研究者データベース

戸田 泰則(トダ ヤスノリ)
工学研究院 応用物理学部門 固体量子物理工学分野
教授

基本情報

所属

  • 工学研究院 応用物理学部門 固体量子物理工学分野

職名

  • 教授

学位

  • 博士(工学)(東京工業大学)

論文上での記載著者名

  • Y. Toda
  • Yasunori Toda

ホームページURL

J-Global ID

研究キーワード

  • コヒーレント分光   偏光渦   軌道角運動量   窒化物半導体   高温超伝導   時間分解分光   光渦   励起子   

研究分野

  • ナノテク・材料 / 光工学、光量子科学
  • 自然科学一般 / 半導体、光物性、原子物理

職歴

  • 2012年04月 - 現在 北海道大学 工学(系)研究科(研究院) 教授
  • 1999年 - 2011年 北海道大学 工学(系)研究科(研究院) 准教授
  • 1996年04月 - 1999年08月 東京大学 生産技術研究所 学振特別研究員、助手

学歴

  • 1993年04月 - 1996年03月   東京工業大学 大学院総合理工学研究科
  • 1991年04月 - 1993年03月   東京工業大学 大学院総合理工学研究科
  • 1987年04月 - 1991年03月   東京工業大学 工学部 有機材料工学科

所属学協会

  • 日本物理学会   応用物理学会   

研究活動情報

論文

  • Koichi Nakagawa, Satoshi Tsuchiya, Hiromi Taniguchi, Yasunori Toda
    JOURNAL OF SUPERCONDUCTIVITY AND NOVEL MAGNETISM 2020年03月 [査読有り][通常論文]
     
    The electronic state of the organic single crystal kappa - [(d-BEDT-TTF)(0.5) (h-BEDT-TTF)(0.5)](2) Cu[N(CN)(2)]Br, in which similar to 50 percent of the BEDT-TTF molecules are substituted by fully deuterated molecules (d-BEDT-TTF), has been characterized by resistance and polarized time-resolved spectroscopy measurements. We found that while the resistance shows no sign of a superconducting transition, the polarized time-resolved spectroscopy reveals a slow decay dynamics associated with the superconducting phase at a low temperature. The result suggests phase separation between the insulating and superconducting phases in the vicinity of the first-order Mott transition. In addition, we found that this slow component shows a steep increase below similar to 18 K, suggesting a fluctuating superconductivity in the conducting domains. Furthermore, the component persists up to at least similar to 50 K, which is almost consistent with the onset temperature of the anomalous magnetic-field-dependent signals observed by the previous Nernst effect measurements (Nam et al. Sci. Rep.3:3390, 2013) of the alloy kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]Cl0.27Br0.73. While interpretation of these anomalies is still debatable, there exists a possibility of the onset of superconducting paring at a very high temperature.
  • Tsuchiya Satoshi, Nakagawa Koichi, Taniguchi Hiromi, Toda Yasunori
    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 88 7 2019年07月15日 [査読有り][通常論文]
  • Masato Suzuki, Keisaku Yamane, Kazuhiko Oka, Yasunori Toda, Ryuji Morita
    SCIENTIFIC REPORTS 9 2019年07月 [査読有り][通常論文]
     
    We demonstrate a comprehensive quantitative analysis of vector beam states (VBSs) by using a vector field reconstruction (VFR) technique integrating interferometry and imaging polarimetry, where the analysis is given by a cylindrically polarized Laguerre-Gaussian (LG) mode expansion of VBSs. From test examples of cylindrically polarized LG mode beams, we obtain the complex amplitude distributions of VBSs and perform their quantitative evaluations both in radial and azimuthal directions. The results show that we generated (l, p) = (1, 0) LG radially polarized state with a high purity of 98%. We also argue that the cylindrically polarized LG modal decomposition is meaningful for the detail discussion of experimental results, such as analyses of mode purities and mode contaminations. Thus the VFR technique is significant for analyses of polarization structured beams generated by lasers and converters.
  • Nakagawa Koichi, Tsuchiya Satoshi, Yamada Jun-ichi, Toda Yasunori
    EPL 122 6 2018年06月 [査読有り][通常論文]
  • Toda Y, Mochizuki H, Tsuchiya S, Kurosawa T, Oda M, Mertelj T, Mihailovic D
    JOURNAL OF SUPERCONDUCTIVITY AND NOVEL MAGNETISM 31 3 753 - 756 2018年03月 [査読有り][通常論文]
  • Masato Suzuki, Keisaku Yamane, Moritsugu Sakamoto, Kazuhiko Oka, Yasunori Toda, Ryuji Morita
    OPTICS EXPRESS 26 3 2584 - 2598 2018年02月 [査読有り][通常論文]
     
    We proposed and constructed a system to realize broadband generation of arbitrary axisymmetrically polarized (AP) pulses with spatial complex amplitude modulation. This system employs the combination of a spatial light modulator in the 4-f configuration (4-f SLM), and a space variant wave plate as a common path interferometer. The 4-f SLM and the common path interferometer offer compensation for spatial dispersion with respect to wavelength and stability to perturbation, respectively. We experimentally demonstrated the various AP pulses generation by applying modulations of fundamental and higher-order Laguerre-Gauss modes, whose radial indices were, respectively, p = 0 and 1, with high purity, which showed that we were able to generate arbitral AP pulses with spatial complex amplitude modulation. This technique is expected to be applied in both classical and quantum communications with higher-order modes. (c) 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
  • I. Madan, V. V. Baranov, Y. Toda, M. Oda, T. Kurosawa, V. V. Kabanov, T. Mertelj, D. Mihailovic
    PHYSICAL REVIEW B 96 18 2017年11月 [査読有り][通常論文]
     
    A systematic temperature-dependent study of the femtosecond optical superconducting (SC) state destruction and recovery in Bi2Sr2CaCu2O8+delta cuprate superconductor by means of the all-optical polarization-sensitive multipulse spectroscopy is presented. At low temperatures and a partial SC state suppression, an anisotropic SC-gap recovery time scale is suggested by the data. The SC state destruction and recovery dynamics are compared to the recent TR-ARPES-inferred SC-gap dynamics and a qualitative agreement is found. Using a phenomenological response function, the experimental data are also compared to time-dependent Ginzburg-Landau model simulations.
  • S. Tsuchiya, K. Nakagawa, J. Yamada, H. Taniguchi, Y. Toda
    PHYSICAL REVIEW B 96 13 2017年10月 [査読有り][通常論文]
     
    In this work, polarized pump-probe spectroscopy was carried out to investigate the effects of a structural ordering of molecules on photoinduced phase separation (PIPS) in the organic conductors k-(BEDT-TTF)(2)X [X = Cu[N(CN)(2)]Br (k-Br) and Cu(NCS)(2) (k-NCS)]. We found that the anisotropic response for the probe polarization appeared at around T-g, where the glasslike structural transition occurs. The anisotropy can be a result of a transient destruction of the local ordering of molecules, indicating a connection between the glasslike transition and PIPS. Moreover, we found that the PIPS response gradually develops with decreasing temperature in k-Br, whereas it steeply increases in k-NCS. This qualitative difference suggests that the structural ordering caused by a PIPS is more crucial in k-NCS than in k-Br.
  • Y. Toda, A. Moriya, K. Yamane, R. Morita, K. Shigematsu, Y. Awaji
    APPLIED PHYSICS LETTERS 111 10 2017年09月 [査読有り][通常論文]
     
    The single orbital angular momentum mode emission from a commercially available broad-area vertical cavity surface emitting laser (VCSEL) is realized with an optical feedback technique using a spatial light modulator, where a computer generated hologram (CGH) is optimized by taking into account the mode properties of the external cavity VCSEL. The flexibility with which the chirality of the orbital angular momentum mode can be selected is also demonstrated simply by changing the chirality of the CGH. The technique is very simple and easily applicable to other VCSELs without processing. (C) 2017 Author(s).
  • Amplification of ultrafast-rotating ring-shaped optical lattices
    Keisaku Yamane, Kohei Iwasa, Kohei Kakizawa, Kazuhiko Oka, Yasunori Toda, Ryuji Morita
    2017 CONFERENCE ON LASERS AND ELECTRO-OPTICS EUROPE & EUROPEAN QUANTUM ELECTRONICS CONFERENCE (CLEO/EUROPE-EQEC) 2017年 [査読有り][通常論文]
  • Y. Toda, H. Moriya, K. Shigematsu, K. Yamane, R. Morita, Y. Awaji
    OPTICAL MANIPULATION CONFERENCE 10252 2017年 [査読有り][通常論文]
     
    Single angular momentum (OAM) mode emissions from a vertical cavity surface emitting laser (VCSEL) were demonstrated by an external optical feedback using computer generated holograms, which are optimized on the OAM modal gain of the free-running VCSEL. Side-mode suppression ratio of more than 23 dB was achieved for the OAM modes with l = +/- 1.
  • Kyohhei Shigematsu, Masato Suzuki, Keisaku Yamane, Ryuji Morita, Yasunori Toda
    APPLIED PHYSICS EXPRESS 9 12 2016年12月 [査読有り][通常論文]
     
    By using radially polarized pulses for excitation in degenerate four-wave mixing (DFWM) spectroscopy, we realize highly sensitive snap-shot measurement of optical anisotropies in semiconductor films. Because the radially polarized pulses exhibit spatially varying linear polarizations, the optical anisotropies in samples can be evaluated from the spatial distributions of DFWM signals without rotating the polarization of the excitation pulses. We measure the excitons in GaN layers and evaluate the energies of uniaxial strain and the spin-exchange interaction constant. Our results prove both the accuracy and sensitivity of the snap-shot measurement. (C) 2016 The Japan Society of Applied Physics
  • K. Nakagawa, S. Tsuchiya, J. Yamada, Y. Toda
    JOURNAL OF SUPERCONDUCTIVITY AND NOVEL MAGNETISM 29 12 3065 - 3069 2016年12月 [査読有り][通常論文]
     
    We investigated photo-excited carrier relaxation dynamics in the strongly correlated organic superconductors kappa-(BEDT-TTF)(2)Cu(NCS)(2) and kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]Br, using different polarizations of pump and probe pulses. Below the glasslike transition temperature (T (g)) anisotropic responses for probe polarization were observed in both compounds. Decomposing the data into anisotropic and isotropic components, we found the anisotropic component shows no pump polarization dependence, meaning that dissipative excitation process was dominant for the anisotropic carrier relaxation. This behavior indicates that the appearance of anisotropic responses can be associated with spatial symmetry breaking due to structural change of BEDT-TTF molecules.
  • Satoshi Tsuchiya, Koichi Nakagawa, Jun-ichi Yamada, Yasunori Toda
    JOURNAL OF SUPERCONDUCTIVITY AND NOVEL MAGNETISM 29 12 3071 - 3074 2016年12月 [査読有り][通常論文]
     
    Photo-induced carrier relaxation dynamics have been investigated for the organic superconductor kappa-(BEDT-TTF)(2)Cu(NCS)(2) with different probe polarization under 1.3 kbar at low temperatures. We successfully observed the isotropic and anisotropic responses for the probe polarization, which were found to appear at 56 K. By comparing the responses with and without applying pressure, we found that those were slightly changed, indicating that the application of pressure has effect on the carrier relaxation dynamics.
  • Masato Suzuki, Keisaku Yamane, Kazuhiko Oka, Yasunori Toda, Ryuji Morita
    PHYSICAL REVIEW A 94 4 2016年10月 [査読有り][通常論文]
     
    We derived the Berry connection of vector vortex states (VVSs) from the "true" Hamiltonian obtained through the Maxwell-Schrodinger equation for an inhomogeneous anisotropic (IA) medium, and we experimentally demonstrated measurement of the corresponding Pancharatnam-Berry (PB) geometrical phase of VVSs. The PB phase (PBP) of VVSs can be divided into two phases: homogeneous and inhomogeneous PBPs. Homogeneous and inhomogeneous PBPs are related to the conventional PBP and the spatially dependent geometric phase given by an IA medium such as a polarization converter, respectively. We theoretically detected that inhomogeneous PBP accumulation originates from the gauge dependence of the index of the hybrid-order Poincare sphere, which provides an alternate method for understanding optical spin-orbital angular momentum conversion. The homogeneous PBP, which is explicitly observed, has implications for quantum-statemanipulation and information processing.
  • Satoshi Tsuchiya, Yohei Kino, Koichi Nakagawa, Daisuke Nakagawa, Jun-ichi Yamada, Yasunori Toda
    REVIEW OF SCIENTIFIC INSTRUMENTS 87 4 2016年04月 [査読有り][通常論文]
     
    To perform the femtosecond pump-probe spectroscopy under high pressure and low temperature, we constructed a measurement system with a piston cylinder type pressure cell installing an optical fiber bundle. The applied pressure was achieved to 6 kbar and the cell was cooled down to 15 K. Several demonstrations revealed that broadening and change of polarization of pulse (duration of similar to 120 fs) owing to the dispersions in the fiber bundle are much small indicating that those have little influence on the measurement of carrier relaxation dynamics. In the measurements of kappa-(BEDT-TTF)(2)Cu(NCS)(2) under 1.3 kbar at 43 K, we have successfully detected the polarization anisotropy of the carrier relaxation dynamics and estimated the decay time in the same way as the normal measurement. (C) 2016 AIP Publishing LLC.
  • 戸田 泰則, 黒澤 徹, 小田 研
    日本物理学会誌 71 12 830 - 835 一般社団法人 日本物理学会 2016年 [査読無し][通常論文]
     
    <p>超伝導は物理学における最も魅力的な現象の一つであり,カマリング・オネスによる発見から今日に至るまで多くの研究者を魅了してきた.なかでも液体窒素温度をはるかに上回る転移温度を示す銅酸化物は,いわゆる高温超伝導体のプロトタイプとして数多くの研究がなされ,今も新たな知見や問題提起が次々と報告されている.</p><p>超伝導状態では電子間に有効な引力が作用して電子対(クーパー対)が形成され,複数の電子対が位相をそろえて量子凝縮する.凝縮が起こると,フェルミ面近傍の電子スペクトルには,対演算子の期待値(ギャップ関数)によって特徴づけられるエネルギーギャップが開く.Bardeen–Cooper–Schrieffer (BCS) の理論によって説明される従来型超伝導では,格子振動(フォノン)を介した引力で形成される電子対がフェルミ面上に等方的なs波対称性を持つギャップを形成する.これに対して電子間に強い斥力の働く銅酸化物高温超伝導体のギャップは,有効な引力を獲得するために異方的なギャップ関数を持つと考えられており,フェルミ面上にギャップの節(ノード)を持つd波対称性を示す.また超伝導転移温度(<i>T</i><sub>c</sub>)以上でフェルミ面の一部に擬ギャップと呼ばれるギャップ構造が形成されることが知られており,特に反強磁性絶縁相に近いアンダードープ(UD)試料では<i>T</i><sub>c</sub>よりはるかに高温から観測される.擬ギャップの存在は銅酸化物高温超伝導体の最大の特徴であり,古くから精力的な研究がなされているが,超伝導の前駆現象から超伝導と共存/ 競合する秩序相など様々な解釈があり,結論を見ていない.また物質や測定手法に応じて多様な性質を示すため,ギャップの起源や形成メカニズムを決定するには,多面的かつ系統的な物性観測が必要とされている.</p><p>このように多彩な顔を持つ擬ギャップおよび超伝導ギャップと両者の関係性について,従来の実空間や運動量空間の観測手法に加え,超短光パルスを使った時間領域からのアプローチが行われるようになってきた.本稿では物性観測に広く使われる時間分解ポンププローブ分光をベースに,この手法が,これまで観測できなかった高温超伝導体のバルク性質を反映した対称性解析,および秩序形成の相関スケール解析へと拡張可能であることを示す.本稿で扱うポンププローブ分光では,ポンプ光を用いてギャップを形成する電子または正孔(キャリア)を瞬時的に破壊し,高エネルギー状態へと励起する.キャリアのエネルギー緩和に伴って生成される非平衡な準粒子やフォノンは(超伝導・擬ギャップに対応する)ギャップ関数の変化を反映し,ポンプ光に対して遅延時間を持つプローブ光の反射率変化として測定される.励起に使用する光子エネルギーはギャップエネルギーよりもはるかに大きいが,結晶内部への侵入長を大きく取れるため,バルク性質を反映する.拡張されたポンププローブ分光は,それぞれ超伝導ギャップおよび擬ギャップに起因する準粒子応答を一意に決定可能であり,超伝導前駆現象を反映する揺らぎ状態を含め,超伝導ギャップと擬ギャップの個性および両者の関係性をダイナミクスの観点から明らかにする.偏光応答に着目した対称性解析では,擬ギャップの形成温度<i>T </i><sup>*</sup>以下で自発的な回転対称性破れが生じることが示される.他方,秩序形成ダイナミクスからは,擬ギャップが長距離相関のない局在状態を反映することが明らかとなる.</p>
  • K. Shigematsu, K. Yamane, R. Morita, Y. Toda
    PHYSICAL REVIEW B 93 4 2016年01月 [査読有り][通常論文]
     
    The coherent dynamics of the exciton center-of-mass motion in bulk GaN are studied using degenerate four-wave-mixing (FWM) spectroscopy with Laguerre-Gaussian (LG) mode pulses. We evaluate the exciton orbital angular momentum (OAM) dynamics from the degree of OAM, which is derived from the distributions of OAM (topological charge) of the FWM signals. When excitons are excited with two single-mode LG pulses, the exciton OAM decay time significantly exceeds the exciton dephasing time, which can be attributed to high uniformity of the exciton dephasing in our bulk sample because the decoherence of the exciton OAM is governed by the angular variation in the exciton dephasing. We also analyze the topological charge (l) dependence of the OAM decay using a multiple-mode LG pump pulse, which allows us to simultaneously observe the dynamics of the exciton OAM for different l values under the same excitation conditions. The OAM decay times of the l = 1 component are usually longer than those of the l = 0 component. The l-dependent OAM decay is supported by a phenomenological model which takes into account the local nonuniformity of the exciton dephasing.
  • Masato Suzuki, Keisaku Yamane, Kazuhiko Oka, Yasunori Toda, Ryuji Morita
    SCIENTIFIC REPORTS 5 2015年12月 [査読無し][通常論文]
     
    Cylindrically polarized (CP) modes are laser beam modes which have rotational symmetry of the polarization distribution around the beam axis. Considerable attention has been paid to CP modes for their various applications. In this paper, by using the extended Stokes parameters and the degree of polarization defined for the spatial distribution (DOP-SD), we fully-quantitatively characterize the spectrally-resolved polarization states of arbitrary CP (axisymmetrically polarized and higher-order cylindrically polarized) broadband pulses generated by coherent beam combining. All the generated pulse states were fully-quantitatively analyzed for the first time and proved to have high symmetry (DOP-SD greater than or similar to 0.95) and low spectral dependence of polarization states. Moreover, we show the DOP-SD, which cannot be defined by the conventional higher-order and hybrid Stokes parameters, enables us to make a quantitative evaluation of small degradation of rotational symmetry of polarization distribution. This quantitative characterization with high precision is significant for applications of precise material processing, quantum information processing, magneto-optical storage and nonlinear spectroscopic polarimetry.
  • K. Watanabe, S. Tsuchiya, Y. Toda, T. Kurosawa, M. Oda, T. Mertelj, I. Madan, V. V. Kabanov, D. Mihailovic
    INTERNATIONAL JOURNAL OF MODERN PHYSICS B 29 25-26 2015年10月 [査読有り][通常論文]
     
    The dynamics of quasi-particle (QP) excitations with different symmetry is investigated in the superconducting (SC) and pseudogap (PG) states of the high-temperature superconductor Bi2Sr2CaCu2O8+d (Bi2212) using a polarization-sensitive optical pump-probe measurement. The observation of distinct anisotropies for SC excitations and for PG excitations by the probe beam polarization and absence of any dependence on the pump beam polarization evidence the existence of a spontaneous spatial symmetry breaking in the PG state.
  • S. Tsuchiya, Y. Sugawara, S. Tanda, Y. Toda
    JOURNAL OF OPTICS 17 8 2015年08月 [査読有り][通常論文]
     
    Photo-induced quasi-particle (QP) relaxation dynamics with different symmetries have been investigated for the multiple charge-density-wave (CDW) compound DyTe3 by using ultrafast polarized pump-probe spectroscopy. By performing symmetry analysis, the QP dynamics with isotropic A(1g) and anisotropic B-2g symmetry were found to show unique anomalies at the first and second CDW transitions. Both the temperature dependence and pump fluence dependence indicate that the B-2g response is very sensitive to the underlying lattice deformation, which provides critical insight into the multiple CDW formations.
  • I. Madan, T. Kurosawa, Y. Toda, M. Oda, T. Mertelj, D. Mihailovic
    NATURE COMMUNICATIONS 6 2015年04月 [査読有り][通常論文]
     
    A 'pseudogap' was introduced by Mott to describe a state of matter that has a minimum in the density of states at the Fermi level, deep enough for states to become localized. It can arise either from Coulomb repulsion between electrons, and/or incipient charge or spin order. Here we employ ultrafast spectroscopy to study dynamical properties of the normal to pseudogap state transition in the prototype high-temperature superconductor Bi2Sr2CaCu2O8+delta. We perform a systematic temperature and doping dependence study of the pseudogap photodestruction and recovery in coherent quench experiments, revealing marked absence of critical behaviour of the elementary excitations, which implies an absence of collective electronic ordering beyond a few coherence lengths on short timescales. The data imply ultrafast carrier localization into a textured polaronic state arising from a competing Coulomb interaction and lattice strain, enhanced by a Fermi surface instability.
  • Masato Suzuki, Keisaku Yamane, Kazuhiko Oka, Yasunori Toda, Ryuji Morita
    OPTICAL REVIEW 22 1 179 - 183 2015年02月 [査読有り][通常論文]
     
    The extended Stokes parameters for arbitrary cylindrically polarized beams are newly introduced to evaluate their quality. A set of the parameters, expressing a cylindrically polarized state, permits the definition of the degree of polarization that indicates the purity of the spatial symmetry of polarization of a light beam. In addition, the Pancharatnam-Berry phase related to the cylindrically polarized states is described by the new Stokes parameters.
  • Quantitative characterization of polarization states of axisymmetrically polarized pulses generated by coherent beam combining
    Masato Suzuki, Keisaku Yamane, Kazuhiko Oka, Yasunori Toda, Ryuji Morita
    2015 CONFERENCE ON LASERS AND ELECTRO-OPTICS (CLEO) 2015年 [査読有り][通常論文]
     
    By using the extended Stokes parameters and the degree of polarization defined for the spatial distribution (DOP-SD), we fully quantitatively characterize the polarization states of arbitrary (DOP-SD> 0.96) axisymmetrically-polarized femtosecond pulses generated by coherent beam combining. (C) 2014 Optical Society of America
  • TODA Yasunori, SHIGEMATSU Kyohhei
    Proc Hiroshima Int Symp Synchrotron Radiat 19th 116 - 120 2015年 [査読無し][通常論文]
  • Shota Ono, Yasunori Toda, Jun Onoe
    PHYSICAL REVIEW B 90 15 155435.1-155435.5  2014年10月 [査読有り][通常論文]
     
    The electron-phonon coupling constant (lambda) of carbon nanotubes (lambda = 0.006) is much smaller than that of alkali-metal-doped C-60 crystals (lambda = 0.6). This difference may be due to the shape of the pi-electron conjugated system: the former has a flat, whereas the latter has a sphere. In order to confirm the shape effects in lambda, we have examined the magnitude of lambda for a one-dimensional (1D) uneven peanut-shaped C-60 polymer that has an intermediate shape of the pi-electron conjugated system between a carbon nanotube and a C-60 system, using femtosecond (fs) time-resolved pump-probe spectroscopy, because it can be expected to have an intermediate value of lambda between them. Theoretical analysis of fs-transient refractivity obtained experimentally found the magnitude of lambda of the 1D C-60 polymer film to be 0.02 as our expectation. This indicates that the shape of the pi-electron conjugated system affects the magnitude of lambda for nanocarbon allotropes significantly.
  • Y. Toda, F. Kawanokami, T. Kurosawa, M. Oda, I. Madan, T. Mertelj, V. V. Kabanov, D. Mihailovic
    PHYSICAL REVIEW B 90 9 2014年09月 [査読無し][通常論文]
     
    The quasiparticle (QP) dynamics with different symmetry is investigated in the superconducting (SC) and normal state of the high-temperature superconductor Bi2Sr2CaCu2O8+delta (Bi2212) using optical pump-probe experiments with different light polarizations at different doping levels. The observation of distinct selection rules for SC excitations present in A(1g) and B-1g symmetries, and for the pseudogap (PG) excitations present in A(1g) and B-2g symmetries by the probe, and absence of any dependence on the pump beam polarization leads to the unequivocal conclusion of the existence of a spontaneous spatial symmetry breaking in the PG state not limited to the sample surface.
  • Masato Suzuki, Keisaku Yamane, Kazuhiko Oka, Yasunori Toda, Ryuji Morita
    OPTICS EXPRESS 22 14 16903 - 16915 2014年07月 [査読無し][通常論文]
     
    Nonlinear propagation of focused axisymmetrically-polarized ultrashort optical pulses along the optic axis in a uniaxial crystal is investigated experimentally and theoretically. The energy transfer between an azimuthally-polarized pulse and a radially-polarized pulse is observed. To analyze the nonlinear propagation, a general paraxial equation with a third-order nonlinearity for axisymmetrically-polarized pulses in a uniaxial crystal is derived and the extended Stokes parameters (ESPs) based on cylindrical coordinates are newly-introduced. The simulation results by using this equation, providing the calculated ESPs, well explain our experimental observations: 1) the energy transfer is attributed to the four-wave-mixing effect, reflecting the overlapping between the axisymmetrically polarized modes, 2) the variations of the polarization defined from the ESPs are clarified to be affected by the self-and the cross-phase modulations, which make the effective propagation length long or short. (C) 2014 Optical Society of America
  • I. Madan, T. Kurosawa, Y. Toda, M. Oda, T. Mertelj, P. Kusar, D. Mihailovic
    SCIENTIFIC REPORTS 4 2014年07月 [査読無し][通常論文]
     
    In classical superconductors an energy gap and phase coherence appear simultaneously with pairing at the transition to the superconducting state. In high-temperature superconductors, the possibility that pairing and phase coherence are distinct and independent processes has led to intense experimental search of their separate manifestations. Using femtosecond spectroscopy methods we now show that it is possible to clearly separate fluctuation dynamics of the superconducting pairing amplitude from the phase relaxation above the critical transition temperature. Empirically establishing a close correspondence between the superfluid density measured by THz spectroscopy and superconducting optical pump-probe response over a wide region of temperature, we find that in differently doped Bi2Sr2CaCu2O8+delta crystals the pairing gap amplitude monotonically extends well beyond T-c, while the phase coherence shows a pronounced power-law divergence as T -> T-c, thus showing that phase coherence and gap formation are distinct processes which occur on different timescales.
  • Keisaku Yamane, Zhili Yang, Yasunori Toda, Ryuji Morita
    NEW JOURNAL OF PHYSICS 16 May 053020  2014年05月 [査読有り][通常論文]
     
    We propose a high-precision method for measuring the orbital angular momentum (OAM) spectrum of ultra-broadband optical-vortex (OV) pulses from fork-like interferograms between OV pulses and a reference plane-wave pulse. It is based on spatial reconstruction of the electric fields of the pulses to be measured from the frequency-resolved interference pattern. Our method is demonstrated experimentally by obtaining the OAM spectra for different spectral components of the OV pulses, enabling us to characterize the frequency dispersion of the topological charge of the OAM spectrum by a simple experimental setup. Retrieval is carried out in quasi-real time, allowing us to investigate OAM spectra dynamically. Furthermore, we determine the relative phases (including the sign) of the topological-charge-resolved electric-field amplitudes, which are significant for evaluating OVs or OV pulses with arbitrarily superposed modes.
  • C. Gadermaier, V. V. Kabanov, A. S. Alexandrov, L. Stojchevska, T. Mertelj, C. Manzoni, G. Cerullo, N. D. Zhigadlo, J. Karpinski, Y. Q. Cai, X. Yao, Y. Toda, M. Oda, S. Sugai, D. Mihailovic
    PHYSICAL REVIEW X 4 1 011056  2014年03月 [査読有り][通常論文]
     
    We use femtosecond optical spectroscopy to systematically measure the primary energy relaxation rate Gamma(1) of photoexcited carriers in cuprate and pnictide superconductors. We find that Gamma(1) increases monotonically with increased negative strain in the crystallographic a axis. Generally, the Bardeen-Shockley deformation potential theorem and, specifically, pressure-induced Raman shifts reported in the literature suggest that increased negative strain enhances electron-phonon coupling, which implies that the observed direct correspondence between a and Gamma(1) is consistent with the canonical assignment of Gamma(1) to the electron-phonon interaction. The well-known nonmonotonic dependence of the superconducting critical temperature T-c on the a-axis strain is also reflected in a systematic dependence T-c on Gamma(1), with a distinct maximum at intermediate values (similar to 16 ps(-1) at room temperature). The empirical nonmonotonic systematic variation of T-c with the strength of the electron-phonon interaction provides us with unique insight into the role of electron-phonon interaction in relation to the mechanism of high-T-c superconductivity as a crossover phenomenon.
  • Spatiotemporal coherence of GaN excitons excited by an optical vortex with multiple orbital angular momentum
    Kyohhei Shigematsu, Keisaku Yamane, Ryuji Morita, Yasunori Toda
    2014 CONFERENCE ON LASERS AND ELECTRO-OPTICS (CLEO) 2014年 [査読有り][通常論文]
     
    The spatiotemporal coherence of excitons is investigated by degenerate four-wavemixing using optical vortices with multiple orbital angular momentum (OAM). The OAM-resolved signal indicates that the topological phase with large OAM is robust against spatial decoherence.
  • Generation of ultra-broadband pulses with axially-symmetric polarization based on coherent beam combining of optical vortices
    Keisaku Yamane, Yasuyuki Shioda, Masato Suzuki, Yasunori Toda, Ryuji Morita
    2014 CONFERENCE ON LASERS AND ELECTRO-OPTICS (CLEO) 2014年 [査読有り][通常論文]
     
    We demonstrated the generation of ultra-broadband optical pulses with axially-symmetric polarization, on the basis of a coherent beam combining technique. The high polarization purity (similar to 95 %) ranging from 750 to 880 nm was obtained. (C) 2014 Optical Society of America
  • Ryuji Morita, Keisaku Yamane, Zhili Yang, Yasunori Toda
    COMPLEX LIGHT AND OPTICAL FORCES VIII 8999 2014年 [査読無し][通常論文]
     
    Subsequently to the previous generation of ultrashort and ultra-broadband optical-vortex (OV) pulses (pulse energy: several tens of mu J) in few-cycle regime, we demonstrate high-power OV pulse generation with topological charge flexibility by employing a 4-f OV converter and pre- and main-amplifications. Our configuration overcomes the low-throughput drawback of the vortex converter, simultaneously compensating for the angular dispersion. It also gives flexibility of OAM- or topological-charge control. Thus, we succeed in generation of mJ-class ultra-broadband OV pulses (similar to 790-similar to 820 nm) with a programmably-controlled topological charge. Moreover, we experimentally exhibit a high-precision method for measuring frequency-resolved OAM spectrum of femtosecond ultra-broadband OV pulses on the basis of electric-field reconstruction in the spatial domain. In addition, we present the generation of ultrashort pulses with axially-symmetric polarization by coherent beam combining of OVs.
  • Y. Toda, T. Mertelj, P. Kusar, T. Kurosawa, M. Oda, M. Ido, D. Mihailovic
    PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS 493 112 - 113 2013年10月 [査読有り][通常論文]
     
    We report nonequilibrium quasiparticle (QP) dynamics in underdoped Bi2212 crystals using ultrafast optical spectroscopy, which allows to analyze the dynamics associated with the superconducting (SC) and psuedogap (PG) QPs independently. In the saturation condition of the SC component, where the SC condensate is fully destroyed within the photoexcited volume, we found a delay of the SC state recovery associated with a transient normal state. The delay increases linearly with increasing the pump fluence. The QP dynamics also shows a contribution of the PG component, whose magnitude at the start of the SC state recovery was almost constant at various pump fluences, suggesting a critical level of PG order before the SC condensate can recover. (C) 2013 Elsevier B.V. All rights reserved.
  • Yasunori Toda, Kyohhei Shigematsu, Akira Hasegawa, Satoru Adachi
    JAPANESE JOURNAL OF APPLIED PHYSICS 52 8 08JL03  2013年08月 [査読有り][通常論文]
     
    We report on the four-wave mixing (FWM) spectroscopy of excitons in tensile-strained GaN films. Three excitonic resonances (A-, B-, and C-excitons) were clearly observed with energy separations of Delta(AB) similar to 4.7 meV and Delta(BC) similar to 14.5 meV. Owing to tensile strain, these values are much smaller than those of free-standing GaN, allowing the simultaneous excitations of A-B and B-C excitons pairs. The time evolutions of the FWM in different polarization configurations resolved the differences in the simultaneous excitonic transitions; the excitation of A-B excitons showed an oscillation with a critical pi-phase shift in the orthogonal polarization configuration as expected in the quantum beat, and the oscillation of B-C excitons pair is independent of the polarization configuration, indicating polarization interference. The polarization-dependent FWM spectra of A-B excitons revealed peaks associated with biexciton transitions with binding energies of E-AA = 4.6 meV, E-AB = 2.1 meV, and E-BB = 1.7 meV. (C) 2013 The Japan Society of Applied Physics
  • Kyohhei Shigematsu, Yasunori Toda, Keisaku Yamane, Ryuji Morita
    JAPANESE JOURNAL OF APPLIED PHYSICS 52 8 08JL08  2013年08月 [査読有り][通常論文]
     
    We report on the coherent dynamics of the photoexcited orbital angular momentum (OAM) states of GaN excitons. The measurements were performed by a pump-probe four-wave mixing (FWM) using optical vortex (OV) pulses, where the pump OV pulse transfers its OAM to the center of mass momentum of excitons and the conversion of OAM via FWM with the probe pulse is detected. Especially, we performed analysis of the signal using an OAM-resolved spectroscopy, which allows us to evaluate the dephasing dynamics of excitons in time and space. The results indicate that near perfect conversion is achieved using the OV pulses with nonzero OAM and its spatial coherence remains high during the dephasing time, clearly indicating that the excitons keep the OAM transferred by the pump OV. In contrast, the component associated with the imperfect conversion becomes significant when one of the incident pulses intentionally includes an OAM-free component. The effect is clearly confirmed by the FWM excited by the pump with multiple OAM. (C) 2013 The Japan Society of Applied Physics
  • Shota Ono, Hiroyuki Shima, Yasunori Toda
    Trans. Mater. Res. Soc. Jpn. 38 2 283 - 286 2013年06月 [査読有り][通常論文]
  • T. Naito, S. Matsuo, T. Inabe, Y. Toda
    Journal of Superconductivity and Novel Magnetism 26 5 1679 - 1682 2013年 [査読有り][通常論文]
     
    The temperature-dependent time-resolved reflectance spectra of the organic superconductors (OSs) κ- and λ- (BETS)2MCl4 (M = Fe, Ga) and κ-(ET)2Cu[N(CN)2]Br are reported. The π electrons of the organic molecules (BETS or ET) are responsible for the electrical conduction in these salts, while only the Fe-containing salts have localized spins on the d electrons in the Fe atoms (S = 5/2) with π–d interaction. Analysis of the spectra provided the relaxation time τ of the carriers, as well as information concerning the number of carriers at the Fermi energy N(EF). The carrier dynamics were classified in terms of high-TC and low-TC OSs. In these salts, the results indicate that (1) the electron-phonon interaction is important in the occurrence and stability of the superconducting phase, and (2) electron correlation can be related to the enhancement of TC.
  • Y. Toda, K. Shigematsu, K. Yamane, R. Morita
    Opt. Commun. 308 147 - 151 2013年 [査読有り][通常論文]
     
    The orbital angular momentum (OAM) resolved spectroscopy is being applied to various research fields associated with optical vortices or Laguerre-Gaussian (LG) beams, in which the key technique is to convert LG beams to a fundamental Gaussian (HG≡HG00) beam. In this work, we propose a method to realize efficient mode conversion by using spiral phase modulations. Numerical simulations and experiments are carried out to compare our proposal with the standard technique using rotational phase modulations. The results reveal that our proposed method improves the uniformity of the conversion efficiency across various OAM, allowing us to obtain precise OAM spectra. © 2013 Elsevier B.V.
  • Nonlinear Coupling Between Radially- And Azimuthally-Polarized Modes of Ultrashort Optical Pulses in an Anisotropic Crystal
    Masato Suzuki, Keisaku Yamane, Yasunori Toda, Ryuji Morita
    2013 CONFERENCE ON LASERS AND ELECTRO-OPTICS PACIFIC RIM (CLEO-PR) 2013年 [査読有り][通常論文]
     
    We investigate nonlinear propagation of ultrashort optical pulses with radially- and azimuthally-polarized modes in a uniaxial crystal. The energy exchange between the modes, ascribed to nonlinear four-wave-mixing effect with Gouy-phase shifts, is discussed.
  • Nonlinear Conversion between Ultrashort Radially- and Azimuthally-Polarized Pulses in an Anisotropic Media
    Masato Suzuki, Keisaku Yamane, Yasunori Toda, Ryuji Morita
    2013 CONFERENCE ON LASERS AND ELECTRO-OPTICS EUROPE AND INTERNATIONAL QUANTUM ELECTRONICS CONFERENCE (CLEO EUROPE/IQEC) 2013年 [査読有り][通常論文]
  • Programmable Ultrashort Optical-vortex Pulse Generation Using Optical Parametric Amplification and 4-f Configuration
    Keisaku Yamane, Asami Honda, Kyohhei Shigematsu, Yasunori Toda, Ryuji Morita
    2013 CONFERENCE ON LASERS AND ELECTRO-OPTICS PACIFIC RIM (CLEO-PR) 2013年 [査読有り][通常論文]
     
    We demonstrate the optical parametric amplification of optical-vortex pulses with the programmable topological-charge control. Combination of optical-parametric amplification and 4-f configuration overcomes the low-throughput drawback of the vortex converter, simultaneously compensating for the angular dispersion.
  • Frequency-resolved Orbital Angular Momentum Spectrum Measurement of Ultra-broadband Optical Vortices
    Zhili Yang, Keisaku Yamane, Yasunori Toda, Ryuji Morita
    2013 CONFERENCE ON LASERS AND ELECTRO-OPTICS PACIFIC RIM (CLEO-PR) 2013年 [査読有り][通常論文]
     
    We experimentally exhibit a high-precision method for measuring frequency-resolved orbital angular momentum (OAM) spectrum of femtosecond ultra-broadband optical-vortex pulses from fork-like interferograms. This method enables us to evaluate topological-charge dispersion of ultra-broadband optical-vortex pulses.
  • Quasi-real-time Measurement of Orbital Angular Momentum Spectra of Ultra-broadband Optical Vortices from Fork-like Interferograms
    Zhili Yang, Keisaku Yamane, Yasunori Toda, Ryuji Morita
    2013 CONFERENCE ON LASERS AND ELECTRO-OPTICS (CLEO) 2013年 [査読有り][通常論文]
     
    We experimentally demonstrate a simple and high-precision method for measuring orbital angular momentum (OAM) spectra of femtosecond ultra-broadband optical-vortex pulses from fork-like interferograms. This method enables quasi-real-time OAM measurement for ultra-short or ultra-broadband optical vortices.
  • Amplification of ultrashort optical-vortex pulses with programmable topological-charge control
    Keisaku Yamane, Yasunori Toda, Ryuji Morita
    2013 CONFERENCE ON LASERS AND ELECTRO-OPTICS (CLEO) 2013年 [査読有り][通常論文]
     
    We demonstrate the amplification of ultrashort optical-vortex pulses through the optical parametric process, where optical-vortex seed pulses with computer-controlled topological charges are employed. In this scheme, the low-throughput drawback of the vortex converter is overcome. (C) 2013 Optical Society of America
  • T. Naito, S. Matsuo, T. Inabe, Y. Toda
    Journal of Superconductivity and Novel Magnetism 1 - 4 2012年12月21日 [査読無し][通常論文]
     
    The temperature-dependent time-resolved reflectance spectra of the organic superconductors (OSs) κ- and λ-(BETS)2MCl4 (M=Fe, Ga) and κ-(ET)2Cu[N(CN)2]Br are reported. The π electrons of the organic molecules (BETS or ET) are responsible for the electrical conduction in these salts, while only the Fe-containing salts have localized spins on the d electrons in the Fe atoms (S=5/2) with π-d interaction. Analysis of the spectra provided the relaxation time τ of the carriers, as well as information concerning the number of carriers at the Fermi energy N(EF). The carrier dynamics were classified in terms of high-TC and low-TC OSs. In these salts, the results indicate that (1) the electron-phonon interaction is important in the occurrence and stability of the superconducting phase, and (2) electron correlation can be related to the enhancement of TC. © 2012 Springer Science+Business Media New York.
  • Shota Ono, Hiroyuki Shima, Yasunori Toda
    PHYSICAL REVIEW B 86 10 104512  2012年09月 [査読有り][通常論文]
     
    We develop a theory to describe the energy relaxation of photoexcited carriers in low-temperature ordered states with a band-gap opening. Carrier relaxation time tau near and below transition temperature T-c is formulated by examining the contributions from different carrier-phonon scatterings to the relaxation rate. Transverse acoustic phonon modes are found to play a crucial role in carrier relaxation; their heat capacity determines the tau divergence near T-c. Remarkable agreements with the theory and experimental data on two different materials which exhibit contrasting tau(T) behaviors are also demonstrated.
  • Keisaku Yamane, Yasunori Toda, Ryuji Morita
    OPTICS EXPRESS 20 17 18986 - 18993 2012年08月 [査読有り][通常論文]
     
    We generated a 2.3-cycle, 5.9-fs, 56-mu J ultrashort optical-vortex pulse (ranging from similar to 650 to similar to 950 nm) in few-cycle regime, by optical parametric amplification. It was performed even by using passive elements (a pair of prisms and chirped mirrors) for chirp compensation. Spectrally-resolved interferograms and intensity profiles showed that the obtained pulses have no spatial or topological-charge dispersion during the amplification process. To the best of our knowledge, it is the first generation of optical-vortex pulses in few-cycle regime. They can be powerful tools for ultrabroadband and/or ultrafast spectroscopy and experiments of high-intensity field physics. (C) 2012 Optical Society of America
  • C. Gadermaier, V. V. Kabanov, A. S. Alexandrov, L. Stojchevska, T. Mertelj, C. Manzoni, G. Cerullo, N. D. Zhigadlo, J. Karpinski, Y. Q. Cai, X. Yao, Y. Toda, M. Oda, S. Sugai, D. Mihailovic
    2012年05月 [査読有り][通常論文]
     
    The origin of high critical temperature (Tc) superconductivity is still
    remarkably elusive. To gain insight into the high- Tc mechanism, we need
    experiments which identify the parameters that determine Tc and link them to
    the interaction(s) that establish the superconducting state. Here we show that
    for pnictides, cuprates, and bismuthates Tc depends systematically on the
    primary electron energy relaxation rate 1/tau_1. We find that 1/tau_1 is a
    direct experimental measure of the strength of the electron-phonon interaction
    (EPI) and correlates with structural parameters, in particular the l...
  • Shota Ono, Hiroyuki Shima, Yasunori Toda
    2012年02月 [査読有り][通常論文]
     
    We develop a theory to describe energy relaxation of photo-excited carriers
    in low-temperature ordered states with band gap opening and formulate carrier
    relaxation time $\tau$ near and below transition temperature $T_{\mathrm{c } }$
    by quantifying contributions from different carrier-phonon scatterings to the
    relaxation rate. The theory explains anomalous experimental observations of
    $\tau$ in gapped systems. Transverse acoustic (TA) phonon modes play a crucial
    role in carrier relaxation; their heat capacity determines $\tau$-divergence
    near $T_{\mathrm{c } }$. The theory is validated by fitti...
  • Shota Ono, Hiroyuki Shima, Yasunori Toda
    Phys. Rev. B 86 10 104512  2012年 [査読有り][通常論文]
  • Jun Onoe, Takahiro Ito, Shin-Ichi Kimura, Hiroyuki Shima, Yasuhiro Toda, Hideo Yoshioka
    FULLERENES NANOTUBES AND CARBON NANOSTRUCTURES 20 1 1 - 16 2012年 [査読有り][通常論文]
     
    When a C-60 film is irradiated with a 3 kV electron-beam in an ultrahigh vacuum (base pressure: 10(-7) Pa), C-60 molecules are coalesced to form a one-dimensional (1D) uneven peanut-shaped C-60 polymer. From a topological viewpoint, since the polymer has both positive and negative Gaussian curvatures, it can be regarded as a new allotrope of nanocarbons different from graphene, fullerenes, and nanotubes. We will present our recent results of the electronic, optical, and electron-transport properties of the 1D uneven peanut-shaped C-60 polymer and discuss its novel electronic properties on the basis of quantum mechanics in Riemannian space.
  • Toshio Naito, Saori Matsuo, Tamotsu Inabe, Yasunori Toda
    JOURNAL OF PHYSICAL CHEMISTRY C 116 3 2588 - 2593 2012年01月 [査読有り][通常論文]
     
    Understanding how electrons behave, i.e., carrier dynamics, in superconductors is indispensable for understanding the mechanism of superconductivity. Recently we have reported the carrier dynamics of kappa- and lambda-(BETS)(2)MCl4 (BETS = bis(ethylenedithio)tetraselenafulvalene, M = Ga, Fe) based on ultrafast spectroscopy, but the interpretation of the results remains an open question. In this paper we interpreted the results with the aid of newly measured magnetic susceptibility, X-ray single crystal structural analysis, and band calculation. Observation of coherent phonons only in the lambda-type salts indicated that e-ph interaction should be characteristically strong in the A-type salts. By comparison of the observed carrier dynamics with a two-temperature model, the temperature-dependence of carrier dynamics is consistently explained by the different strengths of the e-ph interaction between the lambda- and the kappa-type salts. The difference in strength of the e-ph interactions is related to their crystal structures. In conclusion, their carrier dynamics is consistently interpreted and classified by their crystal structures.
  • Jun Onoe, Takahiro Ito, Shin-Ichi Kimura, Hiroyuki Shima, Yasuhiro Toda, Hideo Yoshioka
    FULLERENES NANOTUBES AND CARBON NANOSTRUCTURES 20 1 1 - 16 2012年 [査読有り][通常論文]
     
    When a C-60 film is irradiated with a 3 kV electron-beam in an ultrahigh vacuum (base pressure: 10(-7) Pa), C-60 molecules are coalesced to form a one-dimensional (1D) uneven peanut-shaped C-60 polymer. From a topological viewpoint, since the polymer has both positive and negative Gaussian curvatures, it can be regarded as a new allotrope of nanocarbons different from graphene, fullerenes, and nanotubes. We will present our recent results of the electronic, optical, and electron-transport properties of the 1D uneven peanut-shaped C-60 polymer and discuss its novel electronic properties on the basis of quantum mechanics in Riemannian space.
  • Toshio Naito, Saori Matsuo, Tamotsu Inabe, Yasunori Toda
    JOURNAL OF PHYSICAL CHEMISTRY C 116 3 2588 - 2593 2012年01月 [査読有り][通常論文]
     
    Understanding how electrons behave, i.e., carrier dynamics, in superconductors is indispensable for understanding the mechanism of superconductivity. Recently we have reported the carrier dynamics of kappa- and lambda-(BETS)(2)MCl4 (BETS = bis(ethylenedithio)tetraselenafulvalene, M = Ga, Fe) based on ultrafast spectroscopy, but the interpretation of the results remains an open question. In this paper we interpreted the results with the aid of newly measured magnetic susceptibility, X-ray single crystal structural analysis, and band calculation. Observation of coherent phonons only in the lambda-type salts indicated that e-ph interaction should be characteristically strong in the A-type salts. By comparison of the observed carrier dynamics with a two-temperature model, the temperature-dependence of carrier dynamics is consistently explained by the different strengths of the e-ph interaction between the lambda- and the kappa-type salts. The difference in strength of the e-ph interactions is related to their crystal structures. In conclusion, their carrier dynamics is consistently interpreted and classified by their crystal structures.
  • Y. Toda, T. Mertelj, P. Kusar, T. Kurosawa, M. Oda, M. Ido, D. Mihailovic
    Phys. Rev. B 84 17 174516  2011年11月 [査読有り][通常論文]
     
    We investigate the relaxation dynamics of photoexcited quasiparticles (QPs) in underdoped Bi2Sr2CaCu2O8+delta (T-c = 78 K). By changing the excitation energy and polarization of the probe beam, two different types of relaxation dynamics, associated with superconducting (SC) and pseudogap (PG) QPs, are quantitatively analyzed independently. From the temperature dependencies, we obtained the SC gap, Delta(SC)(0) = 24 meV, using BCS-type temperature-dependent gap and the pseudogap, Delta(PG) = 41 meV. The pump fluence (F) dependence of the SC-dominated transients shows a contribution of the PG component above the saturation condition of the SC component (F-th = 16 mu J/cm(2)), where Cooper pairs with long-range order are fully destroyed within the photoexcited volume. Assuming a temperature-independent PG decay time, we successfully isolate the native SC transient even above F-th by subtracting the PG response from the original data. In the saturation regime, the exponential decay (recovery of SC) is fast (tau(SC) similar to 2-3 ps), suggesting an efficient nonequilibrium phonon relaxation in this compound. We also find a flat-top response preceding the exponential decay at F > F-th, which appears as a delay of SC recovery in the original data. This response is visible over the whole temperature range below T-c and its duration increases with increasing F. The response is attributable to a photoinduced SC to non-SC phase transition arising from excitation by the nonthermal QPs and/or high-frequency phonons. The consistently near-constant magnitude of the PG response at the start of the SC state recovery from the non-SC phase suggests a correlation between the SC and PG QPs.
  • Y. Toda, T. Mertelj, T. Naito, D. Mihailovic
    Phys. Rev. Lett. 107 22 227002  2011年11月 [査読有り][通常論文]
     
    We investigate the relaxation dynamics of nonequilibrium carriers in organic conductors kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]X (X = Br and Cl) using ultrafast time-resolved optical spectroscopy. The dynamics for both salts show similar temperature dependences, which is well characterized by the carrier relaxation across the pseudogap (PG) of the magnitude Delta(PG) approximate to 16 meV for Br salt and 7.0 meV for Cl salt. On the other hand, only the Br salt shows an abrupt increase of the decay time at low temperature, indicating an additional decay component associated with the superconducting (SC) gap below T-c. The fluence dependent dynamics at low temperature evidences the superposition of the SC component onto the PG component. These results indicate a metallic-insulating phase separation in the Br salt triggered by photoexcited nonequilibrium carriers.
  • L. Stojchevska, P. Kusar, T. Mertelj, V. V. Kabanov, Y. Toda, X. Yao, D. Mihailovic
    PHYSICAL REVIEW B 84 18 180507  2011年11月 [査読有り][通常論文]
     
    The processes leading to nonthermal condensate vaporization and charge-density-wave (CDW) melting with femtosecond laser pulses is systematically investigated in different materials. We find that vaporization is relatively slow (tau(v)1 ps) and inefficient in superconductors, exhibiting a strong systematic dependence of the vaporization energy U(v) on T(c). In contrast, melting of CDW order proceeds rapidly (tau(m) = 50-200 fs) and more efficiently. A quantitative model describing the observed systematic behavior in superconductors is proposed based on a phonon-mediated quasiparticle (QP) bottleneck mechanism. In contrast, Fermi-surface disruption by hot QPs is proposed to be responsible for CDW state melting.
  • Y. Toda, T. Mertelj, P. Kusar, T. Kurosawa, M. Oda, M. Ido, D. Mihailovic
    PHYSICAL REVIEW B 84 17 174516  2011年11月 [査読有り][通常論文]
     
    We investigate the relaxation dynamics of photoexcited quasiparticles (QPs) in underdoped Bi2Sr2CaCu2O8+delta (T-c = 78 K). By changing the excitation energy and polarization of the probe beam, two different types of relaxation dynamics, associated with superconducting (SC) and pseudogap (PG) QPs, are quantitatively analyzed independently. From the temperature dependencies, we obtained the SC gap, Delta(SC)(0) = 24 meV, using BCS-type temperature-dependent gap and the pseudogap, Delta(PG) = 41 meV. The pump fluence (F) dependence of the SC-dominated transients shows a contribution of the PG component above the saturation condition of the SC component (F-th = 16 mu J/cm(2)), where Cooper pairs with long-range order are fully destroyed within the photoexcited volume. Assuming a temperature-independent PG decay time, we successfully isolate the native SC transient even above F-th by subtracting the PG response from the original data. In the saturation regime, the exponential decay (recovery of SC) is fast (tau(SC) similar to 2-3 ps), suggesting an efficient nonequilibrium phonon relaxation in this compound. We also find a flat-top response preceding the exponential decay at F > F-th, which appears as a delay of SC recovery in the original data. This response is visible over the whole temperature range below T-c and its duration increases with increasing F. The response is attributable to a photoinduced SC to non-SC phase transition arising from excitation by the nonthermal QPs and/or high-frequency phonons. The consistently near-constant magnitude of the PG response at the start of the SC state recovery from the non-SC phase suggests a correlation between the SC and PG QPs.
  • Sou Ryuzaki, Toshihiro Kai, Yasunori Toda, Satoru Adachi, Jun Onoe
    JOURNAL OF PHYSICS D-APPLIED PHYSICS 44 14 145103  2011年04月 [査読有り][通常論文]
     
    We have examined the structural effects of zinc-octaethylporphyrin [Zn(OEP)] films used as a donor on the external quantum efficiency (EQE) of organic heterojunction photovoltaic (OPV) cells [ITO/Zn(OEP)/C-60/Al], and investigated what exactly causes the improvement of EQE. When the structure of the Zn(OEP) films changed from amorphous to crystalline, the maximum EQE increased from 36% to 42%, which is greater than that of around 35% for previously reported OPV cells using buffer materials (Peumans and Forrest 2001 Appl. Phys. Lett. 79 126). The crystallization of Zn(OEP) films is found to increase the number of inter-molecular charge-transfer (IMCT) excitons and to enlarge the mobility of carriers and IMCT excitons, thus significantly improving the EQE of the photoabsorption band under illumination due to the IMCT excitons.
  • Masato Suzuki, Yasunori Toda, Ryuji Morita
    2011 Conference on Lasers and Electro-Optics Europe and 12th European Quantum Electronics Conference, CLEO EUROPE/EQEC 2011 2011年 [査読有り][通常論文]
     
    Having a helical wavefront, a Laguerre-Gaussian (LG) mode or optical vortex is one of the modes of paraxial solutions to wave equation. The optical vortex has unique properties. The beam has a phase singularity on its center, which shows a dark part on its center of intensity profile, and carries an orbital angular momentum of light well defined by the topological charge ℓ[1]. These characteristics recently attracted much attention. For more versatile applications, high power optical-vortex pulses are desired. Typically, optical vortices are generated by using a spiral plate [2], a spatial light modulator (SLM) [3], a photonic-crystal axially-symmetric polarizer/waveplate [4] or a uniaxial crystal [5]. Among them, the most flexible method is by SLM. However, this method using SLM is power-limited owing to its damage threshold. Hence, as an alternative to generate high power optical-vortex pulses, we choose the method using a uniaxial crystal, which is rather flexible. In this scheme, the nonlinear effects have not been clarified in detail. In the present paper, we investigate nonlinear effects on optical-vortex pulse generation through the spin-orbit interaction and its propagation in an anisotropic crystal. © 2011 IEEE.
  • Chiral Charge-Density Waves
    J. Ishioka, Y. H. Liu, K. Shimatake, T. Kurosawa, K. Ichimura, Y. Toda, M. Oda, S. Tanda
    PHYSICAL REVIEW LETTERS 105 17 2010年10月 [査読有り][通常論文]
     
    We discovered the chirality of charge-density waves (CDW) in 1T-TiSe(2) by using STM and time-domain optical polarimetry. We found that the CDW intensity becomes Ia(1):Ia(2):Ia(3) 1:0.7 +/- 0.1:0.5 +/- 0.1, where Ia(i) (i = 1, 2, 3) is the amplitude of the tunneling current contributed by the CDWs. There were two states, in which the three intensity peaks of the CDW decrease clockwise and anticlockwise. The chirality in CDW results in the threefold symmetry breaking. Macroscopically, twofold symmetry was indeed observed in optical measurement. We propose the new generalized CDW chirality H(CDW) equivalent to q(1) . (q(2) x q(3)), where q(i) are the CDW q vectors, which is independent of the symmetry of components. The nonzero H(CDW)-the triple-q vectors do not exist in an identical plane in the reciprocal space-should induce a real-space chirality in CDW system.
  • Y. Toda, S. Honda, R. Morita
    OPTICS EXPRESS 18 17 17796 - 17804 2010年08月 [査読有り][通常論文]
     
    We study the dynamics of a paired optical vortex (OV) generated by second-harmonic generation (SHG) using sub-picosecond pulses. By changing the position of a thin nonlinear crystal along the propagation direction, we observe a rotation of two vortices with changing separation distance. The dynamics is well explained by SHG with a beam walk-off, which introduces a contamination of zero-order Laguerre-Gaussian beam (LG(0)) together with topological charge doubling. The quantitative analysis indicates that the rotation angle of the OVs manifests the Gouy phase while the splitting provides the walk-off angle of the crystal. We also show that the subtraction of LG(0) is realized by the superposition of LG(0) with an anti-balanced phase in the pump. (C) 2010 Optical Society of America
  • Y. Toda, J. Sekiya, Y. H. Liu, N. Momono, M. Oda, M. Ido
    JOURNAL OF SUPERCONDUCTIVITY AND NOVEL MAGNETISM 23 5 871 - 872 2010年07月 [査読有り][通常論文]
     
    We investigate nonequilibrium quasiparticle dynamics measured by ultrafast optical spectroscopy on underdoped Bi2212 crystals, which provide direct evidence that superconducting (SC) and pseudogap (PG) quasiparticies coexist below T. We verify that the ratio of signals from SC and PG quasiparticles depends on both excitation energy and polarization of the probe beam due to the anisotropy of the probe transition matrix elements and the interband transition probability. Based on this property, we successfully separate the SC or PG component and precisely evaluate the temperature dependence of them across T-c.
  • T. Naito, M. Fukusaku, S. Matsuo, K. Tajima, T. Inabe, Y. Toda
    PHYSICA B-CONDENSED MATTER 405 11 S269 - S272 2010年06月 [査読有り][通常論文]
     
    In order to compare the carrier dynamics in lambda-(BETS)(2)MCl4 (M=Ga Fe) and kappa-(ET)(2)X (X=Cu(NCS)(2) Cu[N(CN)(2)]Br Cu[N(CN)(2)]Cl) their polarized reflectance spectra were measured by a pump-probe method Polarization-dependences were compared among the different materials at varying temperatures The result indicated that each salt had anisotropy in the conduction planes corresponding to its crystallographic axes at all the temperatures of measurements (C) 2009 Elsevier B V All rights reserved
  • J. Ishioka, Y. H. Liu, K. Shimatake, T. Kurosawa, K. Ichimura, Y. Toda, M. Oda, S. Tanda
    PHYSICA B-CONDENSED MATTER 405 11 S214 - S216 2010年06月 [査読有り][通常論文]
     
    We report the chirality of charge density waves (COW) in 1T-TiSe2 by using scanning tunneling microscopy (STM) measurements We found that the COW intensity becomes Ia(1) Ia(2) Ia(3) = 1 07 +/- 0 1 0 5 +/- 0 1 where Ia(1) (I = 1 2 3) is the amplitude of the tunneling current contributed by the CDWs We found two states in which the three intensity peaks of the CDW decrease clockwise and anticlockwise when we index each nesting vector in order of intensity in the Fourier transformation of the STM images We found that this difference arises from COW stacking along the c-axis at intervals of 2c(0)/3 This chirality is attributed to the helical stacking such as cholesteric liquid crystals (C) 2010 Elsevier B V All rights reserved
  • T. Naito, S. Matsuo, T. Inabe, Y. Toda
    JOURNAL OF PHYSICS-CONDENSED MATTER 22 18 185701-1 - 185701-7 2010年05月 [査読有り][通常論文]
     
    In order to compare the carrier dynamics and reveal the origin of differences in their electrical behaviour, time-resolved reflectivity changes were measured on single crystals of the title compounds. The observed relaxation behaviour explained well their electrical behaviour, and indicated that different conduction mechanisms dominated kappa-and lambda-type BETS salts, irrespective of local spins, at about 8-100 K; the electrical behaviour of the kappa-type salts is governed by relaxation times of carriers and/or density of states at the Fermi levels, while that of the lambda-type salts is dominated by the latter.
  • J. Onoe, A. Takashima, Y. Toda
    Appl. Phys. Lett. 97 24 241911  2010年 [査読有り][通常論文]
  • Chiral charge-density-waves
    J. Ishioka, Y.H. Liu, K. Shimatake, T. Kurosawa, K. Ichimura, Y. Toda, M. Oda, S. Tanda
    Phys. Rev. Lett. 105 176401-1 - 176401-4 2010年 [査読有り][通常論文]
  • Yu Tokizane, Katsuhiro Shimatake, Yasunori Toda, Kazuhiko Oka, Masakatsu Tsubota, Satoshi Tanda, Ryuji Morita
    OPTICS EXPRESS 17 26 24198 - 24207 2009年12月 [査読有り][通常論文]
     
    We evaluate the quasi-one-dimensional (1D) electron dynamics in a NbSe3 ring crystal using polarization vortex pulses with various azimuthal distributions. The single particle relaxation component reveals a large anisotropy on the crystal, indicating that the electrons in the ring maintain their 1D character. The results also suggest that the polarization vortex evaluates the global polarization property of the closed-loop electron that plays an important role in the quantum correlation phenomena such as the Aharonov-Bohm effect. (C) 2009 Optical Society of America
  • Y. Ueno, Y. Toda, S. Adachi, R. Morita, T. Tawara
    OPTICS EXPRESS 17 22 20567 - 20574 2009年10月 [査読有り][通常論文]
     
    We demonstrate the coherent transfer of optical orbital angular momentum (OAM) to the center of mass momentum of excitons in semiconductor GaN using a four-wave mixing (FWM) process. When we apply the optical vortex (OV) as an excitation pulse, the diffracted FWM signal exhibits phase singularities that satisfy the OAM conservation law, which remain clear within the exciton dephasing time (similar to 1ps). We also demonstrate the arbitrary control of the topological charge in the output signal by changing the OAM of the input pulse. The results provide a way of controlling the optical OAM through carriers in solids. Moreover, the time evolution of the FWM with OAM leads to the study of the closed-loop carrier coherence in materials. (C) 2009 Optical Society of America
  • Y. Toda, Y. Toda, K. Shimatake, K. Shimatake, S. Tanda, J. Onoe
    Proceedings of SPIE - The International Society for Optical Engineering 7214 2009年06月15日 [査読無し][通常論文]
     
    Due to the recent progress of material science, quasi-one-dimensional (ID) materials provide an opportunity for investigating the influence of topology and dimensionality on their optical and electrical properties. In this study, we report the phase transition properties of such quasi-lD compounds by utilizing an ultrafast optical spectroscopy. Photoexcited nonequilibrium carrier dynamics yield characteristic features around the phase transition temperatures. We also discuss the topological effects on the phase coherence of correlated carriers by using both polarization and excitation energy dependences of the transient signals and their spatial characteristics. © 2009 SPIE.
  • Yasunori Toda, Koshi Nagaoka, Katsuhiro Shimatake, Ryuji Morita
    ELECTRICAL ENGINEERING IN JAPAN 167 4 39 - 46 2009年06月 [査読有り][通常論文]
     
    Topology has become one of the key concepts allowing one to understand the intrinsic, qualitative properties of phenomena throughout various scientific fields. To date, this concept has been extended to the field of material science and technology. On the other hand, we can now utilize the spatially controlled light defined by the topology (so-called "optical vortices") in order to characterize the topological properties of materials. In particular, optical vortices in femtosecond pulses will be invaluable for advanced topological spectroscopy. In this work, the authors created femtosecond optical vortices using a spatial light modulator. Their spatiotemporal properties were evaluated using interferogram and correlation measurements. 0 2009 Wiley Periodicals, Inc. Electr Eng Jpn, 167(4): 39-46, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20791
  • Y. Toda, R. Onozaki, M. Tsubota, K. Inagaki, S. Tanda
    Phys. Rev. B 80 12 121103  2009年 [査読有り][通常論文]
  • Coherent dynamics of exciton fine structure in uniaxially-strained GaN
    T. Ishiguro, Y. Toda, S. Adachi
    J. Phys. Soc. Japan 78 174018-1 - 4 2009年 [査読有り][通常論文]
  • T. Ishiguro, Y. Toda, S. Adachi, K. Tadatomo, K. Hoshino
    PHYSICA STATUS SOLIDI C - CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 6, NO 1 6 1 42 - + 2009年 [査読有り][通常論文]
     
    Dephasing dynamics of exciton fine structure (EFS) caused by long and short range exchange interactions in GaN was investigated by using a four-wave-mixing (FWM) spectroscopy. The type of exchange interactions was selected by samples (uniaxially strained c-plane GaN films and a thick bulk GaN) and their geometries in optical excitation. Since each component of EFS is highly polarized, the polarization-dependent FWM spectra successfully resolve the dephasing dynamics in the individual EFS resonances. The results show a difference in dephasing time, suggesting a contribution of anisotropic exchange interaction (AEI). (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
  • T. Ishiguro, Y. Toda, S. Adachi, S. F. Chichibu
    PHYSICA STATUS SOLIDI C: CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 6, SUPPL 2 6 S719 - S722 2009年 [査読有り][通常論文]
     
    We report on the dephasing dynamics of exciton fine structure (EFS) in a free standing GaN. The EFS was observed and studied by utilizing a four-wave-mixing (FWM) spectroscopy. In the cleavage plane of wurtzite GaN, we can excite the B-exciton fine structure, whose z-polarized state is energetically well separated from the other in-plane polarized exciton states. Due to the large spin exchange interaction in GaN, an energy shift of in-plane polarized fine structure was also observed. Since the each component of EFS is highly polarized, the polarization-dependent FWM successfully resolves their dephasing dynamics. The results show a significant difference in the dephasing time between EFS, suggesting that anisotropic exchange interaction is effective as reported in our previous work using a GaN film on alpha-plane sapphire. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
  • Y. Toda, K. Yamaguchi, T. Ishiguro, K. Hoshino, K. Tadatomo, S. Adachi
    PHYSICA STATUS SOLIDI C: CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 6, SUPPL 2 6 S684 - S686 2009年 [査読有り][通常論文]
     
    Spatial imaging of spectrally-resolved four-wave mixing (SR FWM) was performed on a GaN film grown on c-sapphire substrate. The image obtained from the exciton resonances at zero time delay clearly showed some highly localized reductions in their intensities whose spatial profiles can be mapped out with a spatial resolution of similar to 20 m. The image obtained from the evaluation of the quantum beats indicates these reductions correspond to the phase changes between two exciton transitions, reflecting the spatial properties of exciton-exciton interactions. The line-localization suggests a contribution of line-defect to the anomalous interactions. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
  • Y. H. Liu, Y. Toda, K. Shimatake, N. Momono, M. Oda, M. Ido
    PHYSICAL REVIEW LETTERS 101 13 137003  2008年09月 [査読有り][通常論文]
     
    We report the ultrafast optical response of quasiparticles (QPs) in both the pseudogap ( PG) and superconducting (SC) states of an underdoped Bi2Sr2CaCu2O8+y (Bi2212) single crystal measured with the time-resolved pump-probe technique. At a probe energy hw(pr) = 1.55 eV, it is found that the reflectivity change Delta R/R changes its sign at exactly T-c, which allows the direct separation of the charge dynamics of PG and SC QPs. Further systematic investigations indicate that the transient signals associated with PG and SC QPs depend on the probe beam energy and polarization. By tuning them below T-c, two distinct components can be detected simultaneously, providing evidence for the coexistence of PG and SC QPs.
  • Toshio Naito, Yukari Yamada, Tamotsu Inabe, Yasunori Toda
    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 77 6 064709-1 - 064709-6 2008年06月 [査読有り][通常論文]
     
    The K-type organic superconductors have unique physical properties, many of which are yet to be understood. Most of their physical properties are closely related to carrier dynamics, which can be directly observed by ultrafast optical measurements in their nonequilibrium states. Time-resolved temperature-dependent reflectivity changes were measured on single crystals of an isostructural pair of organic charge transfer salts kappa-(ET)(2)Cu[N(CN)(2)]X, which are ambient- (X = Br) or high-pressure (X = Cl) superconductors. The latter compound in its ground state is a Mott insulator under ambient pressure. The obtained temperature dependences of relaxation times for both salts indicated the existence of an intermediate electronic state between those of metals and insulators at all measurement temperatures. Such an unusual electronic state coexisted with the superconducting and Mott insulating states in X = Br and X = Cl, respectively. In addition, the results for X = Br revealed the formation of a temperature-dependent well-defined gap in the superconducting transition, while those for X = Cl suggested that the Mott transition in this salt does not depend on electron-phonon interactions, consistent with the theory that the Mott transition should originate from electron correlation.
  • Y. Toda, T. Ishiguro, S. Adachi, K. Hoshino, K. Tadatomo
    PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS 245 5 878 - 880 2008年05月 [査読有り][通常論文]
     
    We have previously demonstrated that a spectrally-resolved four-wave mixing highlights the anisotropically polarized excitons in the uniaxially strained GaN films [1], where the enhancement of sensitivity for an uniaxial strain, was achieved by nonlinear response of the exciton polarization. In this work, we have applied this technique to the GaN films on various substrates including isotropic ones, and have examined the sensitivity of the technique. The results show distinct polarizations and energy variations originating from the uniaxial strain, the magnitude of which depends on the sample and its position: The minimum changes of the polarized FWM intensity and exchange energy splittings correspond to a uniaxial strain of 5.0 x 10(-5), which currently gives a lower resolution limit of this technique. (C) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
  • T. Ishiguro, T. Ishiguro, Y. Toda, Y. Toda, S. Adachi, S. F. Chichibu
    Proceedings of SPIE - The International Society for Optical Engineering 6892 2008年04月21日 [査読無し][通常論文]
     
    We perform degenerate four-wave-mixing (FWM) studies of GaN excitons especially for an understanding of the strain-fields in the heteroepitaxial films. The shifts of exciton energies and their beating oscillation variations highlight the biaxial strain, allowing for a precise determination of the strain parameters. The uniaxial strain field can be characterized by the polarization dependence of FWM, which shows distinct polarizations and energy variations depending on the sample and its position. The minimum changes of the polarized FWM intensity and exchange energy splittings correspond to a uniaxial strain of 5.0 × 10-5, which currently gives a lower resolution limit of this technique and is comparable with that of conventional X-ray diffraction. In the time-evolutions, we investigate the strain effects on the phase of the quantum beats (QBs), giving insight into the excitons interactions. By using time-resolved FWM, difference between two-types of exciton transitions is identified. In addition, coherent manipulations of QBs are successfully realized in the FWM with a Michelson interferometer..
  • Y. Toda, S. Ryuzaki, J. Onoe
    APPLIED PHYSICS LETTERS 92 9 094102  2008年03月 [査読有り][通常論文]
     
    Time-resolved temperature-dependent transmission changes were measured for both pristine C-60 and electron-beam-irradiated C-60 polymer films using an optical pump-probe technique. Only the signals obtained for the C-60 polymer show a temperature-dependent slow decay, which appears in the low temperature region below similar to 60 K. This slow decay component exhibits a monotonic increase in both relaxation time and amplitude with decreasing temperature, providing evidence of gap formation associated with ordering fluctuations. (c) 2008 American Institute of Physics.
  • T. Ishiguro, Y. Toda, S. Adachi, K. Hoshino, Y. Arakawa
    PHYSICA STATUS SOLIDI C - CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 5, NO 6 5 6 2114 - + 2008年 [査読有り][通常論文]
     
    Dephasing dynamics of excitons in a GaN film grown on a-plane sapphire were studied by using the spectrally-resolved four-wave-mixing (SR-FWM) spectroscopy. Due to the anisotropic exchange interaction originates from the anisotropic strain field of the substrate, the degenerate exciton states with the total angular moment +/- 1 mix and split into their fine structures. Since each component of the exciton fine structured (EFS) is highly polarized, the polarization-dependent SR-FWM spectra successfully resolve the dephasing dynamics in the individual EFS resonances. The result of each EFS shows a significantly different dephasing time, suggesting that the anisotropic dephasing contribution occurs in each EFS.
  • K. Shimatake, Y. Toda, S. Tanda
    PHYSICAL REVIEW B 75 11 115120  2007年03月 [査読有り][通常論文]
     
    We report a study of the ultrafast single particle dynamics in quasi-one-dimensional NbSe3 whisker crystals using a two-color pump-probe measurement. A selective enhancement of the relaxation dynamics connected with one of the two charge-density-wave (CDW) phases was realized by tuning the probe energy to around 1.5 eV, in which the temperature dependence across the corresponding phase transition shows a pronounced anomaly. A similar selective excitation was possible in the coherent CDW oscillation.
  • Tatsuyuki Onozaki, Yasunori Toda, Satoshi Tanda, Ryuji Morita
    JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS BRIEF COMMUNICATIONS & REVIEW PAPERS 46 2 870 - 872 2007年02月 [査読有り][通常論文]
     
    The coherent control of a charge-density-wave (CDW) collective mode in 1T-TaS2 was performed using a femosecond pulse pair. Selective enhancement and suppression of the oscillation were achieved by optimal pulse separation. The ratio of enhancement to Suppression and pulse-separation dependence of the amplitude show a good correspondence with theoretical evaluation in the pulse-separation time range -0.5 to 1 ps.
  • K. Yamaguchi, Y. Toda, T. Ishiguro, S. Adachi, K. Hoshin, K. Tadatomo
    PHYSICA STATUS SOLIDI C - CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 4 NO 7 2007 4 7 2752 - + 2007年 [査読有り][通常論文]
     
    Quantum beats in the time-resolved four-wave mixing (FWM) signals arising from GaN excitons are observed. Two different types of beats are investigated: beats between the A-exciton (X-A) and B-exciton (X-B) states; beats between the XA and A-biexciton ((XXA)) states. The former exhibits oscillations along all directions in the delay- (T) and real-time (t) plane while the latter exhibits no beat in the direction parallel to the diagonal axis (T = t). The results are well reproduced by a model calculation, showing good homogeneity of the sample.
  • T. Ishiguro, Y. Toda, S. Adachi, K. Hazu, T. Sota, S. F. Chichibu
    PHYSICA STATUS SOLIDI C - CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 4 NO 7 2007 4 7 2776 - + 2007年 [査読有り][通常論文]
     
    An optical coherent manipulation of excitonic polarizations in hexagonal bulk GaN is achieved by means of an interferometric technique combined with spectrally-resolved degenerate four-wave-mixing (FWM). Initial phase of the FWM quantum beats between A and B excitonic polarizations are arbitrarily controlled, in which simultaneous increase (decrease) of the beats in both polarizations are observed. In addition, 80% selective amplification and 40% suppression of one of the excitonic polarizations are realized.
  • T. Ishiguro, Y. Toda, S. Adachi
    APPLIED PHYSICS LETTERS 90 1 11904  2007年01月 [査読有り][通常論文]
     
    The authors studied the exciton spin relaxation of bulk GaN by creating spin polarization gratings using degenerate four-wave mixing spectroscopy. The spectrally resolved analysis achieved with this technique facilitated the direct evaluation of spin polarizations in the individual excitons (A and B excitons). The spin polarizations for each exciton decay very quickly (tau(s)similar to 1 ps) at low temperatures. Moreover the tau(s) is faster than the dephasing time T-2 throughout the measured temperature range, suggesting the existence of fast intrinsic spin relaxation processes, which can be attributed to a large exchange constant characterized in GaN.
  • 戸田 泰則, 長岡 耕志, 島竹 克大, 森田 隆二
    電気学会論文誌C(電子・情報・システム部門誌) 127 9 1308 - 1313 2007年 [査読無し][通常論文]
  • T Ishiguro, Y Toda, S Adachi, T Mukai, K Hoshino, Y Arakawa
    PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS 243 7 1560 - 1563 2006年06月 [査読有り][通常論文]
     
    We have carried out time-integrated and spectrally-resolved four-wave mixing (FWM) measurements to study the effects of built-in strain within GaN films on their exciton structures and on their coherent dynamics. Precise values for exciton energies, linewidths, and oscillation periods of quantum beats were presented. Remarkable phase shifts of the quantum beats were observed in the GaN films on a-sapphire, suggesting the exciton-exciton interaction caused by the reduction of the crystal symmetry.
  • T Ishiguro, Y Toda, S Adachi, M Arita, Y Arakawa
    PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS 243 7 1564 - 1567 2006年06月 [査読有り][通常論文]
     
    We have studied exciton spin relaxation in strained GaN epilayers on various sapphire substrates using spin-dependent transient reflectivity and three-pulse four-wave mixing measurements. Strikingly fast exciton spin relaxation (< 1 ps) at 10 K was observed in the lowest exciton level (A-exciton) both in bulk and GaN layer on c-sapphire. In contrast, the transient signals in the GaN layer on alpha-sapphire show the break down of the spin polarization caused by the exchange interaction.
  • Satoru Adachi, Yasunori Toda, Tetsuro Ishiguro
    PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS 243 7 1568 - 1571 2006年06月 [査読有り][通常論文]
     
    Biexcitons of GaN under uniaxial strain have been investigated by spectrally-resolved four-wave mixing (FWM) technique. Two kinds of biexcitons were observed clearly and the binding energies were obtained. Since the uniaxial strain changes the exciton structure, the biexciton binding is modified and biexciton has also fine structure, which was revealed in the FWM spectra that were obtained by rotating the linearly polarized excitation against the direction of uniaxial strain.
  • K Shimatake, Y Toda, S Tanda
    PHYSICAL REVIEW B 73 15 153403  2006年04月 [査読有り][通常論文]
     
    A comparison of the single-particle (SP) dynamics of whisker and ring NbSe3 crystals provides new insight into the phase transition properties of quasi-one-dimensional charge density wave (CDW) systems. In the incommensurate CDW phase, SP relaxation triggered by an ultrafast laser pulse reflects the formation of collective states, and reveals the divergence of the relaxation time when approaching a transition temperature. The degree of divergence is less pronounced in rings than in whiskers, suggesting a loss of phase coherence in ring crystals characterized by a closed-loop topology.
  • K. Shimatake, Y. Toda, S. Tanda
    Conference on Lasers and Electro-Optics and 2006 Quantum Electronics and Laser Science Conference, CLEO/QELS 2006 2006年 [査読有り][通常論文]
     
    Ultrafast electron dynamics in quasi-one-dimensional NbSe3 conductors was investigated by means of two-color pump-probe reflectivity measurements. The results clearly demonstrate the resonant probing of the charge-density-wave phases with different transition properties. © 2005 Optical Society of America.
  • Satoru Adachi, Yasunori Toda, Tetsuro Ishiguro, Katsuyuki Hoshino, Yasuhiko Arakawa
    PHYSICA STATUS SOLIDI C - CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 3, NO 6 3 6 1595 - 1598 2006年 [査読無し][通常論文]
     
    Exciton fine structures induced by the uniaxial strain have been revealed in gallium nitride grown on an a-plane sapphire substrate by using the spectrally-resolved four-wave mixing technique. Sensitivity on the exciton polarization is enhanced by the nonlinearities of the four wave mixing signals, and therefore the safety alternative tool for mapping of the uniaxial strain field and the resultant anisotropic exchange can be offered. Such a nonlinear spectroscopic measurement gives a kind of crystalline analysis achieved in X-ray diffraction spectroscopy as well as information of the exciton fine structures including their temporal dynamics. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
  • T Minami, Y Toda, K Shimatake, S Tanda
    JOURNAL DE PHYSIQUE IV 131 121 - 122 2005年12月 [査読有り][通常論文]
     
    We have investigated ultrafast carrier dynamics in a quasi-one-dimensional metal m-TaS3 by using pump-probe reflectivity measurements. The temperature dependence of transient signal reveals two critical changes at T-p = 240K and T-c similar to 100K. Below T-c a new low-frequency oscillation mode appears and shows a pronounced softening depending oil the temperature, indicating a collective excitation connected with the commensurability of CDW phase. This soft-mode exhibits remarkable polarization anisotropy with respect to the chain direction.
  • K Shimatake, Y Toda, T Minami, S Tanda
    JOURNAL DE PHYSIQUE IV 131 295 - 298 2005年12月 [査読有り][通常論文]
     
    Influence of the crystal topology on the Peierls transition was investigated by means of ultrafast optical pump-probe measurements. The comparison of the exponential single particle decay between the whisker and ring NbSe3 compounds reveals several pronounced differences around a phase transition attire (T-p). A discrepancy from BCS-like temperature dependence of the gap was observed in the ring, suggesting a loss of phase coherence between adjacent chains in the closed-loop topology.
  • Y. Abe, Y. Toda, K. Hoshino, Y. Arakawa
    Japanese Journal of Applied Physics 44 16-19 L535 - L538 2005年12月01日 [査読有り][通常論文]
     
    Two-photon absorption (TPA) imaging, based on a transient pump-probe measurement, has been demonstrated for characterizing the nonlinear optical properties of GaN films in three-dimensions. The use of near-infrared excitation light demonstrated the capability to probe changes in TPA inside the structure, which is closely associated with fluctuations in defect states. The TPA coefficient at 1.55 eV was evaluated to be ∼0.12 cm/GW for a standard GaN film. A spatial resolution better than λ/2, for an excitation wavelength of 800nm, was achieved. © 2005 The Japan Society of Applied Physics.
  • S Adachi, Y Toda
    PHYSICA STATUS SOLIDI C - CONFERENCES AND CRITICAL REVIEWS, VOL. 2 , NO 11 2 11 3936 - 3940 2005年 [査読有り][通常論文]
     
    Spectrally-resolved four-wave mixing technique has revealed the internal structures of excitons in uniaxially strained gallium nitride films. Nonlinear response of four-wave mixing on the oscillator strength highlights the polarized excitons, and therefore allows us to map out the uniaxial strain field and the resultant anisotropic exchange splitting. Such a measurement gives a kind of crystalline analysis achieved in X-ray diffraction spectroscopy as well as information of the exciton fine structures including their temporal dynamics.
  • Optical diffraction spectroscopy of excitons in uniaxially-strained GaN films
    Y. Toda, S. Adachi, Y. Abe, K. Hoshino, Y. Arakawa
    Physical Review B 71 195315  2005年 [査読有り][通常論文]
  • Satoru Adachi, Yasunori Toda
    Physica Status Solidi C: Conferences 2 11 3936 - 3940 2005年 [査読有り][通常論文]
     
    Spectrally-resolved four-wave mixing technique has revealed the internal structures of excitons in uniaxially strained gallium nitride films. Nonlinear response of four-wave mixing on the oscillator strength highlights the polarized excitons, and therefore allows us to map out the uniaxial strain field and the resultant anisotropic exchange splitting. Such a measurement gives a kind of crystalline analysis achieved in X-ray diffraction spectroscopy as well as information of the exciton fine structures including their temporal dynamics. © 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
  • K Itoh, Y Toda, R Morita, M Yamashita
    JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS SHORT NOTES & REVIEW PAPERS 43 9A 6448 - 6451 2004年09月 [査読有り][通常論文]
     
    By using Raman-induced Kerr-effect spectroscopy, two-pulse excitations with different polarization combinations have been demonstrated for the selective coherent excitation of molecular vibrations in liquid CCl4. Anti correlated selective excitations as well as the suppressions of the two asymmetric vibrational motions were observed in the opposite configuration of polarization between the two-pump pulses. The measurements show that we can manipulate molecular motions by controlling the optical phase of sequential pulses through timing and polarization.
  • Y. Toda, K. Tateishi, S. Tanda
    Physical Reiew B 70 33106  2004年07月 [査読有り][通常論文]
     
    We have investigated the coherent dynamics of the collective motions of phonons in the commensurate phase of quasi-two-dimensional 1T-TaS2 using femtosecond laser pulses. One of the coherent phonons increased in intensity as a function of time, this behavior being strongly correlated with the decay of charge-density-wave oscillations as well as the slow decay component in single-particle relaxation.
  • Y Toda, K Tateishi, S Tanda
    PHYSICAL REVIEW B 70 3 2004年07月 [査読有り][通常論文]
     
    We have investigated the coherent dynamics of the collective motions of phonons in the commensurate phase of quasi-two-dimensional 1T-TaS2 using femtosecond laser pulses. One of the coherent phonons increased in intensity as a function of time, this behavior being strongly correlated with the decay of charge-density-wave oscillations as well as the slow decay component in single-particle relaxation.
  • Y Toda, T Nakaoka, R Morita, M Yamashita, T Inoue, Y Arakawa
    PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES 21 2-4 180 - 183 2004年03月 [査読有り][通常論文]
     
    We carried out optical selective excitation of individual self-assembled quantum dots by using phase-modulated pulses. Based on scattered photoluminescence excitation resonances in individual QDs, the excitation pulses modulated in the spectral region allows for addressing individual ground states emission. The photoluminescence spectra including several QDs showed intensity changes according to both the modulation energies and phases. The results also suggested the individual control of selective QDs even in collective excitation. (C) 2003 Elsevier B.V. All rights reserved.
  • Y Toda, T Matsubara, R Morita, M Yamashita, K Hoshino, T Someya, Y Arakawa
    APPLIED PHYSICS LETTERS 82 26 4714 - 4716 2003年06月 [査読有り][通常論文]
     
    Optical nonlinearity in the yellow luminescence (YL) band of GaN was investigated using thick bulk samples. Transient pump-probe measurements revealed strong transmission changes due to two-photon absorption (TPA) even at the middle of the YL band. The TPA coefficient evaluated reaches similar to5 cm/GW at about 1.3 eV, which was as large as the mid-gap resonance. The TPA spectrum clearly showed that the observed large nonlinearity originated from the YL band. On the basis of efficient TPA in the YL band, relaxation processes in the multiphoton-induced photoluminescence excitation spectrum were also investigated. (C) 2003 American Institute of Physics.
  • Y Toda, T Matsubara, R Morita, K Hoshino, T Someya, Y Arakawa
    5TH INTERNATIONAL CONFERENCE ON NITRIDE SEMICONDUCTORS (ICNS-5), PROCEEDINGS 0 7 2627 - 2630 2003年 [査読有り][通常論文]
     
    Transient transmission changes in the yellow luminescence (YL) band of GaN were investigated using excitation far smaller than the bandgap. At lower excitation power, time-resolved measurements revealed efficient transmission changes due to two-photon absorption (TPA), whose spectrum clearly indicates the contribution of the YL band. Upon increasing the excitation power, the transient signal showed an additional offset signal. The excitation power dependences of the offset suggested a two-step absorption process in conjunction with TPA in the YL band. The excitation energy dependence also supported this conclusion. (C) 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
  • T Inoue, Y Toda, K Hoshino, T Someya, Y Arakawa
    5TH INTERNATIONAL CONFERENCE ON NITRIDE SEMICONDUCTORS (ICNS-5), PROCEEDINGS 0 7 2428 - 2431 2003年 [査読有り][通常論文]
     
    Temperature-dependent Raman spectroscopy was used to analyze the effect of residual strain on optical phonons E-2 (high) and A(1) (LO) in high-quality GaN films grown on C-plane 6H-SiC, A- and C-plane sapphire substrates. By comparison with a free-standing GaN, we found that shifts in the Raman spectrum were consistent with compressive and tensile strains. Experimental results indicated that differences in the thermal expansion coefficients between GaN film and substrate have an effect on these frequency shifts. We also compared frequency shifts of the A(1) (LO) mode with those of the E-2 (high) mode and concluded that the strain-induced frequency shift of the A(1) (LO) mode was smaller than that of the E-2 (high) mode. The strain-tensor-dependences of each phonon mode are also discussed. (C) Wiley-VCH Verlag GmbH & Co. KgaA, Weinheim
  • Y. Toda, T. Inoue, T. Nakaoka, S. Ishida, Y. Arakawa
    Pacific Rim Conference on Lasers and Electro-Optics, CLEO - Technical Digest 2 448  2003年 [査読有り][通常論文]
     
    Phonon-assisted transitions in single self-assembled InGaAs quantum dots are investigated by photoluminescence (PL) and PL excitation (PLE) spectroscopy. Under nearly resonant conditions of the excitonic ground state, a broad sideband is observed in both PL and PLE. The adiabatic description with the localized acoustic phonons well explains the observed spectra.
  • Y. Toda, T. Matsubara, R. Morita, K. Hoshino, T. Someya, Y. Arakawa
    Physica Status Solidi C: Conferences 7 2627 - 2630 2003年 [査読有り][通常論文]
     
    Transient transmission changes in the yellow luminescence (YL) band of GaN were investigated using excitation far smaller than the bandgap. At lower excitation power, time-resolved measurements revealed efficient transmission changes due to two-photon absorption (TPA), whose spectrum clearly indicates the contribution of the YL band. Upon increasing the excitation power, the transient signal showed an additional offset signal. The excitation power dependences of the offset suggested a two-step absorption process in conjunction with TPA in the YL band. The excitation energy dependence also supported this conclusion. © 2003 WILEY-VCH Verlag GmbH & Co. KGaA.
  • T. Inoue, Y. Toda, K. Hoshino, T. Someya, Y. Arakawa
    Physica Status Solidi C: Conferences 7 2428 - 2431 2003年 [査読有り][通常論文]
     
    Temperature-dependent Raman spectroscopy was used to analyze the effect of residual strain on optical phonons E2 (high) and A1 (LO) in high-quality GaN films grown on C-plane 6H-SiC, A- and C-plane sapphire substrates. By comparison with a free-standing GaN, we found that shifts in the Raman spectrum were consistent with compressive and tensile strains. Experimental results indicated that differences in the thermal expansion coefficients between GaN film and substrate have an effect on these frequency shifts. We also compared frequency shifts of the A1 (LO) mode with those of the E2 (high) mode and concluded that the strain-induced frequency shift of the A1 (LO) mode was smaller than that of the E2 (high) mode. The strain-tensor-dependences of each phonon mode are also discussed. © 2003 WILEY-VCH Verlag GmbH & Co. KGaA.
  • Y Toda, S Kako, M Nishioka, Y Arakawa
    JAPANESE JOURNAL OF APPLIED PHYSICS PART 2-LETTERS & EXPRESS LETTERS 41 12B L1464 - L1466 2002年12月 [査読有り][通常論文]
     
    Photoluminescence excitation resonances in the multiphonon energy region of single self-assembled quantum dots are investigated. Several multiphonon resonances exhibit dip structures under the saturation condition, whereas the corresponding biexciton resonances have a sharp linewidth. The observed dip structures and inversely correlated biexciton resonances suggest that the photoluminescence excitation resonances include line broadening consisting of several relaxation pathways with different relaxation rates.
  • S. Kako, T. Sugimoto, Y. Toda, S. Ishida, M. Nishioka, Y. Arakawa
    Physica E: Low-Dimensional Systems and Nanostructures 13 2-4 151 - 154 2002年 [査読有り][通常論文]
     
    We have observed dip structures in photoluminescence excitation (PLE) spectra of a highly excited single InGaAs/GaAs self-assembled quantum dot. Some of the exciton PLE peaks lying in continuum states turn into the dips with increasing excitation power. The most remarkable feature is that the biexciton-PLE shows peaks at the excitation energies where the exciton PLE shows the dip structures. Using a model based on the excitation power dependence of the photoluminescence intensity, we discuss the origin of the dip structures. © 2002 Published by Elsevier Science B.V.
  • Y. Toda, M. Takahashi, T. Sugimoto, M. Nishioka, S. Ishida, Y. Arakawa
    Technical Digest - Summaries of Papers Presented at the Quantum Electronics and Laser Science Conference, QELS 2001 20 - 21 2001年 [査読有り][通常論文]
     
    Summary form only given. The study of quantum mechanical interactions between excited carriers in the semiconductor quantum dots (QDs) provides better understanding of zero-dimensional (0D) systems. One of the remarkable features of QDs is their atomic like density of states (DOS). Recently, we performed single dot spectroscopy on InGaAs self-assembled quantum dots (SAQDs) grown by Stranski-Krastnow mode, and found that their photoluminescence excitation (PLE) spectra exhibit 2D-like continuum states as well as discrete DOS structures. Because SAQDs are connected with 2D wetting layer (WL), it is supposed that the observed continuum originate from a crossover from 0D to 2D nature in their DOS. On the other hand, excited carriers in a single discrete state within a continuum show asymmetric resonance in their optical spectrum. This asymmetry is produced by interference between the optical transitions of a discrete energy state with continuum, the so-called Fano interference. In this paper, we carried out PLE spectroscopy with high excitation power and observed asymmetric resonances due to the existence of continuum states.
  • Y Toda, T Sugimoto, M Nishioka, Y Arakawa
    APPLIED PHYSICS LETTERS 76 26 3887 - 3889 2000年06月 [査読有り][通常論文]
     
    Using near-field optical microscopy, we have performed coherent excitation spectroscopy of self-assembled quantum dots (SAQDs). A pair of coherent pulses with a time delay between them allows measurement of the temporal coherence of the carrier wave function in single quantum dots. The observed decoherence time is about 15 ps and is well explained by resonant Raman scattering of phonons. Furthermore, quantum beats originating from the superposition of two closely spaced coherent states have been observed. This opens up possibilities of quantum mechanical control of the carrier wave function in SAQDs. (C) 2000 American Institute of Physics. [S0003-6951(00)01426-1].
  • Y. Toda, O.Moriwaki, M. Nishioka, Y. Arakawa
    PhysicaE 8 328  2000年 [査読有り][通常論文]
  • Y. Toda, Y. Arakawa
    IEEE Journal of Selected Topics in Quantum electronics 6 3 528 - 533 2000年 [査読有り][招待有り]
  • Y Toda, K Suzuki, S Shinomori, Y Arakawa
    MICROELECTRONIC ENGINEERING 47 1-4 111 - 113 1999年06月 [査読有り][通常論文]
     
    We have investigated carrier relaxations in single InAs/GaAs self-assembled quantum dots using near-field optical spectroscopy. Polarized PL spectroscopy of Zeeman splitting peaks with an external magnetic field allows us to investigate spin relaxation in the excited states. The excitation energy dependence of the polarization of each spin component shows suppression of spin flipping during the relaxation of carriers inside the quantum dots. The result also suggests different spin relaxation processes between in the quantum dots and in a wetting layer.
  • Y Toda, O Moriwaki, M Nishioka, Y Arakawa
    PHYSICAL REVIEW LETTERS 82 20 4114 - 4117 1999年05月 [査読有り][通常論文]
     
    Comparison of near-field and far-field photoluminescence excitation (PLE) spectra gives new insight into the carrier relaxation process in InGaAs/GaAs self-assembled quantum dots. The near-field PLE spectra of single quantum dots clearly show 2D-like continuum states and a number of sharp lines, between a large zero-absorption region due to the quasi-0D density of states and the 2D wetting layer absorption edge. The results reveal an efficient intradot relaxation mechanism, proceeding as follows: The carriers can relax easily within continuum states, and make transitions to the excitonic ground state by resonant emission of localized phonons.
  • Polarized photoluminescence spectroscopy of single self-assembled InAs quantum dots
    Y Toda, S Shinomori, K Suzuki, Y Arakawa
    PHYSICAL REVIEW B 58 16 R10147 - R10150 1998年10月 [査読有り][通常論文]
     
    We have investigated the spin relaxation of carriers in single InAs/GaAs self-assembled quantum dots using near-field optical spectroscopy. Polarized photoluminescence spectroscopy of Zeeman split peaks in an external magnetic field allows us to investigate spin relaxation in the excited states. The excitation energy dependence of the polarization of each spin component shows the suppression of spin relaxation to be quenched at higher relaxation energies. This effect suggests a change in relaxation processes in the self-assembled dot systems. [S0163-1829(98)51740-7].
  • Y Toda, S Shinomori, K Suzuki, Y Arakawa
    APPLIED PHYSICS LETTERS 73 4 517 - 519 1998年07月 [査読有り][通常論文]
     
    We report observations of Zeeman spin splitting in photoluminescence from single InAs/GaAs self-assembled quantum dots (SAQDs) fabricated by Stranski-Krastanow growth. A low-temperature near-field optical microscope incorporating a superconducting magnet enabled us to resolve the luminescence from individual SAQDs. We have measured Zeeman split components from single SAQDs in magnetic fields up to 10 T. Circularly-polarized photoluminescence resolved carriers of different spin polarity. Dependence of the splitting on the QD structures was also investigated. (C) 1998 American Institute of Physics.
  • Y Toda, S Shinomori, T Arakawa, Y Arakawa
    PHYSICA E 2 1-4 987 - 990 1998年07月 [査読有り][通常論文]
     
    Using near-field optical microscopy we present experimental results on spatially resolved PL spectra of GaAs quantum wire (QWR) structures with a lateral width of 15 nm fabricated by selective growth method. The geometries of the samples make it possible to observe the luminescence from a single QWR structure. The typical fluctuations of the PL peak energy from different QWRs is estimated to be about 5 meV. To reduce the diffusion of carriers from barrier regions, PL spectra are observed by selective excitation of the QWR. A PL linewidth of 5 meV, which is about ten times narrower than that of the conventional PL measurement, is observed. (C) 1998 Elsevier Science B.V. All rights reserved.
  • Near-field optical spectroscopy of selfassembled quantum dots: NSOM apparatus for measuring the features of single dots
    Y Toda, S Shinomori, K Suzuki, Y Arakawa
    SOLID-STATE ELECTRONICS 42 7-8 1083 - 1086 1998年07月 [査読有り][通常論文]
     
    We discuss the realization of a low-temperature near-held optical microscope incorporating a superconducting magnet. Experimental results on spatially resolved photoluminescence (PL) spectra of a single quantum dot (QD) fabricated by Stranski-Krastanow growth demonstrate the performance of this apparatus. The luminescence linewidth was estimated to be less than 0.4 meV -the limit of our spectrometer. The PL due to individual QDs was spatially and spectrally resolved. PL peaks from excited states of single QDs were identified. Small diamagnetic energy shifts (< 100 mu eV@10 T) were observed for the single QDs. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.
  • Y. Toda, S. Shinomori, K. Suzuki, Y. Arakawa
    Physical Review B 58 16 R10147 - R10150 1998年 [査読有り][通常論文]
  • Y. Toda, S. Shinomori, K. Suzuki, Y. Arakawa
    Physical Review B - Condensed Matter and Materials Physics 58 16 R10147 - R10150 1998年 [査読有り][通常論文]
     
    We have investigated the spin relaxation of carriers in single InAs/GaAs self-assembled quantum dots using near-field optical spectroscopy. Polarized photoluminescence spectroscopy of Zeeman split peaks in an external magnetic field allows us to investigate spin relaxation in the excited states. The excitation energy dependence of the polarization of each spin component shows the suppression of spin relaxation to be quenched at higher relaxation energies. This effect suggests a change in relaxation processes in the self-assembled dot systems. © 1998 The American Physical Society.
  • Spectroscopy of a single quantum dot by NSOM for photonic device applications
    Y Arakawa, Y Toda, S Shinomori
    PHYSICS AND SIMULATION OF OPTOELECTRONIC DEVICES VI, PTS 1 AND 2 3283 785 - 792 1998年 [査読有り][通常論文]
     
    We report photoluminescence (PL) studies of zero-dimensional excitonic confinement in InAs/GaAs self-assembled quantum dots (SAQDs) fabricated by Stranski-Krastanow growth. A low-temperature near-field optical microscope is employed to identify individual PL peaks due to single SAQDs. Highly spatial and spectral evaluation of each PL peak identify the Zeeman spin splitting with the magnetic field up to 10 T. The comparison of different confinement structures was also discussed.
  • Y Toda, M Kourogi, M Ohtsu, Y Nagamune, Y Arakawa
    APPLIED PHYSICS LETTERS 69 6 827 - 829 1996年08月 [査読有り][通常論文]
     
    We present experimental result on spatially and spectrally resolved imaging of GaAs quantum dot (QD) structures using a near-field optical microscope. Three photoluminescence (PL) peaks which originate from QD, quantum well (QW), and bulk regions were observed at liquid-He temperature. Carriers were observed to diffuse effectively from the AlGaAs barriers to the QD and QW regions. The region of the intension carrier capture manifests itself by emitting a sharp PL peak, and is estimated to be about 300 nm. (C) 1996 American Institute of Physics.
  • Y TODA, M OHTSU
    IEEE PHOTONICS TECHNOLOGY LETTERS 7 1 84 - 86 1995年01月 [査読有り][通常論文]
     
    We propose a high-resolution diagnostics technique for optical waveguides using a photon-scanning tunneling microscope (P-STM). This technique has the advantageous capabilities of nondestructive measurement and informative analysis of guided mode. The capability of this technique was evaluated by measuring the characteristics of an LiTaO3 waveguide. Scattered light spot of 500-nm diameter caused by defects was measured, and a normalized width was estimated to be 3.7 mu m. Moreover, we obtained the power dispersion in Y-branch waveguides, which were in good agreement with the theoretical plots calculated by the beam propagation method (BPM).
  • Frequency Stabilization of 1.5m Diode Laser Using Nonlinear Optical Freauency Conversion in Organic Fiber
    Y. Toda, T. Enami, M. Ohtsu
    Japanese Journal of Applied Physics 32 L1233  1993年 [査読有り][通常論文]
  • 1.5 MU-M DIODE LASER-BASED NONLINEAR FREQUENCY CONVERSIONS BY USING POTASSIUM TITANYL PHOSPHATE
    WZ WANG, K NAKAGAWA, Y TODA, M OHTSU
    APPLIED PHYSICS LETTERS 61 16 1886 - 1888 1992年10月 [査読有り][通常論文]
     
    Both second harmonic and sum-frequency generations by using 1.5 mum diode lasers in a type II phase matched potassium titanyl phosphate are demonstrated for the first time. A maximum second harmonic output power of 1.0 muW is obtained with the use of an external builtup cavity, and a 0.23 muW sum-frequency output power at 0.54 mum is obtained by using 1.5 and 0.82 mum diode lasers.

書籍

  • Laser Spectroscopy Using Topological Light Beams
    Yasunori Toda (担当:共著)
    Springer Series in Progress in Nanophotonics 3 2014年
  • 光物性入門
    戸田泰則 (担当:共著)
    朝倉書店 2009年
  • ナノオプティクス:ナノスケールの光物性
    戸田泰則 (担当:共著)
    オーム社 2004年
  • Optical Characterization of In(Ga)As/GaAs Self-assembled Quantum Dots Using Near-Field Spectroscopy
    戸田泰則 (担当:共著)
    Springer Series in Optical Sciences Volume 86 2003年
  • ナノ光工学ハンドブック
    戸田泰則 (担当:共著)
    朝倉書店 2002年

その他活動・業績

受賞

  • 2015年 応用物理学会 第37回優秀論文賞
     
    受賞者: 戸田泰則

共同研究・競争的資金等の研究課題

  • 光の軌道角運動量を用いた巨視的量子相関ダイナミクス探索
    日本学術振興会:基盤研究(B)
    研究期間 : 2016年 -2018年 
    代表者 : 戸田泰則
  • 偏光渦パルスを用いた時間分解コヒーレントポラリメトリ
    日本学術振興会:挑戦的萌芽研究
    研究期間 : 2015年 -2016年 
    代表者 : 戸田泰則
  • トポロジカル変換を用いた縮退四光波混合分光の多次元化
    日本学術振興会:挑戦的萌芽研究
    研究期間 : 2013年 -2014年 
    代表者 : 戸田泰則
  • ビスマス系高温超伝導体における光誘起超伝導相転移ダイナミクスの研究
    日本学術振興会:基盤研究(B)
    研究期間 : 2012年 -2014年 
    代表者 : 戸田泰則
  • 時空間制御光波を用いた電子相関物質における巨視的量子コヒーレンスの探索研究
    日本学術振興会:優秀若手研究者海外派遣事業
    研究期間 : 2010年03月 -2011年01月 
    代表者 : 戸田泰則

教育活動情報

主要な担当授業

  • Optical Properties of Materials(光物性E)
    開講年度 : 2018年
    課程区分 : 修士課程
    開講学部 : 工学院
    キーワード : 反射・屈折、光の吸収・放出、誘電関数、ローレンツ・モデル、ポラリトン、光学遷移、ラビ振動、半導体、金属、超伝導、励起子、スピン、光誘起相転移
  • 光物性特論
    開講年度 : 2018年
    課程区分 : 修士課程
    開講学部 : 工学院
    キーワード : 反射・屈折、光の吸収・放出、誘電関数、ローレンツ・モデル、ポラリトン、光学遷移、ラビ振動、半導体、金属、超伝導、励起子、スピン、光誘起相転移
  • Optical Properties of Materials(光物性E)
    開講年度 : 2018年
    課程区分 : 博士後期課程
    開講学部 : 工学院
    キーワード : 反射・屈折、光の吸収・放出、誘電関数、ローレンツ・モデル、ポラリトン、光学遷移、ラビ振動、半導体、金属、超伝導、励起子、スピン、光誘起相転移
  • 光物性特論
    開講年度 : 2018年
    課程区分 : 博士後期課程
    開講学部 : 工学院
    キーワード : 反射・屈折、光の吸収・放出、誘電関数、ローレンツ・モデル、ポラリトン、光学遷移、ラビ振動、半導体、金属、超伝導、励起子、スピン、光誘起相転移
  • 応用物理学
    開講年度 : 2018年
    課程区分 : 学士課程
    開講学部 : 工学部
    キーワード : ナノテク・ナノサイエンス、トポロジー理工学, 元素、量子力学
  • 応用物理学
    開講年度 : 2018年
    課程区分 : 学士課程
    開講学部 : 工学部
    キーワード : ナノテク・ナノサイエンス、光科学、量子力学、トポロジー理工学
  • 応用数学Ⅰ
    開講年度 : 2018年
    課程区分 : 学士課程
    開講学部 : 工学部
    キーワード : 常微分方程式 ラプラス変換 フーリエ解析 偏微分方程式
  • 応用数学演習Ⅰ
    開講年度 : 2018年
    課程区分 : 学士課程
    開講学部 : 工学部
    キーワード : 常微分方程式、偏微分方程式、フーリエ級数、ラプラス変換   
  • 電子工学
    開講年度 : 2018年
    課程区分 : 学士課程
    開講学部 : 工学部
    キーワード : 電子回路、過渡応答、周波数解析、半導体、増幅回路

大学運営

委員歴

  • 2015年03月 - 現在   応用物理学会   代議員


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