUROPE/EQEC 2011, 2011  Peer-reviewed
    In disordered materials, the combination of multiple light scattering and optical interference induces the localization of light. This phenomenon is called Anderson localization which is widely observed in electron systems with random potentials. Experimental studies of Anderson localization of light have been performed over the last three decades [1]. However, most of those studies attempted to secure the evidence of light localization by macroscopic observation of the light scattered by disordered materials. Recently, we proposed the direct observation of light localization using GaN nanocolumn samples by near-field scanning optical microscopy (NSOM), and presented the evidence of the light localization by a histogram analysis of NSOM image and by showing the wavelength dependence of the position of the localized spots in NSOM images [2,3]. We also observed random lasing in GaN nanocolumn samples and discussed the lasing property from a viewpoint of Anderson localization [4]. In this study, we extracted the two-dimensional (2D) spatial dependence from the NSOM image to estimate the localization length in GaN nanocolumns. © 2011 IEEE.
  • Shota Tsuchida, Kazuhiro Ema, Hiroshi Sakama, Hideyuki Kunugita
    2011 Conference on Lasers and Electro-Optics Europe and 12th European Quantum Electronics Conference, CLEO EUROPE/EQEC 2011, 2011  Peer-reviewed
    Anatase TiO2 is well known to exhibit high photocatalytic activity. It is believed that photo-excited carriers create activated species by reacting with water and oxygen at the crystal surface, and such activated species decompose organic pollutants. However, the relaxation and diffuse process of the photo-excited carriers is not understood sufficiently. In this study, we investigated such dynamics of photo-excited carriers by transient absorption spectroscopy in anatase TiO2 epitaxial thin film. Moreover, the transient absorption signal and photoluminescence (PL) results are compared for the first time. © 2011 IEEE.
  • Y. Inose, M. Sakai, K. Ema, A. Kikuchi, K. Kishino, T. Ohtsuki
    PHOTONIC AND PHONONIC PROPERTIES OF ENGINEERED NANOSTRUCTURES, 7946 794629, 2011  
    In the two-dimensional random system composed of a disordered array of a dielectric cylindrical column ensemble, Anderson localization of light is possible. We show localization parameter maps for the light localization adopting parameters of gallium nitride nanocolumn samples, which consist of random arrays of parallel nanosized columnar semiconductor crystals. The maps indicate parametric dependence of the localization characteristics on the light frequency, the radius of the columns, and the filling fraction of the columns. To obtain the maps, we have simulated temporal light diffusion in random media using the two-dimensional finite-difference time-domain method and analyzed the simulation results by Fourier transformation. We conclude that the main mechanism for localization varies continuously with the column filling fraction from Mie resonance of single column to Bragg-like diffraction of the column ensemble.
  • M. Hashimoto, K. Fukunaga, H. Kunugita, J. Kamimura, A. Kikuchi, K. Kishino, K. Ema
    ULTRAFAST PHENOMENA IN SEMICONDUCTORS AND NANOSTRUCTURE MATERIALS XV, 7937 793712, 2011  
    The relaxation dynamics of photo-excited carriers of indium nitride (InN) films and nanocolumns were examined using degenerate pump-probe measurements at room temperature. We measured two InN films and nanocolumns with different background carrier densities, and performed numerical calculations incorporating band-filling and bandgap-renormalization effects, as well as LO phonon scattering. We found that the intrinsic relaxation properties of InN can be understood by considering the density of states and electron occupation number of the conduction band. It was also revealed that the decay dynamics of InN are not affected by the carrier recombination time under the appropriate conditions. In addition, we examined the differences in carrier relaxation properties between films and nanocolumns.
  • Takemasa Miura, Yuko Takeoka, Mikio Sato, Hideyuki Kunugita, Kazuhiro Ema
    PHYSICA STATUS SOLIDI C: CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 8, NO 9, 8(9) 2752-2755, 2011  
    Optical properties in a single crystal of the organic-inorganic hybrid perovskite quantum-well material (C4H9NH3)(2)(CH3NH3)Pb2Br7 have been investigated. This material has double-layer sheets of corner-sharing [PbBr6](4-) octahedrons constituting the inorganic well. The estimated longitudinal-transverse (LT) splitting, exciton exchange energy, and biexciton binding energy are about 60 meV, 17 meV and 26 meV respectively. We can also study well-width dependence in angstroms by comparing with (C4H9NH3)(2)PbBr4 which has single-layer sheets comprising the inorganic well. This is the first time the well-width dependence of excitonic properties in hybrid organic-inorganic quantum materials is investigated. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
  • S. Mitsutake, M. Kujiraoka, J. Ishi-Hayase, K. Akahane, N. Yamamoto, K. Ema, M. Sasaki
    PHYSICA STATUS SOLIDI C: CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 8, NO 9, 8(9) 2601-2604, 2011  
    We numerically investigated optical Rabi oscillations of excitonic polarization in a quantum dot ensemble, taking into consideration the spatial distribution of both the excitation electric field and the coherent signal field. We show that the spatial distributions of the signal fields change drastically by increasing the pulse area for excitation pulses with a Gaussian spatial distribution. We found that the ensemble average of the signal intensity is affected by the size of the aperture located in front of the detector, depending on the spatial distribution of the signals on the detector. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
  • Yuta Inose, Masaru Sakai, Kazuhiro Ema, Akihiko Kikuchi, Katsumi Kishino, Tomi Ohtsuki
    PHYSICAL REVIEW B, 82(20) 205328, Nov, 2010  
    In the two-dimensional random system composed of a disordered array of dielectric cylindrical columns, Anderson localization of light occurs. To obtain frequency dependence of the light localization characteristics, we have simulated temporal diffusion of electromagnetic waves in such a random system adopting parameters of actual nanosized semiconductor samples with a high filling fraction of the columns, using the finite-difference time-domain method. We have investigated diffusion length, autocorrelation function of light energy density, and time variation in total energy within the system at several frequencies. We obtain universal behavior of light localization phenomenon as a function of the light localization length and system size, from which we estimate frequency dependence of the localization length. In addition, we show that the frequency dependence of the localization effect depends on the degree of wave interference due to Bragg-like diffraction, rather than on the magnitude of the light scattering cross section of a single scatterer.
  • Masaru Sakai, Yuta Inose, Kazuhiro Ema, Tomi Ohtsuki, Hiroto Sekiguchi, Akihiko Kikuchi, Katsumi Kishino
    Applied Physics Letters, 97(15) 151109, Oct, 2010  
  • T. Kouno, K. Kishino, M. Sakai, Y. Inose, A. Kikuchi, K. Ema
    ELECTRONICS LETTERS, 46(9) 644-U65, Apr, 2010  
    Triangular GaN nanocolumn arrays, in which InGaN/GaN multiple quantum wells were integrated at the top region of each nanocolumn, were grown by Ti-mask selective-area growth of RF-plasma-assisted molecular beam epitaxy. In the nanocolumn arrays, a strong enhancement in light intensity at the specific wavelength of 479.4 nm was observed under low optical excitation, and then optically pumped stimulated emission occurred at near the specific wavelength (476.3 nm) under high optical excitation with a threshold excitation density of 400 kW/cm(2). The light response of the nanocolumn array was numerically analysed by two-dimensional (2D) finite-difference time-domain and 2D plane-wave expansion methods, proving that the stimulated emission occurred in the 2D distributed feedback scheme.
  • Yuta Inose, Masaru Sakai, Kazuhiro Ema, Akihiko Kikuchi, Katsumi Kishino, Tomi Ohtsuki
    THIRD INTERNATIONAL WORKSHOP ON THEORETICAL AND COMPUTATIONAL NANOPHOTONICS - TACONA-PHOTONICS 2010, 1291 118-+, 2010  Peer-reviewed
    We present frequency dependence of Anderson localization of light in finite-sized two-dimensional random systems. We have investigated time variation of total energy within a system composed of dielectric columns, by adopting parameters of actual nano-sized semiconductor crystals with a high filling fraction of the columns. We have succeeded in obtaining single parameter scaling of the localization effect. In addition, we show that the frequency dependence of the localization in the system depends on the degree of wave interference due to Bragg-like diffraction, rather than on the magnitude of the light scattering cross section for a single column.
  • Mamiko Kujiraoka, Junko Ishi-Hayase, Kouichi Akahane, Naokatsu Yamamoto, Kazuhiro Ema, Masahide Sasaki
    APPLIED PHYSICS EXPRESS, 3(9) 092801-1-092801-3, 2010  
    We have investigated Rabi oscillations of exciton polarization in a self-assembled InAs quantum dot ensemble. The four-wave mixing signals measured as a function of the average of the pulse area showed the large in-plane anisotropy and nonharmonic oscillations. The experimental results can be well reproduced by a two-level model calculation including three types of inhomogeneities without any fitting parameter. The large anisotropy can be well explained by the anisotropic dipole moments. We also find that the nonharmonic behaviors partly originate from the polarization interference. (c) 2010 The Japan Society of Applied Physics
  • Kazuya Fukunaga, Kazuaki Kouyama, Masafumi Hashimoto, Jumpei Kamimura, Hideyuki Kunugita, Kazuhiro Ema, Akihiko Kikuchi, Katsumi Kishino
    JOURNAL OF LUMINESCENCE, 129(12) 1735-1738, Dec, 2009  
    We have measured the optical properties of wurtzite InN nanocolumns and film by photoluminescence (PL) Measurements at temperatures from 5 to 300 K and analyzed the PL spectra by fitting with the free-electron recombination bound (FERB) model. For the top-linked InN nanocolumns, we observed strong PL intensity compared to the InN film sample. The PL spectra were asymmetrical with low-energy tails and a red-shift of the PL peak energy position was observed with increasing temperature. However, For the separated InN nanocolumns, we observed weak PL intensity and symmetrical PL spectra. Analyzing the spectra shape of the top-linked InN nanocolumns at 5 K using the FERB model, we evaluated the intrinsic bandgap energy and carrier density of InN nanocolumns to be 0.69 eV and 2.5 x 10(17) cm(-3), respectively. (C) 2009 Published by Elsevier B.V.
  • Yohei Kasai, Daisuke Suzuki, Hideyuki Kunugita, Kazuhiro Ema
    JOURNAL OF LUMINESCENCE, 129(12) 1820-1823, Dec, 2009  
    We report the first observations of resonantly excited coherent optical phonons in bulk ZnTe using noncollinear optical parametric amplifiers As the pulse photon energy approaches the bandgap, the amplitudes of coherent phonons increase Since pulse fluence and polarization dependencies show that generation mechanism of resonantly excited coherent phonons IS impulsive stimulated Raman scattering, this enhancement can be concluded to be the result of the resonant Raman effect (C) 2009 Published by Elsevier B V.
  • K. Fukunaga, M. Hashimoto, H. Kunugita, J. Kamimura, A. Kikuchi, K. Kishino, K. Ema
    APPLIED PHYSICS LETTERS, 95(23) 232114-1-232114-3, Dec, 2009  
    Fast and slow photocarrier dynamics in indium nitride films have been investigated using femtosecond transient measurements at room temperature. The behavior of the decay dynamics is found to be strongly dependent on the excitation energy and the background carrier density. We have found that the slow decay component disappears under optimized conditions due to equilibrium between band filling and bandgap renormalization effects. (C) 2009 American Institute of Physics. [doi: 10.1063/1.3272916]
  • Naoki Suzuki, Kazuaki Kouyama, Kazuaki Kouyama, Yuta Insose, Hideyuki Kunugita, Hideyuki Kunugita, Kazuhiro Ema, Kazuhiro Ema, Hiroto Sekiguchi, Hiroto Sekiguchi, Akihiko Kikuchi, Akihiko Kikuchi, Katumi Kisino, Katumi Kisino
    Physics Procedia, 2(2) 327-333, Aug 1, 2009  Peer-reviewed
    We have studied the emission mechanism of InGaN/GaN nanocolumns. We extracted only the effect of localized states and investigated the difference between InGaN/GaN single-quantum-disc (SQD) and InGaN nanocolumns by means of photoluminescence and photoluminescence excitation measurements. The difference between the localized states was interpreted by the band tail model. We conclude that the InGaN nanocolumn has widely distributed localized states, while the InGaN/GaN SQD has a single localized state..
  • Mamiko Kujiraoka, Junko Ishi-Hayase, Kouichi Akahane, Naokatsu Yamamoto, Kazuhiro Ema, Masahide Sasaki
    PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE, 206(5) 952-955, May, 2009  
    We observed excitonic Rabi oscillations (ROs) in quantum dots (QDs) by four-wave mixing (FWM) and pump-probe techniques. When a large number of QDs are excited by a spatially nonuniform laser, the inhomogeneous distribution of the transition dipole moments of the QD excitons and the spatial distribution of the input electric field induce distribution of the Rabi frequencies. We investigated this ensemble effect on the ROs of the excitonic polarization and the excitonic population observed in the FWM and pump-probe signals. By calculating the FWM and pump-probe signals including the Gaussian distribution of the pulse area, we first showed that a part of the ensemble effect can be cancelled out only in case of the ROs of the excitonic polarization. This reflects the differences between the characteristics of excitonic polarization and population. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
  • Kazuaki Kouyama, Kazuaki Kouyama, Masaya Inoue, Yuta Inose, Naoki Suzuki, Hideyuki Kunugita, Hideyuki Kunugita, Kazuhiro Ema, Kazuhiro Ema, Hiroto Sekiguchi, Hiroto Sekiguchi, Akihiko Kikuchi, Akihiko Kikuchi, Katsumi Kishino, Katsumi Kishino
    Physica Status Solidi (C) Current Topics in Solid State Physics, 6(1) 141-143, Apr 6, 2009  Peer-reviewed
    We have investigated how the morphology and diameter of GaN nanocolumn affect the optical properties of exciton and biexciton. We measured photoluminescence (PL) of excitons in GaN nanocohinms at low temperature and found that the PL spectra depend on the column diameter. Changing the distance between the sample and the collimatmg lens, it was established that this phenomenon is due to the z-polarized PL radiated from the side surface. This result proves that the nano-scale morphology produces a change in the optical properties. In addition, to elucidate the mechanism causing the increase of the biewciton binding energy observed in a previous experiment, we calculated the exciton binding energies by the variational method and also estimated their sizes. From the calculated results we can expect that the only biexcitons experience spatial confinement effect even when the nanocohimn width is much larger than the exciton Bohr radius. © 2009 WILEY-VCH Verlag GmbH & Co. KGaA.
  • K. Fukunaga, M. Hashimoto, H. Kunugita, J. Kamimura, A. Kikuchi, K. Kishino, K. Ema
    2009 LASERS & ELECTRO-OPTICS & THE PACIFIC RIM CONFERENCE ON LASERS AND ELECTRO-OPTICS, VOLS 1 AND 2, 595-596, 2009  Peer-reviewed
    Ultrafast and slow photo-earner dynamics in InN films were investigeted using femtosecond transient measurements. We found the slow component disappears under the optimized condition due to the equilibrium of band filling and bandgap renormalization effects
  • Naoki Suzuki, Kazuaki Kouyama, Yuta Insose, Hideyuki Kunugita, Kazuhiro Ema, Hiroto Sekiguchi, Akihiko Kikuchi, Katumi Kisino
    2008 INTERNATIONAL CONFERENCE ON LUMINESCENCE AND OPTICAL SPECTROSCOPY OF CONDENSED MATTER, 2(2) 327-333, 2009  Peer-reviewed
    We have studied the emission mechanism of InGaN/GaN nanocolumns. We extracted only the effect of localized states and investigated the difference between InGaN/GaN single-quantum-disc ( SQD) and InGaN nanocolumns by means of photoluminescence and photoluminescence excitation measurements. The difference between the localized states was interpreted by the band tail model. We conclude that the InGaN nanocolumn has widely distributed localized states, while the InGaN/GaN SQD has a single localized state.
  • Hideyuki Kunugita, Daisuke Suzuki, Yohei Kasai, Kazuhiro Ema
    2009 LASERS & ELECTRO-OPTICS & THE PACIFIC RIM CONFERENCE ON LASERS AND ELECTRO-OPTICS, VOLS 1 AND 2, 601-602, 2009  Peer-reviewed
    We studied resonant generation of coherent phonon in ZnTe by a nondegenerate EO sampling measurement. The pump energy dependence of the initial amplitude can be explained by the intense absorption of the pump pulse
  • Ishi-Hayase Junko, Akahane Kouichi, Yamamoto Naokatsu, Kujiraoka Mamiko, Ema Kazuhiro, Sasaki Masahide
    PHYSICA STATUS SOLIDI C - CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 6, NO 1, 6(1) 162-+, 2009  Peer-reviewed
  • Junko Ishi-Hayase, Kouichi Akahane, Naokatsu Yamamoto, Mamiko Kujiraoka, Kazuhiro Ema, Masahide Sasaki
    Phys. Status Solidi (c), 6(1) 162-167, Jan, 2009  
  • Kazuaki Kouyama, Masaya Inoue, Yuta Inose, Naoki Suzuki, Hideyuki Kunugita, Kazuhiro Ema, Hiroto Sekiguchi, Akihiko Kikuchi, Katsumi Kishino
    physica status solidi (c), 6(1) 141-143, Jan, 2009  
  • M. Kujiraoka, J. Ishi-Hayase, K. Akahane, N. Yamamoto, K. Ema, M. Sasaki
    16TH INTERNATIONAL CONFERENCE ON ELECTRON DYNAMICS IN SEMICONDUCTORS, OPTOELECTRONICS AND NANOSTRUCTURES (EDISON 16), 193(1) 012125, 2009  
    We propose a new method for controlling macroscopic coherent phenomena by taking advantage of the characteristics of quantum dot (QD) ensembles. We demonstrate that the ensemble average of Rabi oscillations of excitonic polarization can be changed drastically by selecting the spatial distributions of input electric fields. We show that the changes in ensemble averages are caused by polarization interference; the results of our previous experiments agree with our calculations. This result demonstrates that a macroscopic coherent response from a QD ensemble can be controlled by changing the spatial distribution of excitation beams.
  • Takahiro Soya, Yuta Inose, Hideyuki Kunugita, Kazuhiro Ema, Kouji Yamano, Akihiko Kikuchi, Katsumi Kishino
    16TH INTERNATIONAL CONFERENCE ON ELECTRON DYNAMICS IN SEMICONDUCTORS, OPTOELECTRONICS AND NANOSTRUCTURES (EDISON 16), 193(1) 012056, 2009  
    We observed large enhancement of reflected second harmonic generation (SHG) using the one-dimensional photonic effect in regularly arranged InGaN/GaN single-quantum-well nanowalls. Using the effect when both fundamental and SH resonate with the photonic mode, we obtained enhancement of about 40 times compared with conditions far from resonance.
  • Yuta Inose, Tomi Ohtsuki, Hideyuki Kunugita, Kazuhiro Ema, Masaru Sakai, Akihiko Kikuchi, Katsumi Kishino
    16TH INTERNATIONAL CONFERENCE ON ELECTRON DYNAMICS IN SEMICONDUCTORS, OPTOELECTRONICS AND NANOSTRUCTURES (EDISON 16), 193(1) 012055, 2009  
    We show pseudogap maps of Anderson localization of light adopting the parameters of self-organized nanocolumn samples, which consist of random arrays of parallel nanosized columnar semiconductor crystals. The maps indicate the parametric dependence of the localization effect. To obtain the maps, we simulated light propagation in open random media using the two-dimensional finite-difference time-domain method and analyzed the simulation results by Fourier transformation. We found that the shape of the pseudogaps is close to the one of bandgaps in photonic crystals. We conclude that strong localization of light occurs because of interference by average Bragg diffraction, not strong Mie resonant peaks.
  • Masafumi Hashimoto, Jumpei Kamimura, Hideyuki Kunugita, Kazuhiro Ema, Akihiko Kikuchi, Katsumi Kishino
    16TH INTERNATIONAL CONFERENCE ON ELECTRON DYNAMICS IN SEMICONDUCTORS, OPTOELECTRONICS AND NANOSTRUCTURES (EDISON 16), 193(1) 012054, 2009  
    We analyzed photoluminescence (PL) spectra of In-rich In(x)Al(1-x)N nanocolumns (NCs; x = 0.80) and film (x = 0.82) using the free-electron recombination to bound (FERB) model. The FERB model fits well to the PL spectra of the In-rich In(x)Al(1-x)N film and NCs, which implies that light-emitting properties of In-rich In(x)Al(1-x)N are similar to those of InN. We obtained bandgap energies and electron carrier densities from the results of fitting the PL spectra.
  • Masafumi Hashimoto, Kazuya Fukunaga, Kazuaki Kouyama, Jumpei Kamimura, Hideyuki Kunugita, Kazuhiro Ema, Akihiko Kikuchi, Katsumi Kishino
    16TH INTERNATIONAL CONFERENCE ON ELECTRON DYNAMICS IN SEMICONDUCTORS, OPTOELECTRONICS AND NANOSTRUCTURES (EDISON 16), 193(1) 012053, 2009  
    Photo-excited carrier relaxation dynamics in InN films were investigated using femtosecond transient measurements. We measured two samples with different background carrier densities, and discussed the influence of the carrier densities on the relaxation dynamics. We also found that the slow decay component disappears under the optimized condition because of the effects of equilibrium band filling and bandgap renormalization.
  • Daisuke Suzuki, Hideyuki Kunugita, Kazuhiro Ema
    16TH INTERNATIONAL CONFERENCE ON ELECTRON DYNAMICS IN SEMICONDUCTORS, OPTOELECTRONICS AND NANOSTRUCTURES (EDISON 16), 193(1) 012052, 2009  
    We report the first observation of E(2) coherent phonon modes in CdS using a noncollinear optical parametric amplifier. We studied the intensity, temperature and excitation energy dependences of the high- and low-frequency E(2) modes. We obtain that generation mechanism under non- and near resonant regions is impulsive stimulated Raman scattering (ISRS) and E(2) modes are barely affected by FrOhlich interaction. As temperature increases, E(2) modes are affected by anharmonic effect. The observed phonon lifetime of the low- and high-E(2) modes in CdS is 243 +/- 37 and 11.4 +/- 0.4 ps, respectively, at 83 K. From fitting, we can estimate the lifetime of the high-E(2) mode at the low-temperature limit to be 49.3 ps.
  • Taki Hashimoto, Keisuke Takahashi, Hideyuki Kunugita, Hiroshi Sakama, Kazuhiro Ema
    16TH INTERNATIONAL CONFERENCE ON ELECTRON DYNAMICS IN SEMICONDUCTORS, OPTOELECTRONICS AND NANOSTRUCTURES (EDISON 16), 193(1) 012051, 2009  
    We measured transient absorption spectra with pump-probe technique to investigate the dynamics of photo-excited carriers at 300 K and 12 K in anatase TiO(2) epitaxial film. Although the photoluminescence (PL) originated from self-trapped excitons (STEs) vanished at 300 K, we observed the change in probe signal. On the other hand, the intensity of the transient absorption signal at 12 K was much weaker than that at 300K. Therefore the carriers at 300 K become trapped at the surface, leading to a pump-probe signal, while at 12 K, the remained carriers inside the bulk arise from STE recombination. These results suggest that the pump-probe signal is derived from the trap state(s) near the surface.
  • K. Ema, M. Inomata, Y. Kato, H. Kunugita
    PHYSICAL REVIEW LETTERS, 100(25) 257401, Jun, 2008  
    We report the observation of extremely efficient energy transfer (greater than 99%) in an organic-inorganic hybrid quantum-well structure consisting of perovskite-type lead bromide well layers and naphthalene-linked ammonium barrier layers. Time-resolved photoluminescence measurements confirm that the transfer is triplet-triplet Dexter-type energy transfer from Wannier excitons in the inorganic well to the triplet state of naphthalene molecules in the organic barrier. Using measurements in the 10-300 K temperature range, we also investigated the temperature dependence of the energy transfer.
  • Ryo Tamaki, Yuko Arai, Daisuke Ichikawa, Masaya Inoue, Hideyuki Kunugita, Kazuhiro Ema
    JOURNAL OF LUMINESCENCE, 128(5-6) 842-844, May, 2008  
    Exciton polariton in an organic-inorganic multiple-quantum-well (MQW) single crystal (C4H9NH3)(2)PbBr4 at low temperature has been investigated by photoluminescence excitation (PLE), reflection, and time-resolved photoluminescence measurements. Since (C4H9NH3)(2)PbBr4 has ideal two-dimensional excitons with an extremely large oscillator strength and forms self-organized MQW with a very short well-period (d similar to 10 angstrom), polaritonic coupling among excitons is strong and extends over a large number of wells. Therefore, observed MQW polariton features were the same as those of bulk polaritons. We have also investigated relaxation dynamics of the MQW polariton in the same framework as discussions on bulk polaritons. (C) 2007 Elsevier B.V. All rights reserved.
  • M. Kujiraoka, J. Ishi-Hayase, K. Akahane, Y. Yamamotoa, K. Ema, M. Sasaki
    JOURNAL OF LUMINESCENCE, 128(5-6) 972-974, May, 2008  
    We have investigated the population dynamics of excitons in strain-compensated InAs quantum dots (QDs) using a pump-probe technique under resonant excitation. Precise control of polarization directions of incident pulses enabled us to selectively estimate population lifetimes for two orthogonally polarized exciton ground states according to polarization selection rules. Measured decay times of the probe transmissions were highly dependent on the polarization directions of the exciton states. We found that the ratio of the decay times for the orthogonally polarized states is in quantitative agreement with the ratio of square of the transition dipole moments. This indicates that radiative recombination processes have a dominant effect on the population dynamics and that non-radiative and spin relaxations are negligible in our QDs. As a result, we can estimate the radiative lifetimes to be 1.0+/-0.1 and 1.7+/-0.2ns for orthogonally polarized exciton ground states. (C) 2007 Elsevier B.V. All rights reserved.
  • Kaiichi Tanaka, Keita Ikuno, Yom Kasai, Kazuya Fukunaga, Hideyuki Kunugita, Kazuhlro Ema, Akihiko Kikuchi, Katsumi Kishino
    JOURNAL OF LUMINESCENCE, 128(5-6) 1084-1086, May, 2008  
    We observed intersubband transition in GaN/AlN multiple quantum disk (MQD) in nanocolumns by absorption spectra. We also investigated ultrafast relaxation dynamics at 1.55 mu m by time-resolved saturable absorption. We have found that the temporal response of nanocolumns is almost the same as the multiple-quantum-well (MQW) sample. This indicates that the delay caused by multiple scatterings in MQD nanocolumns is negligible. The saturation intensity for the Brewster configuration in MQD was also estimated. (C) 2007 Elsevier B.V. All rights reserved.
  • Koji Iijima, Masako Goto, Shogo Enomoto, Hideyuki Kunugita, Kazuhiro Ema, Masanori Tsukamoto, Noriya Ichikawa, Hiroshi Sakama
    JOURNAL OF LUMINESCENCE, 128(5-6) 911-913, May, 2008  
    This paper presents an investigation into the relaxation process of photo-excited carriers in anatase TiO2 thin films, in which the concentration of oxygen vacancy is controlled by annealing at various temperatures in an oxygen atmosphere. The influence of oxygen vacancies on absorption spectra, photoluminescence (PL) spectra, and PL decay dynamics are discussed. (c) 2007 Elsevier B.V. All rights reserved.
  • J. Ishi-Hayase, K. Akahane, Y. Yamamoto, M. Kujiraoka, K. Ema, M. Sasaki
    JOURNAL OF LUMINESCENCE, 128(5-6) 1016-1018, May, 2008  
    We performed a transient four-wave mixing experiment on a strain-compensated InAs quantum dot (QD) ensemble over a wide range of excitation intensities. Under the resonant excitation of an exciton ground state, an extremely long dephasing time of 1 ns was found. By increasing the areas of the excitation pulses, Rabi oscillations of excitonic polarizations were clearly observed. The corresponding Rabi frequency is three orders of magnitude higher than the measured dephasing rate. For larger pulse areas, we found that the deviation of experimental data from two-level predictions became significant. The deviations cannot be explained by taking into account, as has been suggested in other research, excitation density-dependent dephasing or Hartree-Fock-type Coulomb interactions between excitons. (C) 2008 Elsevier B.V. All rights reserved.
  • Kazuaki Kouyama, Masaya Inoue, Yuta Inose, Naoki Suzuki, Hiroto Sekiguchi, Hideyuki Kunugita, Kazuhiro Ema, Akihiko Kikuchi, Katsumi Kishino
    JOURNAL OF LUMINESCENCE, 128(5-6) 969-971, May, 2008  
    We measured photoluminescence (PL) from excitons and biexcitons in GaN nanocolumns at low temperature and found that the PL spectra of excitons depend on the nanocolumn diameter. Taking into account the polaritonic effect of the excitons, calculation of the PL spectra revealed that the dependence on diameters causes a difference of PL intensity from side surfaces of the nanocolumns. At high excitation intensities, we also observed biexciton emissions and found that the biexciton binding energies are higher than those in bulk samples. Although the mechanism for the increase in the binding energy is not clear at present, we suppose that it arises from a spatial confinement effect due to the nanocolumn morphology. (C) 2007 Elsevier B.V. All rights reserved.
  • Kazuaki Kouyama, Yoshinori Kubo, Kazuhiro Ema, Hideki Kuwahara
    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, 76(12) 123702-1-123702-3, Dec, 2007  
    We investigate the time-dependent reflectivity of the layered manganite La0.5Sr1.5MnO4 at low temperature. We find that the reflectivity oscillates with an anomalously long period, e.g., several tens of hours. The period varies with wavelength and the character of the oscillation depends on the illumination condition. The oscillation is clearly observed below T similar to 20 K and almost disappears above this temperature. Furthermore, in the temperature-cycle measurements, the reflectivity oscillation shows a particularly peculiar behavior.
  • Junko Ishi-Hayase, Kouichi Akahane, Naokatsu Yamamoto, Mamiko Kujiraoka, Kazuhiro Ema, Masahide Sasaki
    JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS BRIEF COMMUNICATIONS & REVIEW PAPERS, 46(9B) 6352-6354, Sep, 2007  
    We measured the dephasing time and radiative lifetime of excitons in InAs quantum dots fabricated using the strain compensation technique. The dephasing time at 3 K was as long as 2.86 ns using transient four-wave mixing measurements at an excitation wavelength of 1.468 mu m. This ultralong dephasing time was due to the significant suppression of pure dephasing.
  • J. Ishi-Hayase, K. Akahane, N. Yamamoto, M. Sasaki, M. Kujiraoka, K. Ema
    APPLIED PHYSICS LETTERS, 91(10) 103111, Sep, 2007  
    The extremely long dephasing time of excitons in strain-compensated quantum dots at telecommunications wavelengths was measured using a polarization-dependent four-wave mixing technique. The use of a 150-layer-stacked structure enabled them to measure a four-wave mixing signal with a high signal-to-noise ratio, in spite of the fact that a high-sensitive heterodyne detection was not used. The large anisotropy of the dephasing time indicates the dominance of the radiative recombination process on dephasing. By simultaneously measuring the radiative lifetime using a pump-probe technique, they could directly estimate pure dephasing with an accuracy of better than 0.1 mu eV.
  • J. Ishi-Hayase, K. Akahane, N. Yamamoto, M. Kujiraoka, K. Ema, M. Sasaki
    28th International Conference on the Physics of Semiconductors (ICPS2006),AIP Conference Proceedings 893, 973-974, Jul, 2007  Peer-reviewed
    歪補償量子ドットにおいて、これまでに観測されたことのない量子ビートが観測されることを見出した。
  • Naomi Harada, Masako Goto, Koji Iijima, Hiroshi Sakama, Noriya Ichikawa, Hideyuki Kunugita, Kazuhiro Ema
    JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS BRIEF COMMUNICATIONS & REVIEW PAPERS, 46(7A) 4170-4171, Jul, 2007  
    The dynamics of photoexcited carriers in standard TiO2 photocatalytic powders has been studied by investigating the time decay and temperature dependence of the photoluminescence. A biexponential decay curve in a time-resolved signal suggests that there are different formation processes of the self-trapped exciton states. The peak position of the luminescence spectrum shifted to a higher energy at room temperature. On the basis of these experimental results, we propose a relaxation model of photoexcited carriers.
  • 江馬一弘
    光アライアンス, 18(2) 21-26, Feb, 2007  
  • K. Ikuno, J. Hamazaki, K. Tanaka, H. Kunugita, K. Ema, A. Kikuchi, K. Kishino
    PHYSICS OF SEMICONDUCTORS, PTS A AND B, 893 477-+, 2007  Peer-reviewed
    We have investigated the detailed ultrafast electronic relaxation dynamics and coherent nonlinearity at optical communication wavelength in GaN/AlN multiple quantum wells (MQWs), using spectral-integrated and resolved two-color pump-probe techniques in a wide energy range. First, we found that the ISBT relaxation dynamics consists of the ultrafast (similar to 140 fs) intersubband relaxation and slow (similar to 1.3 ps) carrier cooling process. Second, we have observed coherent spectrum oscillations, which show that the ISBT dephasing time is more than 100 fs at a room temperature. Finally, we have observed the polarization-dependent absorption in waveguide structures.
  • Masako Goto, Naomi Harada, Koji Iijima, Hideyuki Kunugita, Kazuhiro Ema, Masanori Tsukamoto, Noriya Ichikawa, Hiroshi Sakama
    PHYSICS OF SEMICONDUCTORS, PTS A AND B, 893 161-+, 2007  Peer-reviewed
    We have investigated the relaxation process of photo-excited carriers in anatase TiO2 films, in which the concentration of the oxygen vacancy is controlled by annealing in ultra high vacuum. A broad luminescence band peaked at 2.32 eV is ascribed to a recombination of self-trapped excitons (STEs). The luminescence decay curve was composed of several time constants corresponding to the direct and indirect formation of STEs. We have confirmed that the longer annealing, i.e. higher concentration of the oxygen vacancy, leads to an increase in the indirect STEs.
  • Yuko Arai, Chieko Yajima, Masaya Inoue, Hideyuki Kunugita, Kazuhiro Ema
    PHYSICS OF SEMICONDUCTORS, PTS A AND B, 893 435-+, 2007  Peer-reviewed
    We have investigated the 2s-2p splitting of excitons in an organic-inorganic hybrid quantum well material, (C4H9NH3)PbBr4, by measuring one-photon and two-photon excitation spectra. Generally, energy level of 2s is lower than that of 2p for usual Wannier excitons because of the polaron effect. For the hybrid system, however, we have found that the energy level of 2p is lower by 20 meV than that of 2s. This large and opposite splitting of 2s-2p excitons can be explained by considering image charge effect.
  • Hideyuki Kunugita, Kanji Yabumoto, Kazuhiro Ema
    PHYSICS OF SEMICONDUCTORS, PTS A AND B, 893 165-+, 2007  Peer-reviewed
    We observed coherent phonon oscillations in zinc blende and wurtzite wide-gap semiconductors (ZnTe, ZnS, ZnO). We clarified the different relaxation mechanism of the coherent oscillations between LO phonon in zinc blendes and non polar mode in ZnO. Resonant effect of coherent phonon generation is also studied.
  • M. Kujiraoka, J. Ishi-Hayase, K. Akahane, N. Yamamoto, K. Ema, M. Sasaki
    28th International Conference on the Physics of Semiconductors (ICPS2006),AIP Proceedings 893, 893 975-976, 2007  Peer-reviewed
    歪補償量子ドットにおける四光波混合信号の面内異方性を明らかにした。
  • Ishi-Hayase Junko, Akahane Kouichi, Yamamoto Naokatsu, Kujiraoka Mamiko, Ema Kazuhiro, Sasaki Masahide
    NANOPHOTONICS FOR COMMUNICATION: MATERIALS, DEVICES, AND SYSTEMS IV, 6779, 2007  Peer-reviewed

Misc.

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Professional Memberships

 4

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Industrial Property Rights

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 3
  • 1998
    文化系学生向けに，物理の考え方と面白さを，身近な題材を用いて講義している．90分の講義を3テーマくらいに分けて，30分程度で一つの話題に時間を抑えている．これは，もし興味の沸かないテーマだった場合にも切り替えが速いことで対応できること，および興味が沸いた学生にとっては，もっと深く知りたいと感じたところで終わり，自分で調べる意欲を沸かせるという教育上の効果を狙っている．
  • 1994
    すべての講義において，自作の補助教材を用意し，事前に学生に配布している．図表や細かい式などは，その教材に記載されているので，学生は講義の最中に長い式や複雑な図表のノート取りの必要はなく，講義の内容を聞くことに集中できる．また，動画を見た方がわかりやすいものは，ノートＰＣを使って動画を見せたり，わかりやすいスライドを見せることも行っている．
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