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Yasunori Toda
Faculty of Engineering　Applied Physics　Solid State Physics and Engineering
Professor
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Last Updated :2024/05/09

Researcher Profile and Settings

Affiliation

  • Faculty of Engineering　Applied Physics　Solid State Physics and Engineering

Job Title

  • Professor

URL

J-Global ID

Research Interests

  • optical vortex   coherent spectroscopy   time-resolved spectroscopy   high-Tc   exciton   

Research Areas

  • Natural sciences / Semiconductors, optical and atomic physics
  • Nanotechnology/Materials / Optical engineering and photonics

Educational Organization

Academic & Professional Experience

  • 2012/04 - Today 北海道大学 工学(系)研究科(研究院) 教授
  • 1999 - 2011 北海道大学 工学(系)研究科(研究院) 准教授
  • 1996/04 - 1999/08 The University of Tokyo Institute of Industrial Science

Education

  • 1993/04 - 1996/03  Tokyo Institute of Technology  Interdisciplinary Graduate School of Science and Engineering
  • 1991/04 - 1993/03  Tokyo Institute of Technology  Interdisciplinary Graduate School of Science and Engineering
  • 1987/04 - 1991/03  Tokyo Institute of Technology  School of Engineering  Dept. of Organic and Polymeric Materials

Association Memberships

  • THE PHYSICAL SOCIETY OF JAPAN   THE JAPAN SOCIETY OF APPLIED PHYSICS   

Research Activities

Published Papers

  • Satoshi Tsuchiya, Masato Katsumi, Ryuhei Oka, Toshio Naito, Yasunori Toda
    Condensed Matter 8 (4) 88 - 88 2023/10/10 
    Photo-induced carrier dynamics were measured in the organic Dirac electron candidate α-(BETS)2I3 to investigate why resistivity increases below TMI = 50 K. We found a change in carrier dynamics due to an insulating gap formation below T′ = 50 K. On the other hand, the relaxation time and polarization anisotropy of the observed dynamics differ from those in the charge-ordering (CO) state of the isostructural salt α-(ET)2I3. Based on the difference, it can be concluded that the insulating phase has a different origin than the CO state.
  • Satoshi Tsuchiya, Naoki Kanai, Ryuhei Oka, Toshio Naito, Yasunori Toda
    Journal of the Physical Society of Japan 92 (9) 094703/1 - 7 0031-9015 2023/09/15 [Refereed]
  • Yasunori Toda, Satoshi Tsuchiya, Keisaku Yamane, Ryuji Morita, Migaku Oda, Tohru Kurosawa, Tomaz Mertelj, Dragan Mihailovic
    Optics Express 31 (11) 17537 - 17537 2023/05/10 [Refereed]
     
    We report an experimental approach to produce spatially localized photoinduced superconducting state in a cuprate superconductor using optical vortices with ultrafast pulses. The measurements were carried out using coaxially aligned three-pulse time-resolved spectroscopy, in which an intense vortex pulse was used for coherent quenching of superconductivity and the resulting spatially modulated metastable states were analyzed by the pump-probe spectroscopy. The transient response after quenching shows a spatially localized superconducting state that remains unquenched at the dark core of the vortex beam for a few picoseconds. Because the quenching is instantaneously driven by photoexcited quasiparticles, the vortex beam profile can be transferred directly to the electron system. By using the optical vortex-induced superconductor, we demonstrate spatially resolved imaging of the superconducting response and show that the spatial resolution can be improved using the same principle as that of super-resolution microscopy for fluorescent molecules. The demonstration of spatially controlled photoinduced superconductivity is significant for establishing a new method for exploring novel photoinduced phenomena and applications in ultrafast optical devices.
  • K. Nakagawa, S. Tsuchiya, H. Taniguchi, Y. Toda
    Physical Review Research 5 (1) 2023/01/19 [Refereed][Not invited]
  • Asami Honda, Keisaku Yamane, Kohei Iwasa, Kazuhiko Oka, Yasunori Toda, Ryuji Morita
    Scientific reports 12 (1) 18674 - 18674 2022/11/04
  • Asami Honda, Keisaku Yamane, Kohei Iwasa, Kazuhiko Oka, Yasunori Toda, Ryuji Morita
    Scientific reports 12 (1) 14991 - 14991 2022/09/02 
    As an extension of pulse shaping techniques using the space-time coupling of ultrashort pulses or chirped pulses, we demonstrated the ultrafast beam pattern modulation by the superposition of chirped optical vortex pulses with orthogonal spatial modes. The stable and robust modulations with a modulation frequency of sub-THz were carried out by using the precise phase control technique of the constituent pulses in both the spatial and time/frequency domains. The performed modulations were ultrafast ring-shaped optical lattice modulation with 2, 4 and 6 petals, and beam pattern modulations in the radial direction. The simple linear fringe modulation was also demonstrated with chirped spatially Gaussian pulses. While the input pulse energy of the pulses to be modulated was 360 [Formula: see text]J, the output pulse energy of the modulated pulses was 115 [Formula: see text]J with the conversion efficiency of [Formula: see text] 32%. Demonstrating the superposition of orthogonal spatial modes in several ways, this ultrafast beam pattern modulation technique with high intensity can be applicable to the spatially coherent excitation of quasi-particles or collective excitation of charge and spin with dynamic degrees of freedom. Furthermore, we analyzed the Poynting vector and OAM of the composed chirped OV pulses. Although the ring-shaped optical lattice composed of OV pulse with topological charges of [Formula: see text] is rotated in a sub-THz frequency, the net orbital angular momentum (OAM) averaged over one optical period is found to be negligible. Hence, it is necessary to require careful attention to the application of the OAM transfer interaction with matter by employing such rotating ring-shaped optical lattices.
  • Asami Honda, Keisaku Yamane, Masato Suzuki, Jin Kawaguchi, Yasunori Toda, Takashige Omatsu, Ryuji Morita
    2022 Conference on Lasers and Electro-Optics, CLEO 2022 - Proceedings 2022 
    We demonstrated ultrafast snapshot imaging with a sub-GHz framerate on the basis of our newly proposed recirculation-filtering method, which enables us to generate a wavelength-division multiplexed pulse train with a time interval in nanosecond regime.
  • K. Nakagawa, T. Sato, S. Tsuchiya, J. Yamada, Y. Toda
    Europhysics Letters 136 (5) 57001 - 57001 0295-5075 2021/12/01 
    Abstract In this work, in order to investigate the origin of the anomalous polarization dynamics observed in the normal conducting state of the organic superconductor κ-(BEDT-TTF)2Cu(NCS)2, we performed optical pump probe measurements by varying the probe pulse energy from $\hbar\omega_{pr} = 1.44$ to 1.59 eV. We found that the polarization-dependent transient signal was most enhanced at ${\sim}1.51\ \text{eV}$ , which was close to the intramolecular excitation energy ${\sim}1.41\ \text{eV}$ of the BEDT-TTF molecules but is not exactly matched. This difference is linked to the formation of an energy gap at the Fermi energy at low temperatures, which results in an increase or decrease in pump-induced absorption between the states near the Fermi energy and the intramolecular excited states depending on probe polarization. In other words, the intramolecular excitation at 1.41 eV, which has a strong polarization dependence, serves as a probe of the dynamics involved in gap formation. A simple calculation shows that the size of the gap is ${\sim}0.20\ \text{eV}$ , which is close to that of the BEDT-TTF based Mott insulators. It is suggested that the gap arises from the phase-separated Mott state caused by photoinduced modulation of effective electron correlation.
  • S. Tsuchiya, H. Taniguchi, J. Yamada, Y. Toda, D. Mihailovic, T. Mertelj
    Physical Review B 104 (11) 2469-9950 2021/09/24 [Refereed]
  • Y. Toda, S. Tsuchiya, M. Oda, T. Kurosawa, S. Katsumata, M. Naseska, T. Mertelj, D. Mihailovic
    Physical Review B 104 (9) 2469-9950 2021/09/07 [Refereed]
  • Satoshi Tsuchiya, Tomaz Mertelj, Dragan Mihailovic, Jun-ichi Yamada, Yasunori Toda
    Journal of Superconductivity and Novel Magnetism 33 (8) 2299 - 2303 1557-1939 2020/08 [Refereed]
     
    In this work, spectrally resolved optical pump-probe measurements were carried out on strongly correlated organic superconductor.-(BEDT-TTF)2Cu[N(CN)2]Br. We find two different types of carrier relaxation dynamics, one of which appears at 1.82 eV and 1.94 eV, and another at 2.17 eV. The former is characterized by short decay time similar to 1-2 ps and appears below similar to 60 K. From the comparison with the previous studies, the dynamics arises from the photo-induced pseudogap (PG) formation of the partially appeared Mott phase. The latter has long relaxation time similar to 9 ps at 5 K and developed as temperature decreases below 30 K. The slow dynamics is consistent with appearance of superconducting (SC) gap. Such separate observation of PG and SC dynamics enables us to proceed further investigation of a relationship between them.
  • Satoshi Tsuchiya, Ryota Kuwae, Takumi Kodama, Yusuke Nakamura, Megumi Kurihara, Takashi Yamamoto, Toshio Naito, Yasunori Toda
    Journal of the Physical Society of Japan 89 (6) 064712 - 064712 0031-9015 2020/06/15 [Refereed]
     
    In this study, to investigate the electronic state of organic molecular crystal kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]I (kappa-1), polarized pump probe spectroscopy was conducted. From the observation of photoinduced carrier relaxation dynamics, we found that the carrier dynamics in kappa-I are qualitatively different from those in other kappa-type salts and inhomogeneous electronic states are realized in the single crystal. As temperature decreases, the transient polarization anisotropy along the c axis associated with a charge order develops below T-MI = 40K, which is consistent with the metal insulator transition observed in the resistivity measurement. Moreover, we found the position where the transient polarization anisotropy is enhanced along the 1/2 (a + c) direction. Remarkably, this signal shows no clear changes at T-MI = 40 K; instead, it shows an anomalous increase in the signal amplitude at T similar to 8 K. A possible origin of this anomaly is the locally induced superconducting state.
  • Koichi Nakagawa, Satoshi Tsuchiya, Hiromi Taniguchi, Yasunori Toda
    JOURNAL OF SUPERCONDUCTIVITY AND NOVEL MAGNETISM 33 2427 - 2433 1557-1939 2020/03 [Refereed][Not invited]
     
    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.
  • Laguerre-Gaussian vortex mode generation from astigmatic semiconductor microcavity
    K. Nakagawa, K.Yamane, R. Morita, Y. Toda
    Appl. Phys. Express 13 042001/1 - 5 2020/03 [Refereed][Not invited]
  • Rin Sasaki, Keisaku Yamane, Kazuhiko Oka, Yasunori Toda, Takashige Omatsu, Ryuji Morita
    Proceedings of SPIE - The International Society for Optical Engineering 11522 0277-786X 2020 
    We demonstrated that ultrafast azimuth rotation of linearly polarized beam by use of a chirped optical pulse pair, and the rotational frequency of sub-THz was realized.
  • Tsuchiya Satoshi, Nakagawa Koichi, Taniguchi Hiromi, Toda Yasunori
    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 88 (7) 0031-9015 2019/07/15 [Refereed][Not invited]
  • Masato Suzuki, Keisaku Yamane, Kazuhiko Oka, Yasunori Toda, Ryuji Morita
    SCIENTIFIC REPORTS 9 2045-2322 2019/07 [Refereed][Not invited]
     
    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.
  • Keisaku Yamane, Kohei Iwasa, Rin Sasaki, Kazuhiko Oka, Yasunori Toda, Ryuji Morita
    2019 Conference on Lasers and Electro-Optics Europe and European Quantum Electronics Conference, CLEO/Europe-EQEC 2019 2019/06 
    When two optical vortices with different topological charges mutually interfere, azimuthally periodic structure called a ring-shaped optical lattice is formed [1]. The petal-like beam profile can be flexibly rotated by controlling the intermodal phase difference, which offers us rotational degree of freedom in applications such as optical manipulations of micro/nano particles and micro machines. However, their maximum rotational frequency has been limited to GHz regime owing to their response time or bandwidth. Recently, we succeeded in drastically increasing the rotational frequency up to THz regime by introducing a new technique based on spatial mode multiplexing of chirped optical-vortex pair [2].
  • Keisaku Yamane, Yuho Watanabe, Kazuhiko Oka, Yasunori Toda, Ryuji Morita
    2019 Conference on Lasers and Electro-Optics Europe and European Quantum Electronics Conference, CLEO/Europe-EQEC 2019 2019/06 
    Recently, the spatial mode multiplexing based on optical vortices (OVs) attracts much attentions as a promissing technology for high-capacity optical communications. For such OV-based telecommunications, optical components for mode decomposition with respect to azimuth mode indices (topological charges) - so-called orbital angular momentum (OAM) sorters - are key devices, and various kinds of OAM sorters have been demonstrated such as computer-generated holograms as far. Among them, the geometric transformation (GT) technique has great advantages that the insertion loss is quite low and it supports inverse transformation (multiplexing) [1]. In earlier studies, GT-based OAM sorters have been realized by using spatial light modulators (SLMs) [1], and more recently, transparent refractive elements produced by using such as diamond machining [2] and Pancharatnam Berry phase [3] have been demonstrated. However, the GT-based method has the serious disadvantage of relatively large crosstalk between the neighboring modes. In order to overcome the problems, the beam copying technique by using diffractive optics was demonstrated [4]. The novel technique can greatly reduce the crosstalk between the OAM modes, but spoils the advantage that the OAM sorter can support the low-loss inverse transformation. In this paper, we demonstrate the new beam duplication technique by using a Sagnac interferometer, which can improve the resolution in the OAM mode decomposition and also support low-loss inverse transformation.
  • Keisaku Yamane, Kohei Iwasa, Rin Sasaki, Kazuhiko Oka, Yasunori Toda, Ryuji Morita
    Proceedings of SPIE - The International Society for Optical Engineering 11141 0277-786X 2019 
    We developed the high-efficiency beam converter for ultrafast spatio-temporal control of light waves based on Sagnac interferometer. The flexible control both in the azimuth and radial directions in the ultrafast regime was also realized.
  • Keisaku Yamane, Kohei Iwasa, Rin Sasaki, Kazuhiko Oka, Yasunori Toda, Ryuji Morita
    Optics InfoBase Conference Papers Part F140-CLEO_Europe 2019 2019 
    When two optical vortices with different topological charges mutually interfere, azimuthally periodic structure called a ring-shaped optical lattice is formed [1]. The petal-like beam profile can be flexibly rotated by controlling the intermodal phase difference, which offers us rotational degree of freedom in applications such as optical manipulations of micro/nano particles and micro machines. However, their maximum rotational frequency has been limited to GHz regime owing to their response time or bandwidth. Recently, we succeeded in drastically increasing the rotational frequency up to THz regime by introducing a new technique based on spatial mode multiplexing of chirped optical-vortex pair [2].
  • Y. Toda, K. Nakagawa, K. Yamane, R. Morita, Y. Awaji
    Proceedings of SPIE - The International Society for Optical Engineering 11141 0277-786X 2019 
    Based on our previous work, in which we demonstrated single orbital angular momentum (OAM) mode emissions from a vertical cavity surface emitting laser (VCSEL) with an external optical feedback, the conditions of the vortex mode emission are investigated in terms of feedback efficiency and polarization. There are two orthogonal polarization configurations for efficient single OAM mode emission, one of which corresponds to the dominant polarization in freerunning VCSEL. Nearly identical emission properties of the two polarizations indicate an independence between the spin and OAM. On the other hand, the stability of the OAM mode emission, which is characterized by the dependencies of the feedback efficiency and injection current, is changed between the two polarizations, suggesting different contributions of the broken rotational symmetry to the spin and OAM.
  • Keisaku Yamane, Yuho Watanabe, Kazuhiko Oka, Yasunori Toda, Ryuji Morita
    Optics InfoBase Conference Papers Part F140-CLEO_Europe 2019 2019 
    Recently, the spatial mode multiplexing based on optical vortices (OVs) attracts much attentions as a promissing technology for high-capacity optical communications. For such OV-based telecommunications, optical components for mode decomposition with respect to azimuth mode indices (topological charges) - so-called orbital angular momentum (OAM) sorters - are key devices, and various kinds of OAM sorters have been demonstrated such as computer-generated holograms as far. Among them, the geometric transformation (GT) technique has great advantages that the insertion loss is quite low and it supports inverse transformation (multiplexing) [1]. In earlier studies, GT-based OAM sorters have been realized by using spatial light modulators (SLMs) [1], and more recently, transparent refractive elements produced by using such as diamond machining [2] and Pancharatnam Berry phase [3] have been demonstrated. However, the GT-based method has the serious disadvantage of relatively large crosstalk between the neighboring modes. In order to overcome the problems, the beam copying technique by using diffractive optics was demonstrated [4]. The novel technique can greatly reduce the crosstalk between the OAM modes, but spoils the advantage that the OAM sorter can support the low-loss inverse transformation. In this paper, we demonstrate the new beam duplication technique by using a Sagnac interferometer, which can improve the resolution in the OAM mode decomposition and also support low-loss inverse transformation.
  • Nakagawa Koichi, Tsuchiya Satoshi, Yamada Jun-ichi, Toda Yasunori
    EPL 122 (6) 0295-5075 2018/06 [Refereed][Not invited]
  • Masato Suzuki, Keisaku Yamane, Moritsugu Sakamoto, Kazuhiko Oka, Yasunori Toda, Ryuji Morita
    OPTICS EXPRESS 26 (3) 2584 - 2598 1094-4087 2018/02 [Refereed][Not invited]
     
    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
  • Keisaku Yamane, Yuho Watanabe, Isamu Wakisaka, Kazuhiko Oka, Yasunori Toda, Ryuji Morita
    Optics InfoBase Conference Papers Part F114-FIO 2018 2018 
    New mode decomposition devices for optical vortices were developed. Their performances were experimentally verified and in good agreement with the corresponding simulations, showing precise spatial-phase control based on Pancharatnam-Berry phase.
  • Morita Ryuji, Suzuki Masato, Yamane Keisaku, Oka Kazuhiko, Toda Yasunori
    COMPLEX LIGHT AND OPTICAL FORCES XII 10549 0277-786X 2018 [Refereed][Not invited]
     
    Subsequently to the previous generation of ultrashort and ultra-broadband optical-vortex (OV) pulses (pulse energy: several tens of μJ) in few-cycle regime, we performed the generation of axisymmetrically polarized pulses with spatial complex amplitude modulation, which is suitable for a broadband spectrum. The generation system employs the combination of spatial light modulators in the 4-f configuration and a space variant wave plate as a common path interferometer. The spatial light modulators in the 4-f configuration offer the spatial dispersion compensation with respect to wavelength; in addition, the common path interferometer provides the stability to perturbation. We experimentally demonstrated the generation of various axisymmetrically polarized pulses by applying modulations of fundamental and higher-order Laguerre-Gauss modes with high purity. The experimental results show that we are able to generate arbitral axisymmetrically polarized pulses with spatial complex amplitude modulation.
  • I. Madan, V. V. Baranov, Y. Toda, M. Oda, T. Kurosawa, V. V. Kabanov, T. Mertelj, D. Mihailovic
    PHYSICAL REVIEW B 96 (18) 2469-9950 2017/11 [Refereed][Not invited]
     
    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.
  • Y. Toda, H. Mochizuki, S. Tsuchiya, T. Kurosawa, M. Oda, T. Mertelj, D. Mihailovic
    Journal of Superconductivity and Novel Magnetism 31 (3) 1 - 4 1557-1947 2017/10/11 [Refereed][Not invited]
     
    We develop an orbital angular momentum (OAM) excitation pump-probe (Pp) spectroscopy and perform experiments on a Bi2Sr2CaCu2O8+δ (Bi2212) superconductor. The OAM modulation for the pump is realized by using a transverse mode conversion technique which allows to modulate the chirality of the optical OAM with a fixed polarization. The result reveals a difference of the OAM-dependent transient reflectivity changes below Tc, indicating that the pump OAM can break the chiral symmetry of the superconducting (SC) state. The OAM dependences of the transient signal imply that the quasi-particle (QP) excitation with OAM enhances the SC with opposite chirality and/or suppress the SC with equal chirality. The temperature dependence of the OAM dynamics shows virtually no pairing fluctuation that usually appears in the QP dynamics observed by conventional Pp, suggesting that the nonequilibrium OAM dynamics is sensitive to the delocalized OAM associated with long range order.
  • S. Tsuchiya, K. Nakagawa, J. Yamada, H. Taniguchi, Y. Toda
    PHYSICAL REVIEW B 96 (13) 2469-9950 2017/10 [Refereed][Not invited]
     
    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) 0003-6951 2017/09 [Refereed][Not invited]
     
    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).
  • 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 [Refereed][Not invited]
  • Y. Toda, H. Moriya, K. Shigematsu, K. Yamane, R. Morita, Y. Awaji
    OPTICAL MANIPULATION CONFERENCE 10252 0277-786X 2017 [Refereed][Not invited]
     
    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.
  • Keisaku Yamane, Kohei Iwasa, Kohei Kakizawa, Kazuhiko Oka, Yasunori Toda, Ryuji Morita
    OPTICAL MANIPULATION CONFERENCE 10252 0277-786X 2017 [Refereed][Not invited]
     
    A new method for the ultrafast rotation of ring-shaped optical lattices based on frequency-chirping of optical pulses was demonstrated in THz regime, which is three orders of magnitude faster than those by the conventional methods. Our optical lattice generator with a spatial light modulator is robust thanks to the 4-f configuration and enables us to flexibly control their rotational symmetry. The generated ultrafast-rotating ring-shaped optical lattices with a rotational frequency of 0.59 THz were successfully boosted from 5 mu J up to 125 mu J by using a home-built 4-pass Ti:sapphire amplifier without any limitation by optical damage to the spatial light modulator.
  • Kyohhei Shigematsu, Masato Suzuki, Keisaku Yamane, Ryuji Morita, Yasunori Toda
    APPLIED PHYSICS EXPRESS 9 (12) 1882-0778 2016/12 [Refereed][Not invited]
     
    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 1557-1939 2016/12 [Refereed][Not invited]
     
    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 1557-1939 2016/12 [Refereed][Not invited]
     
    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 - Atomic, Molecular, and Optical Physics 94 1050-2947 2016/10/26 [Not refereed][Not invited]
     
    © 2016 American Physical Society.We derived the Berry connection of vector vortex states (VVSs) from the "true" Hamiltonian obtained through the Maxwell-Schrödinger 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 Poincaré 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-state manipulation and information processing.
  • Masato Suzuki, Keisaku Yamane, Kazuhiko Oka, Yasunori Toda, Ryuji Morita
    PHYSICAL REVIEW A 94 (4) 2469-9926 2016/10 [Refereed][Not invited]
     
    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) 0034-6748 2016/04 [Refereed][Not invited]
     
    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.
  • K. Shigematsu, K. Yamane, R. Morita, Y. Toda
    Physical Review B - Condensed Matter and Materials Physics 93 1098-0121 2016/01/15 [Not refereed][Not invited]
     
    © 2016 American Physical Society. 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.
  • Uchikoga R., Shigematsu K., Yamane K., Morita R., Toda Y.
    Meeting Abstracts of the Physical Society of Japan 一般社団法人 日本物理学会 71 1414 - 1414 2016 

    ビーム断面上の偏光分布が二回対称で、直線偏光と円偏光を同時に持つポアンカレビームパルスを励起光に用いると、異なるスピン状態を持つ励起子の同時励起が可能である。本研究では、縮退四光波混合分光を用いたバルクGaN励起子励起およびストークス分解検出を行い、励起子スピンのコヒーレントダイナミクス測定を実現した。得られた信号光の空間偏光分布はT2時間内で励起光を良く反映しており、励起子スピンのコヒーレント状態保持を示すことができた。

  • Toda Yasunori, Kurosawa Tohru, Oda Migaku
    Butsuri 一般社団法人 日本物理学会 71 (12) 830 - 835 2016 [Not refereed][Not invited]
     
    <p>We present polarization and coherent quench analyses of the gap dynamics in Bi-based high-<i>T</i><sub>c</sub> cuprates (Bi2212) using femtosecond optical pump-probe spectroscopy. The polarization analysis has addressed the rotational symmetry breakings of the dynamics, which are suppressed at room temperature and appears below <i>T</i><sup>*</sup>. The coherent quench experiment of the pseudogap has indicated an absence of long-range electronic order beyond a few coherence lengths on short timescales.</p>
  • K. Shigematsu, K. Yamane, R. Morita, Y. Toda
    PHYSICAL REVIEW B 93 (4) 2469-9950 2016/01 [Refereed][Not invited]
     
    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 2045-2322 2015/12 [Not refereed][Not invited]
     
    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) 0217-9792 2015/10 [Refereed][Not invited]
     
    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) 2040-8978 2015/08 [Refereed][Not invited]
     
    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 2041-1723 2015/04 [Refereed][Not invited]
     
    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 1340-6000 2015/02 [Refereed][Not invited]
     
    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.
  • Toda Yasunori
    Meeting Abstracts of the Physical Society of Japan 一般社団法人日本物理学会 70 1210 - 1211 2189-079X 2015
  • Aramaki M., Yoshimura S., Morisaki T., Terasaka K., Toda Y., Tanaka M.
    Meeting Abstracts of the Physical Society of Japan 一般社団法人日本物理学会 70 875 - 875 2189-079X 2015
  • Masato Suzuki, Keisaku Yamane, Kazuhiko Oka, Yasunori Toda, Ryuji Morita
    2015 CONFERENCE ON LASERS AND ELECTRO-OPTICS (CLEO) 2160-9020 2015 [Refereed][Not invited]
     
    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 [Not refereed][Not invited]
  • S. Ono, Y. Toda, J. Onoe
    Physical Review B 90 155435  1550-235X 2014/11 [Refereed][Not invited]
  • Shota Ono, Yasunori Toda, Jun Onoe
    PHYSICAL REVIEW B 90 (15) 155435.1-155435.5  1098-0121 2014/10 [Refereed][Not invited]
     
    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) 1098-0121 2014/09 [Not refereed][Not invited]
     
    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 1094-4087 2014/07 [Not refereed][Not invited]
     
    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 2045-2322 2014/07 [Not refereed][Not invited]
     
    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  1367-2630 2014/05 [Refereed][Not invited]
     
    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  2160-3308 2014/03 [Refereed][Not invited]
     
    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.
  • Kyohhei Shigematsu, Keisaku Yamane, Ryuji Morita, Yasunori Toda
    2014 CONFERENCE ON LASERS AND ELECTRO-OPTICS (CLEO) 2160-9020 2014 [Refereed][Not invited]
     
    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.
  • Keisaku Yamane, Yasuyuki Shioda, Masato Suzuki, Yasunori Toda, Ryuji Morita
    2014 CONFERENCE ON LASERS AND ELECTRO-OPTICS (CLEO) 2160-9020 2014 [Refereed][Not invited]
     
    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 0277-786X 2014 [Not refereed][Not invited]
     
    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 0921-4534 2013/10 [Refereed][Not invited]
     
    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  0021-4922 2013/08 [Refereed][Not invited]
     
    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  0021-4922 2013/08 [Refereed][Not invited]
     
    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. The Materials Research Society of Japan 38 (2) 283 - 286 1382-3469 2013/06 [Refereed][Not invited]
     
    We theoretically demonstrate that the temperature (T)-dependence of photo-excited carrier relaxation time (τ) in energy-gapped systems strongly depends on the rate of energy transfer from a longitudinal to transverse acoustic phonon mode. By considering the anharmonic coupling between them, we resolve an ambiguity in how to interpret the T-dependence of τ in alkali-doped C60 superconductors.
  • T. Naito, S. Matsuo, T. Inabe, Y. Toda
    Journal of Superconductivity and Novel Magnetism 26 (5) 1679 - 1682 1557-1947 2013 [Refereed][Not invited]
     
    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
    Optics Communications 308 147 - 151 0030-4018 2013 [Refereed][Not invited]
     
    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.
  • Masato Suzuki, Keisaku Yamane, Yasunori Toda, Ryuji Morita
    2013 CONFERENCE ON LASERS AND ELECTRO-OPTICS PACIFIC RIM (CLEO-PR) 2013 [Refereed][Not invited]
     
    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.
  • 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 [Refereed][Not invited]
  • Keisaku Yamane, Asami Honda, Kyohhei Shigematsu, Yasunori Toda, Ryuji Morita
    2013 CONFERENCE ON LASERS AND ELECTRO-OPTICS PACIFIC RIM (CLEO-PR) 2013 [Refereed][Not invited]
     
    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.
  • Zhili Yang, Keisaku Yamane, Yasunori Toda, Ryuji Morita
    2013 CONFERENCE ON LASERS AND ELECTRO-OPTICS PACIFIC RIM (CLEO-PR) 2013 [Refereed][Not invited]
     
    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.
  • Zhili Yang, Keisaku Yamane, Yasunori Toda, Ryuji Morita
    2013 CONFERENCE ON LASERS AND ELECTRO-OPTICS (CLEO) 2160-9020 2013 [Refereed][Not invited]
     
    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.
  • Keisaku Yamane, Yasunori Toda, Ryuji Morita
    2013 CONFERENCE ON LASERS AND ELECTRO-OPTICS (CLEO) 2160-9020 2013 [Refereed][Not invited]
     
    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 26 (5) 1679 - 1682 1557-1947 2013 [Not refereed][Not invited]
     
    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.
  • Shota Ono, Hiroyuki Shima, Yasunori Toda
    PHYSICAL REVIEW B 86 (10) 104512  1098-0121 2012/09 [Refereed][Not invited]
     
    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 1094-4087 2012/08 [Refereed][Not invited]
     
    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
    2012/05 [Refereed][Not invited]
     
    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
    2012/02 [Refereed][Not invited]
     
    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  1098-0121 2012 [Refereed][Not invited]
  • Jun Onoe, Takahiro Ito, Shin-Ichi Kimura, Hiroyuki Shima, Yasunori Toda, Hideo Yoshioka
    FULLERENES NANOTUBES AND CARBON NANOSTRUCTURES 20 (1) 1 - 16 1536-383X 2012 [Refereed][Not invited]
     
    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 1932-7447 2012/01 [Refereed][Not invited]
     
    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.
  • Toshio Naito, Saori Matsuo, Tamotsu Inabe, Yasunori Toda
    JOURNAL OF PHYSICAL CHEMISTRY C 116 (3) 2588 - 2593 1932-7447 2012/01 [Refereed][Not invited]
     
    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.
  • J. Onoe, T. Ito, S. Kimura, H. Shima, Y. Toda, H. Yoshioka
    Fullerene, Nanotubes, and Carbon Nanostructures 20 (1) 1-16  1536-4046 2012 [Refereed][Invited]
  • MORITA Ryuji, TODA Yasunori, OMATSU Takashige
    J. Surf. Sci. Soc. Jpn. The Surface Science Society of Japan 32 (12) 748 - 754 0388-5321 2011/12/10 
    Together with the introduction of topological light waves (light with phase or polarization singularities), their generation and applications were described. Using a photonic crystal, axially-symmetric polarizer or mirror, 1) supercontinuum optical vortex generation ranging ∼500 to ∼800 nm without spatial nor topological-charge dispersions, 2) generation of ultrabroadband axially-symmetric polarization modes ranging ∼600 to ∼800 nm without polarization dispersion, and 3) a high power radially-polarized light generation directly from a diode-pumped Nd:YVO4 bounce amplifier were experimentally demonstrated. As applications of generated topological light waves, 4) laser ablation of Ta plates using nanosecond optical vortex pulses and, in particular, using spin-orbit interactions of light, microneedle fabrication on a metal surface by laser ablation were performed.
  • Y. Toda, T. Mertelj, P. Kusar, T. Kurosawa, M. Oda, M. Ido, D. Mihailovic
    PHYSICAL REVIEW B 84 (17) 174516  1098-0121 2011/11 [Refereed][Not invited]
     
    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
    PHYSICAL REVIEW LETTERS 107 (22) 227002  0031-9007 2011/11 [Refereed][Not invited]
     
    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  1098-0121 2011/11 [Refereed][Not invited]
     
    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  1098-0121 2011/11 [Refereed][Not invited]
     
    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  0022-3727 2011/04 [Refereed][Not invited]
     
    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 [Refereed][Not invited]
     
    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.
  • K. Watanabe, S. Tsuchiya, Y. Toda, T. Kurosawa, M. Oda, T. Mertelj, I. Madan, V. V. Kabanov, D. Mihailovic
    Int. J. Mod. Phys. B 29, 1542031(1-7) (2015) 29 1542031  2011 [Refereed][Not invited]
  • J. Onoe, A. Takashima, Y. Toda
    APPLIED PHYSICS LETTERS 97 (24) 241911  0003-6951 2010/12 [Refereed][Not invited]
     
    The infrared (IR) phonon properties of one-dimensional (1D) metallic peanut-shaped C-60 polymers, which exhibit an energy gap below 60 K [Y. Toda et al., Appl. Phys. Lett. 92, 094102 (2008)] that is similar to the Peierls transition of quasi-1D metallic materials, have been examined in the temperature range of 30-300 K by high-resolution in-situ IR spectroscopy. We observed an IR phonon anomaly in which two new IR modes appear at 50 K and become increasingly anomalous with decreasing temperature in a similar manner as phase phonons associated with charge density waves in quasi-1D condensates. (C) 2010 American Institute of Physics. [doi:10.1063/1.3527961]
  • J. Ishioka, Y. H. Liu, K. Shimatake, T. Kurosawa, K. Ichimura, Y. Toda, M. Oda, S. Tanda
    PHYSICAL REVIEW LETTERS 105 (17) 176401-1 - 176401-4 0031-9007 2010/10 [Refereed][Not invited]
     
    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.
  • J. Ishioka, Y. H. Liu, K. Shimatake, T. Kurosawa, K. Ichimura, Y. Toda, M. Oda, S. Tanda
    PHYSICAL REVIEW LETTERS 105 (17) 0031-9007 2010/10 [Refereed][Not invited]
     
    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 1094-4087 2010/08 [Refereed][Not invited]
     
    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 1557-1939 2010/07 [Refereed][Not invited]
     
    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 0921-4526 2010/06 [Refereed][Not invited]
     
    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 0921-4526 2010/06 [Refereed][Not invited]
     
    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 0953-8984 2010/05 [Refereed][Not invited]
     
    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.
  • Yoshinari Awaji, Naoya Wada, Yasunori Toda
    2010 23rd Annual Meeting of the IEEE Photonics Society, PHOTINICS 2010 551 - 552 2010 [Refereed][Not invited]
     
    Spatial mode division multiplexing was performed by using Laguerre-Gaussian mode beam (Optical vortex). We demonstrated multiplexing of two channels of 10Gbit/s signal. Error free condition was successfully achieved. This technique can be utilized for free space and multi-mode fiber transmission. ©2010 IEEE.
  • S. Adachi, Y. Toda
    J. Vac. Soc. Jpn. The Vacuum Society of Japan 53 (6) 387 - 392 1882-2398 2010 [Refereed][Invited]
     
    By using four-wave-mixing technique, we demonstrated the possibility of the real space mapping of exciton-exciton interaction and strain in GaN films grown on various substrates. The image obtained from the evaluation of the quantum beats indicates that 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.
  • J. Onoe, A. Takashima, Y. Toda
    Appl. Phys. Lett. 97 241911  1077-3118 2010 [Refereed][Not invited]
  • Yu Tokizane, Katsuhiro Shimatake, Yasunori Toda, Kazuhiko Oka, Masakatsu Tsubota, Satoshi Tanda, Ryuji Morita
    OPTICS EXPRESS 17 (26) 24198 - 24207 1094-4087 2009/12 [Refereed][Not invited]
     
    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
  • Tetsuro Ishiguro, Yasunori Toda, Satoru Adachi
    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 78 (10) 174018-1 - 4 0031-9015 2009/10 [Refereed][Not invited]
     
    We study the influence of the anisotropic exchange interaction (AEI) on exciton coherence by using four-wave-mixing spectroscopy. The exciton fine structure (FS) of uniaxially-strained GaN exhibits linearly-polarized FS-splitting at low temperatures, which enables us to resolve the difference between the coherent dynamics of the FS; the FWM transient in each FS exhibits extended/contracted dephasing compared with that obtained in uniformly-strained GaN. The origin of this asymmetry is associated with the anisotropic pseudospin induced by AEI.
  • Y. Ueno, Y. Toda, S. Adachi, R. Morita, T. Tawara
    OPTICS EXPRESS 17 (22) 20567 - 20574 1094-4087 2009/10 [Refereed][Not invited]
     
    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, R. Onozaki, M. Tsubota, K. Inagaki, S. Tanda
    PHYSICAL REVIEW B 80 (12) 121103  1098-0121 2009/09 [Refereed][Not invited]
     
    We investigate the spectrally resolved transient reflectivity changes Delta R(T,tau,lambda) in the charge density wave (CDW) conductor o-TaS3. A distinct near-infrared resonance in this compound emphasizes the characteristic Delta R(lambda) resonances, and allows a selection of coexisting CDW phases with different nonequilibrium carrier dynamics. Furthermore, the spectrally resolved Delta R(tau) characterizes the collective oscillations associated with the individual states. We believe that this demonstration paves the way for the optical selection of the multiphase order that plays an important role in various macroscopic quantum systems.
  • Yasunori Toda, Koshi Nagaoka, Katsuhiro Shimatake, Ryuji Morita
    ELECTRICAL ENGINEERING IN JAPAN 167 (4) 39 - 46 0424-7760 2009/06 [Refereed][Not invited]
     
    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
  • Yu Tokizane, Katsuhiro Shimatake, Yasunori Toda, Kazuhiko Oka, Masakatsu Tsubota, Satoshi Tanda, Ryuji Morita
    The European Conference on Lasers and Electro-Optics (CLEO/Europe) 2009 2009/06 [Refereed][Not invited]
     
    Electron in a quasi one-dimensional NbSe3 whisker crystal form a collective state at low temperature. It is called charge density wave (CDW) state. Under certain condition, NbSe3 grows as a ring crystal with closed loop structure [1]. This kind of global structural change brings an effect on a formation of the collective state [2]. For observation of a global electronic property on the entire ring crystal, it is necessary to apply a probe method appropriate for the closed loop structure of a ring crystal. The light with a polarization of a closed loop structure, called a polarization vortex, is appropriate for this measurement.
  • 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 - + 1862-6351 2009 [Refereed][Not invited]
     
    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 1862-6351 2009 [Refereed][Not invited]
     
    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 1862-6351 2009 [Refereed][Not invited]
     
    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. Toda, K. Shimatake, S. Tanda, J. Onoe
    ULTRAFAST PHENOMENA IN SEMICONDUCTORS AND NANOSTRUCTURE MATERIALS XIII 7214 0277-786X 2009 [Not refereed][Not invited]
     
    Due to the recent progress of material science, quasi-one-dimensional (1D) 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-1D compounds by utilizing an ultrafast optical spectroscopy. Photoexcited nonequilibrium carrier dynamics yield characteristic feature around the phase transition temperatures. We also discuss the topological effects on the phase coherence of correlated carrieres by using both polarization and excitation energy dependences of the transient signals and their spatial characteristics.
  • Y. H. Liu, Y. Toda, K. Shimatake, N. Momono, M. Oda, M. Ido
    PHYSICAL REVIEW LETTERS 101 (13) 137003  0031-9007 2008/09 [Refereed][Not invited]
     
    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 0031-9015 2008/06 [Refereed][Not invited]
     
    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 0370-1972 2008/05 [Refereed][Not invited]
     
    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.
  • Y. Toda, S. Ryuzaki, J. Onoe
    APPLIED PHYSICS LETTERS 92 (9) 094102  0003-6951 2008/03 [Refereed][Not invited]
     
    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 - + 1862-6351 2008 [Refereed][Not invited]
     
    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.
  • T. Ishiguro, Y. Toda, S. Adachi, S. F. Chichibu
    ULTRAFAST PHENOMENA IN SEMICONDUCTORS AND NANOSTRUCTURE MATERIALS XII 6892 0277-786X 2008 [Not refereed][Not invited]
     
    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 x 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.
  • K. Shimatake, Y. Toda, S. Tanda
    PHYSICAL REVIEW B 75 (11) 115120  1098-0121 2007/03 [Refereed][Not invited]
     
    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 0021-4922 2007/02 [Refereed][Not invited]
     
    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 - + 1862-6351 2007 [Refereed][Not invited]
     
    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 - + 1862-6351 2007 [Refereed][Not invited]
     
    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  0003-6951 2007/01 [Refereed][Not invited]
     
    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.
  • Yasunori Toda, Koshi Nagaoka, Katsuhiro Shimatake, Ryuji Morita
    IEEJ Transactions on Electronics, Information and Systems 127 (9) 4 - 1313 1348-8155 2007 [Not refereed][Not invited]
     
    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. Especially, optical vortices in the femtosecond pulses will be invaluable for advanced topological spectroscopy. In this work, we created femtosecond optical vortices using spatial light modulator. Their spatio-temporal properties were evaluated using interferogram and correlation measurements.
  • T Ishiguro, Y Toda, S Adachi, T Mukai, K Hoshino, Y Arakawa
    PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS 243 (7) 1560 - 1563 0370-1972 2006/06 [Refereed][Not invited]
     
    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 0370-1972 2006/06 [Refereed][Not invited]
     
    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 0370-1972 2006/06 [Refereed][Not invited]
     
    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  1098-0121 2006/04 [Refereed][Not invited]
     
    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 [Refereed][Not invited]
     
    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 1862-6351 2006 [Not refereed][Not invited]
     
    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 1155-4339 2005/12 [Refereed][Not invited]
     
    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 1155-4339 2005/12 [Refereed][Not invited]
     
    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 Toda, S Adachi, Y Abe, K Hoshino, Y Arakawa
    PHYSICAL REVIEW B 71 (19) 195315  1098-0121 2005/05 [Refereed][Not invited]
     
    The degenerate four-wave mixing (FWM) spectroscopy of uniaxially strained GaN layers is demonstrated using colinearly polarized laser pulses. The nonlinear response of FWM signal on exciton oscillator strength enhances the sensitivity for polarized exciton, allowing for mapping out the in-plane anisotropy of the strain field. The observed high-contrast spectral polarization clearly shows fine structure splittings of excitons, which are also confirmed in the change of quantum beating periods of time.
  • S Adachi, Y Toda
    PHYSICA STATUS SOLIDI C - CONFERENCES AND CRITICAL REVIEWS, VOL. 2 , NO 11 2 (11) 3936 - 3940 1862-6351 2005 [Refereed][Not invited]
     
    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.
  • Y Abe, Y Toda, K Hoshino, Y Arakawa
    JAPANESE JOURNAL OF APPLIED PHYSICS PART 2-LETTERS & EXPRESS LETTERS 44 (16-19) L535 - L538 0021-4922 2005 [Refereed][Not invited]
     
    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 similar to 0.12 cin/GW for a standard GaN film. A spatial resolution better than lambda/2, for an excitation wavelength of 800 nm, was achieved.
  • Satoru Adachi, Yasunori Toda
    Physica Status Solidi C: Conferences 2 (11) 3936 - 3940 1610-1634 2005 [Refereed][Not invited]
     
    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 0021-4922 2004/09 [Refereed][Not invited]
     
    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 Review B - Condensed Matter and Materials Physics 70 (3) 1 - 33106 0163-1829 2004/07 [Refereed][Not invited]
     
    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) 1098-0121 2004/07 [Refereed][Not invited]
     
    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 1386-9477 2004/03 [Refereed][Not invited]
     
    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 0003-6951 2003/06 [Refereed][Not invited]
     
    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 1862-6351 2003 [Refereed][Not invited]
     
    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 1862-6351 2003 [Refereed][Not invited]
     
    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 [Refereed][Not invited]
     
    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 1610-1634 2003 [Refereed][Not invited]
     
    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 1610-1634 2003 [Refereed][Not invited]
     
    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 0021-4922 2002/12 [Refereed][Not invited]
     
    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 1386-9477 2002 [Refereed][Not invited]
     
    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 [Refereed][Not invited]
     
    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, O Moriwaki, M Nishioka, Y Arakawa
    PHYSICA E 8 (4) 328 - 332 1386-9477 2000/12 [Refereed][Not invited]
     
    We have investigated the carrier relaxation mechanism in InGaAs/CaAs quantum dots by photoluminescence excitation (PLE) spectroscopy. Near-field scanning optical microscope successfully shows that a PLE resonance at a relaxation energy of 36 meV can be seen in all single-dot luminescence spectra, and thus can be attributed to resonant Raman scattering by a GaAs LO phonon to the excitonic ground state. In addition, a number of sharp resonances observed in single-dot PLE spectra can be identified as resonant Raman features due to localized phonons, which are observed in the conventional Raman spectrum. The results reveal the mechanism for the efficient relaxation of carriers observed in self-assembled quantum dots: the carriers can relax within the continuum states, and make transitions to the excitonic ground state by phonon emission. (C) 2000 Elsevier Science B.V. All rights reserved.
  • Y Toda, T Sugimoto, M Nishioka, Y Arakawa
    APPLIED PHYSICS LETTERS 76 (26) 3887 - 3889 0003-6951 2000/06 [Refereed][Not invited]
     
    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, Y Arakawa
    IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS 6 (3) 528 - 533 1077-260X 2000/05 [Refereed][Invited]
     
    Relaxation mechanism of excited carriers in In-GaAs-GaAs self-assembled quantum dots has been investigated. Near-field photoluminescence excitation (PLE) spectra show some sharp resonance lines whose energies match those of the LO phonons observed in far-field PLE, The results suggest that the PLE resonant peaks are predominantly due to resonant Raman scattering from phonons, This assignment is consistent with the absence of magnetic field dependence of the resonances. Single dot coherent spectroscopy shows the dephasing time of resonant carriers to be as fast as several tens of picoseconds, This fast dephasing time agrees with the phonon-electron interactions being strong, These results allow better understanding of the carrier relaxation process in InGaAs-GaAs self-assembled quantum dots.
  • Y Toda, K Suzuki, S Shinomori, Y Arakawa
    MICROELECTRONIC ENGINEERING 47 (1-4) 111 - 113 0167-9317 1999/06 [Refereed][Not invited]
     
    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 0031-9007 1999/05 [Refereed][Not invited]
     
    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.
  • Y Toda, S Shinomori, K Suzuki, Y Arakawa
    PHYSICAL REVIEW B 58 (16) R10147 - R10150 2469-9950 1998/10 [Refereed][Not invited]
     
    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 0003-6951 1998/07 [Refereed][Not invited]
     
    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 1386-9477 1998/07 [Refereed][Not invited]
     
    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.
  • Y Toda, S Shinomori, K Suzuki, Y Arakawa
    SOLID-STATE ELECTRONICS 42 (7-8) 1083 - 1086 0038-1101 1998/07 [Refereed][Not invited]
     
    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 0163-1829 1998 [Refereed][Not invited]
  • Y. Toda, S. Shinomori, K. Suzuki, Y. Arakawa
    Physical Review B - Condensed Matter and Materials Physics 58 (16) R10147 - R10150 1550-235X 1998 [Refereed][Not invited]
     
    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.
  • Y Arakawa, Y Toda, S Shinomori
    PHYSICS AND SIMULATION OF OPTOELECTRONIC DEVICES VI, PTS 1 AND 2 3283 785 - 792 0277-786X 1998 [Refereed][Not invited]
     
    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 0003-6951 1996/08 [Refereed][Not invited]
     
    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 1041-1135 1995/01 [Refereed][Not invited]
     
    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).
  • Y TODA, T ENAMI, M OUTSU
    JAPANESE JOURNAL OF APPLIED PHYSICS PART 2-LETTERS & EXPRESS LETTERS 32 (9A) L1233 - L1235 0021-4922 1993/09 [Refereed][Not invited]
     
    Second-harmonic generation using 1.5 mum diode lasers in a nonlinear organic fiber with Cerenkov radiation phase matching is demonstrated for the first time. This second harmonic frequency was locked to the center of the rubidium spectral line to stabilize the fundamental frequency of the 1.56 mum diode laser. The resultant frequency fluctuation, which was evaluated by the error signal from the lock-in amplifier, was maintained at less than 0.3 MHz for one hour.
  • WZ WANG, K NAKAGAWA, Y TODA, M OHTSU
    APPLIED PHYSICS LETTERS 61 (16) 1886 - 1888 0003-6951 1992/10 [Refereed][Not invited]
     
    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.

Books etc

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

MISC

Awards & Honors

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

Research Grants & Projects

  • 日本学術振興会：科学研究費助成事業
    Date (from‐to) : 2022/04 -2025/03 
    Author : 土屋 聡, 戸田 泰則, 谷口 弘三
  • 日本学術振興会：科学研究費助成事業
    Date (from‐to) : 2022/04 -2025/03 
    Author : 戸田 泰則, 小田 研, 土屋 聡
  • 日本学術振興会：科学研究費助成事業 新学術領域研究(研究領域提案型)
    Date (from‐to) : 2019/06 -2024/03 
    Author : 小林 研介, 戸田 泰則, 戸川 欣彦, 石坂 香子, 岡崎 浩三, 有馬 孝尚
     
    小林は、量子ドットにおける近藤効果を用いて、平衡非線形領域にある微分伝導度について調べ、平衡から遠く離れた状況にあったとしても、対称性を通じて近藤効果が輸送現象に姿を表すことを微視的に実証した。さらに、GaAs/AlGaAsトンネル結合二重量子井戸構造におけるラシュバ効果、スピン軌道トルクによる磁化の準安定状態の実現や、電流雑音測定のための低温増幅器の開発などを行った。有馬は、希土類を含むいくつかの合金系について共鳴Ｘ線散乱や中性子散乱を行い、磁場や温度およびそれらの履歴によって多彩な磁気ナノ超構造の間を転移する様子を逆空間で観測することでスピン液晶状態を確定させるという成果を得た。戸川はキラルスピン物質CrNb3S6が示す異方的磁気共鳴の研究を行い、励起配置に強く依存した共鳴応答を見出し、共鳴スイッチングやキラルソリトン共鳴モードの切替えなど、高度な制御法に関する成果を得た。戸田はトポロジカル光波を用いた光誘起相制御開拓の研究を行い、光の伝播がもたらす幾何学位相（グイ位相）の制御性を活用した光渦発振を半導体微小共振器に対して実現した。石坂は遷移金属ダイカルコゲナイドVTe2を対象とし、数ピコ秒の時間分解能による電子回折とイメージング実験を行い、電荷密度波の超高速融解とそれに伴う特異な偏極を持つ音響フォノンの生成・伝播の計測に成功した。岡﨑は、高次高調波レーザーを用いた時間分解光電子分光による光照射による物性制御の研究を行い、鉄系超伝導体FeSeにおいて、光誘起超伝導による超伝導ギャップの観測に成功する、という成果を得た。
  • Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research (B)
    Date (from‐to) : 2020/04 -2023/03 
    Author : 森田 隆二, 戸田 泰則
     
    本研究の目的は，光の持つ位相・偏光の空間的特異性・非一様性に着目し，申請者らが現在までに独自に開発したモノサイクル域光パルス発生・制御技術，超短光パルスコヒーレント合成技術，光のスピン軌道相互作用変換・制御技術を用いることにより， THzの速さでビームパターンが変化する光格子（光渦ペア）を発生させること，さらに，物質系において，発生光渦ペアによる擬粒子励起を行い，その伝播過程を利用してコヒーレントな制御・増強を行うことである。既に申請者らは，THz回転周波数の超高速回転光格子（光渦ペア）発生に成功しているが，物質系における擬粒子励起などの応用を考えると，十分な光出力とはいえない。そこで，まず，THz回転周波数の超高速回転リング状光格子（光渦ペア）の高強度化をめざし，当該年度は以下を行った。
    光源系の改良としては，光損失が少なく安定でロバストなコヒーレントビーム合成光学系としてサニャック型干渉計光路を用い，変調前のパルスエネルギー360 μJ，変調後のパルスエネルギー115 μJ，32%の高い変換効率を得ている。また，チャープされた光渦の重ね合わせ実験において，絶対値が等しく符号の異なるトポロジカルチャージを有する光渦パルスの重ね合わせ手法を拡張し，方位方向指数の異なる光渦の重ね合わせによる動径方向に変化する高効率超高速ビーム変調，方位方向指数および動径方向指数の異なる光渦の重ね合わせによる複雑な超高速ビームパターン変調に成功している。
  • Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research Fund for the Promotion of Joint International Research (Fostering Joint International Research (B))
    Date (from‐to) : 2018/10 -2023/03 
    Author : 荒巻 光利, 吉村 信次, 寺坂 健一郎, 矢澤 翔大, 小林 弘和, 戸田 泰則, 鹿野 豊
     
    本研究の目的は，光渦分光法をプロセスプラズマに適用することでプラズマの制御性を格段に向上させ，そのプラズマ源を材料開発に応用することである．2019年度は，2020年度以降のドイツおよびデンマークでの実験に備えた装置開発および予備実験を行った． ドイツ・ルール大学との共同研究では，容量結合プラズマ（CCP）と基板の境界であるシース内における，イオン流速の測定を目的としている．CCPは広くプラズマプロセスで用いられており，シース内で基板に垂直方向に加速されたイオンのエネルギーは，膜質を左右する重要なパラメータである．従来は，基板に穴をあけてレーザーを基板に垂直方向にレーザーを通すか，イオンを引き出してエネルギーアナライザで検出するなどし基板への入射エネルギーが測定されてきた．本研究では，基板に平行な方向から入射した光渦によって，基板に垂直な方向のイオン流速を測定することを目指している．従って，基板や電極を加工する必要がなく，実機への応用も可能となる．2019年度は，光渦を用いたレーザー誘起蛍光測定系を開発し，HYPER-II（九州大学）での予備実験を開始した． デンマーク工科大学との共同研究では，希少金属を用いない透明電極の高性能化に関する研究を進めている．現時点で得られている導電性透明薄膜は，電気特性の位置依存性が大きく，何が特性を決めているのか明らかになっていない．本共同研究では，光渦分光により，基板直前のイオン・原子のエネルギー分布を測定し，薄膜の電気的特性との関係を明らかにすることを目指している．2019年度は，予備的実験として，デンマーク工科大学にて，DCスパッタリングプラズマの発光分光測定を開始し，観測可能なAr, Zn, Aｌ等の発光線を検討した．
  • 日本学術振興会：科学研究費助成事業 挑戦的研究(萌芽)
    Date (from‐to) : 2019/06 -2022/03 
    Author : 戸田 泰則
     
    光渦は高次の空間モード光波に対応し、その特徴である波面の回転位相勾配は勾配の方向（符号）に応じたカイラリティを持つ。したがって異なるカイラリティを持つ同一次数の光渦対を用いると、完全同軸の光励起に対してカイラリティ選択による渦対の分離が可能となる。本研究ではこの光渦カイラリティに着目し、モード変換を利用した光波分離にもとづく全く新しい方式の時間分解分光を開発することを目的としている。 提案手法を確立するため、昨年度は主にカイラリティの異なる光渦の生成分離に対する光学系の確立と最適化に取り組んだ。解決課題として、（i）光渦の空間モード純度、（ii）検出側の空間モード選択性、（iii）試料反射特性、を設定し、 （項目A）半導体面発光レーザー（VCSEL）に対する空間モード選択制御 （項目B）高温超伝導体試料（Bi2212）の完全同軸型ポンププローブ分光 を実施した。項目Aでは光波の空間モード純度（課題i）と選択性（課題ii）の向上を図り、光注入型VCSELにおけるカイラリティ選択可能な光渦発振を実現した。特に共振器の幾何学位相（グイ位相）にもとづく空間モード変換を用いることにより、非対称共振器に対しても光渦発振可能であることを明らかにし、汎用性の高い光渦レーザー実現に向けた原理実証を行った。並行して項目Bでは空間モードカイラリティを活用した時間分解分光開発に取り組み、直交偏光条件下の完全同軸型時間分解ポンププローブ分光によるカイラリティ選択型過渡応答検出を実現した（課題ii）。
  • Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research
    Date (from‐to) : 2019/04 -2022/03 
    Author : TODA YASUNORI
     
    Excitation of all superconducting (SC) Cooper pairs with ultrashort laser pulses induces a coherent quench to the normal state in the photoexcited region. In this study, we have performed coherent quenching of high-Tc SCs using optical vortex pulses with unique spatial characteristics in the beam cross-section. Our newly developed time-resolved spectroscopy demonstrated the generation as well as the observation and control of a novel SC state modulated in space and time. This achievement demonstrates a new type of photo-induced phase transition utilizing the spatial controllability of light waves, showing promise for applications in nano-scale imaging and the development of SC devices.
  • 日本学術振興会：科学研究費助成事業 基盤研究(C)
    Date (from‐to) : 2019/04 -2022/03 
    Author : 小田 研, 戸田 泰則
     
    本研究では、代表的な銅酸化物高温超伝導体の一つであるBi2212で走査トンネル顕微鏡分光（STM/STS）とポンプ・プローブ時間分解分光（PPTS）を行い、実空間と時間空間の双方から「高温超伝導体に固有の擬ギャップ（PG）現象を担う電子系と高温超伝導の発現との関係性」の解明にアプローチする。２０１９年度は不足キャリア（ホール）濃度のBi2212単結晶試料でSTM/STS実験とPPTS実験を行い、以下に記す成果を得た。 １．STM/STSでは、PGにかかる電子系のチェッカーボード様変調構造が超伝導にかかる準粒子の干渉による変調構造と実空間の同じ領域において共存することが明らかとなった。また、両者の共存関係を相互相関関数の手法により調べた結果、明確な実空間相関が観測され、チェッカーボード様変調構造の起源やPG状態と超伝導に関係する電子状態間の相互作用の可能性を明らかにする上で有用な知見が得られた。 ２．PPTSでは、高強度ポンプパルス光の照射によりPG状態と超伝導相を局所的に破壊し、その後の両者の回復過程をプローブ光の反射応答を通して調べた。その結果、PG状態がほぼ回復した後に超伝導状態の形成が始まること、すなわち、両者の間に明確な時間相関のあることが分かった。これはPG状態が高温超伝導の発現に本質的に関わっていることの一つの証拠に成り得ると考えられる。 ２０２０年度以降の研究では、最適ホール濃度および過剰ホール濃度領域のBi2212試料にも研究を広げ、これらの試料で得られた結果も含めて総合的に考察し、PGを担う電子状態と超伝導の発現との関わりについて明らかにする。
  • Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research (B)
    Date (from‐to) : 2016/04 -2019/03 
    Author : TODA YASUNORI
     
    Using ultrafast optical vortex pulses, we have demonstrated the observation of the orbital angular momentum (OAM) dynamics of carriers for high-Tc superconductors (Bi2212) and obtained the following results: (a) OAM-dependent transient reflectivity changes are observed below Tc, indicating that the optical vortex can transfer its OAM to the superconducting state (quasi-particles and/or cooper-pairs); (b) recovery dynamics of the superconducting state shows an efficient contribution of the pairing fluctuation in its early stage; (c) temperature dependence of the OAM dynamics shows virtually no pairing fluctuation, suggesting that the optical vortex induced OAM dynamics reflects a spatially coherent dynamics associated with a global quantum correlation.
  • 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)
    Date (from‐to) : 2016 -2018 
    Author : 戸田泰則
  • Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research Grant-in-Aid for Challenging Exploratory Research
    Date (from‐to) : 2015/04 -2017/03 
    Author : Toda Yasunori
     
    We have developed a novel type of coherent spectroscopy (time-resolved polarimetry using nonlinear laser spectroscopy known as degenerate four-wave-mixing) using polarization vortices characterized by a vector singularity on the beam cross-section, and performed the spectroscopy on semiconductor GaN excitons (electron-hole pairs). We successfully obtained the results concerned as follows: (a) quantitative snap-shot evaluations of exciton spin exchange and uniaxial strain energies; especially for small residual anisotropies in isotropic samples; (b) coherent dynamics of the polarization singularities resulting from broken rotational symmetries of excitons, (c) spin and pseudo-spin dynamics and their correlations resulting from excitons interactions.
  • Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research (B)
    Date (from‐to) : 2014/04 -2017/03 
    Author : MORITA Ryuji
     
    Over 1 mJ pulse intense optical vortex pulses (duration of 27 fs) were generated with programmable topological-charge control. Ultra-broadband polarization vortex pulses (wavelength range of 750 to 880 nm) were successfully synthesized from two orthogonally polarized optical vortex pulses by using coherent beam combining. Moreover, the extended Stokes parameters were newly introduced, enabling one to precisely evaluate polarization symmetry and spatial polarization purity for ultra-broadband polarization vortex pulses. Furthermore, snap-shot polarization spectroscopy through four-wave mixing signals from GaN by use of radially polarized ultrashort pulses was performed, giving high sensitivity and high precision.
  • Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research (B)
    Date (from‐to) : 2013/04 -2017/03 
    Author : ARAMAKI Mitsutoshi, ITO Kiyokazu
     
    Optical vortex (OV) beams are a set of solutions of the paraxial Helmholtz equation in the cylindrical coordinates, and its wave front has a spiral shape. The observer in the OV beam feels the three-dimensional Doppler effect, since the OV beam has the three-dimensional spiral wave front. In this study, spectroscopy methods which have the sensitivity to the beam crossing gas flow have been developed. Using the optical vortex spectroscopy methods, the beam-crossing gas flow was successfully detected as the transverse Doppler shift of the Doppler absorption spectrum and the Lamb dip shift of the saturated absorption spectrum, respectively. The dependence of the transverse Doppler shift on the distance from the phase singularity qualitatively agrees with the theory, however, the further improvement in accuracy of the transverse Doppler shift detection is required for the quantitative evaluation.
  • 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
    Date (from‐to) : 2015 -2016 
    Author : 戸田泰則
  • Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research Grant-in-Aid for Challenging Exploratory Research
    Date (from‐to) : 2013/04 -2015/03 
    Author : TODA YASUNORI, ADACHI Satoru, NAKAYAMA Masaaki
     
    Spatiotemporal four-wave mixing (FWM) spectroscopy of semiconductor excitons was developed by using (A) variably controlled Laguerre-Gaussian (LG) pulses, (B) orbital angular momentum (OAM) resolved spectroscopy with high precision. The observed exciton OAM dynamics indicates that (1) near perfect OAM transfer to the exciton center-of-mass motion is achieved using the LG pulses with nonzero OAM, (2) imperfect conversion becomes significant when the excitation pulses include an OAM-free component (3) an extremely long OAM decay was observed in the bulk sample, which implies a small spatial variation of the exciton dephasing. Moreover, the topological charge dependence of the OAM dynamics suggests a robustness of the exciton OAM. For the application of the topological transformation, a novel spatially modulated optical feedback in a vertical-cavity surface-emitting laser was demonstrated for the first time, where the results show high selectivity of the OAM in the transverse mode.
  • Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research (B)
    Date (from‐to) : 2012/04 -2015/03 
    Author : TODA YASUNORI, ODA Migaku
     
    Photoinduced phase transition dynamics in the superconducting and pseudogap states of Bi-based high-Tc superconductors have been investigated using ultrashort pulses and the following conclusions are obtained: 1) existence of a spontaneous symmetry breaking in the psuedogap state, 2) distinct relationship between phase coherence and gap formation, 3) absence of long-range electronic ordering and carrier lorcalization in the pseudogap formation, 4) enhanced contribution of the pseudogap to the gap formation in the underdoped sample.
  • Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research (B)
    Date (from‐to) : 2011/04 -2014/03 
    Author : MORITA Ryuji, TODA Yasunori
     
    Few-cycle optical-vortex pulse generation and over-mJ intense, 27 fs optical-vortex pulse generation with programmable topological-charge control were first performed. Furthermore, the measurement of the orbital-angular-momentum spectra of ultra-broadband optical-vortex pulses with high precision was demonstrated in quasi-real time. The time-resolved nonlinear spectroscopy technique using optical vortices as a pump and a probe was developed, enabling us to evaluate the relaxation time of orbital angular momentum of the GaN exciton. In addition, chiral nanostructures of metal were first fabricated by using spin- and orbital-angular momentum controlled optical-vortex pulses.
  • 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
    Date (from‐to) : 2013 -2014 
    Author : 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)
    Date (from‐to) : 2012 -2014 
    Author : 戸田泰則
  • Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research (C)
    Date (from‐to) : 2010 -2012 
    Author : ODA Migaku
     
    In this research project, we performed STM/STS experiments under magnetic fields and pump-probe optical experiments in one of the typical high-Tc cuprates Bi2212 to clarify the origin of the electronic charge order accompanied by a pseudogap and its relation to the high-Tc superconductivity. STM/STS experiments suggested that the electronic charge order will be understood in terms of the charge-spin stripes model, and dynamically fluctuating in nature. In pump-probe optical experiments, furthermore, it was found that the electronic charge order and pseudogap do not only compete with but also is needed for the high-Tc superconductivity. Employing these results, we pointed out that the fluctuating stripes would be responsible for the formation of high-Tc superconductivity.
  • 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
    Date (from‐to) : 2010/03 -2011/01 
    Author : 戸田泰則
  • Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research on Innovative Areas (Research a proposed research project)
    Date (from‐to) : 2009 -2011 
    Author : ONOE Jun, SHIMA Hiroyuki, TODA Yasunori
     
    A. Einstein first applied Riemannian geometry to develop the general theory of relativity almost one hundred years ago (1916) and succeeded in understanding astronomical-scale phenomena such as the straining of time-space by a gravitational field (1920). Whether or not Riemannian space affects the electronic properties of condensed matters on a much smaller scale is of great interest. Although Riemannian geometry has been applied to quantum mechanics since the 1950s, nobody has yet answered this question, because the electronic properties of materials with Riemannian geometry have not been examined experimentally. By examining in situ high-resolution ultraviolet photoemission spectra of a one-dimensional metallic C_<60> polymer with an uneven periodic peanut-shaped structure, we first observed Riemannian geometrical effects on the electronic properties of materials such as Tomononaga-Luttinger liquids, which were previously theoretically predicted by our group.
  • Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research (B)
    Date (from‐to) : 2008 -2010 
    Author : MORITA Ryuji, TODA Yasunori
     
    Azobenzene single molecules before and after photo-isomerization were observed by a scanning tunneling microscopy. On the metal substrate with strong adsorption interaction, in-plane trans-azobenzene molecules were found, implying the inversion mechanism is dominant. Furthermore, for controlling photo-isomerization or molecular conformation change, optical pulses with spatially-dependent phase and polarization were generated with high efficiency. In addition, using the pulses, nonlinear spectroscopy along a ring shape was established. Metal surface ablation experiments were performed, demonstrating total angular momentum effects.
  • Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research (B)
    Date (from‐to) : 2005 -2007 
    Author : ADACHI Satoru, TODA Yasunori, MUTO Shunichi
     
    In this project, we aimed the development of the technique to transfer the spin between the Zeeman-split electron states in a single quantum dot (QD). Then, we tried the following two methods; use of teraheltz wave and two-photon process via half-localized state in a QD. As a consequence, by the latter method, we succeeded the control of the exciton spin state and the details were published in the academic journals. In the experiments, depending on the light polarizations of the co-linearly propagating delay-controlled double pulses, we observed the in-phase and out-of-phase oscillations of photoluminescence (PL) from the Zeeman-split two exciton states. In the case of the out-of-phase oscillations, the oscillation of the Zeeman splitting energy as well as the magnitude of the PLs was observed. This is because the nuclear spin polarization in a QD was formed and followed the modulation of the electron spin polarization. This result indicates that the half-localized state of QDs is available as the intermediate state in the Lambda-type transition in order to control of the electron and nuclear spins. In addition, we investigated the hyperfine interaction between electron and nuclear spins in a single QD, and the bistability and hysteretic response of the nuclear spin polarization were observed in the dependence on excitation power, polarization, and external magnetic field. By using this nuclear spin switching, we proved that the effective g factor of the QD electron could be tuned widely from negative to positive and be set to zero.
  • 日本学術振興会：科学研究費助成事業 萌芽研究
    Date (from‐to) : 2006 -2006 
    Author : 戸田 泰則
     
    本研究では、結晶のトポロジカルな空間対称性の破れに起因する電子状態の空間位相変化を検出する手法の確立を目的とした。本年度の成果として、1)四光波混合(FWM)分光を用いた位相干渉計測の確立、2)結晶トポロジーを反映した電子状態の時間特性変化の観測、3)トポロジー光パルスの発生と解析評価の確立、を達成した。FWMをベースとした時空間位相計測1)は、空間的な指向性を有するFWM光と励起光から分離した参照光との間で干渉系を構築することで実現した。干渉パターンをCCDにより検出し、2次元フーリエ変換を通して時間分解型のFWM信号を観測する。測定試料として、励起子束縛エネルギーが大きく、理想的な多準位系として異種励起子を扱える窒化ガリウム薄膜を用いた。異種励起子量子ビートと励起子-励起子分子量子ビートを観測したところ、2準位系と3準位系の励起子光学遷移にもとづくビート信号の違いを明らかにすることができた。並行して2)結晶のトポロジー変化に起因する電子状態の変化を明らかにすることを試みた。試料として特異なトポロジーを有する低次元導体を用いた。この物質は低温で電荷密度波と呼ばれる電子の集団励起状態を形成し、金属-絶縁体相転移を生じる。時間分解分光を用いて巨視的量子秩序形成を反映した電子応答変化を確認し、その温度依存性から結晶トポロジーによる有意な差を確認することに成功した。以上の結果にもとづき、3)より発展的なトポロジー計測を目的とした光渦パルスの発生を試みた。光渦パルスの評価は1)で確立した空間干渉をベースにし、時空間位相解析を実現した。光渦パルスとして50fs程度の短パルス生成に成功し、ドーナッツ状の強度分布と位相特異点の存在を明らかにした。また干渉振幅に着目すると、最大振幅を取る遅延時間が方位角に応じて変化していくことを見出した。
  • Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research (B)
    Date (from‐to) : 2004 -2006 
    Author : TODA Yasunori, ADACHI Satoru, ARAKAWA Yasuhiko
     
    Since strain and defects in semiconductors directly connect with the device performance, the influence has been studied extensively so far. Generally, there are various types of defects ranging from an atomic to a microscopic scale depending on the substrate, structure and growth conditions. In addition, the local strain fields due to mismatches in the lattice constants and the defects/impurities are also important for the thin films grown on foreign substrates. Therefore, three-dimensional (3D) analysis with a high spatial resolution is necessary for the precise characterization. One approach to the optical characterization in 3D is the use of nonlinear processes, in which excitation well below the band-gap allows for cross-sectional analysis at any depth. Furthermore, the use of nonlinear spectroscopy often provides significant enhancement of the signal sensitivity. In this work, we demonstrated i) multiphoton induced absorption imaging and ii) four-wave mixing (FWM) spectroscopy for the precise characterization of Gallium Nitride (GaN) in terms of strain and defects. Both techniques developed here allow for nondestructive and noninvasive optical characterization not only for GaN but also for other semiconductor materials including nanostructures, which would be useful for diagnosing complex device structures. In the two-photon absorption (TPA) imaging, we successfully mapped out the 3D distribution of defects with high spatial resolution. Moreover, using transient TPA measurements, we were able to obtain precise TPA coefficient that is one of the useful parameters for evaluating the optical damage threshold. In the GaN films, we showed that localized regions exist with significantly higher TPA coefficients, possibly due to the concentrations of defect states on a microscopic scale. In another nonlinear spectroscopy based on degenerate FWM technique, we successfully mapped out the local strain-field very precisely. Since the diffraction signal in FWM increases as the fourth power of the exciton oscillator strength, the exciton spectra show strong polarizations resulting from the uniaxial strain in the sample. The highly polarized spectra allow us to determine the precise splitting energy imposed by the local strain and defects. We measured various samples including GaN films on isotropic substrates, and examined the lower limit of detectable uniaxial strain, which is comparable with the resolution of conventional X-ray diffraction analysis. Since the diffraction signal in FWM increases as the fourth power of the exciton oscillator strength, the exciton spectra show strong polarizations resulting from the uniaxial strain in the sample. The highly polarized spectra allow us to determine the precise splitting energy imposed by the local strain and defects. We measured various samples including GaN films on isotropic substrates, and examined the lower limit of detectable uniaxial strain, which is comparable with the resolution of conventional X-ray diffraction analysis.
  • Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research (B)
    Date (from‐to) : 2004 -2006 
    Author : MORITA Ryuji, GOHARA Kazutoshi, TODA Yashinori
     
    1.An over-one-octave spanning (460-1060 nm) ultra-broadband optical pulse was generated by self-modulation effect. It was spectrally-phase-compensated and characterized to be a 2.8 fs, 1.5 cycle-Fourier-transform-limited pulse by the modified spectral phase interferometry for direct electric-field reconstruction. It is the shortest visible and infrared pulse whose spectral phase is well determined. 2.A second-harmonic laser microscope was developed for 3-dimensional spatially-resolved observation of living cells. As an example, neurons of a rat were observed. Strong signals from nuclei and axons owing to inversion-symmetry breaking were detected. Spatial resolution was less than 1.5 μm. 3.A coherent anti-Stokes Raman scattering laser microscope was developed using a femtosecond laser amplifier as a light source. For onion cells, two pulses with wavelengths of 800 and 1040 nm were employed to excite the C-H vibration mode of 2900 cm-1 and the corresponding spatially-resolved signal with a wavelength of 650 nm was detected. Similarly, two pulses with wavelengths of 800 and 1100 nm were employed to excite the N-H vibration mode of 3400 cm-1 and the corresponding signal with a wavelength of 630 nm was detected. From comparison between those signals, C-H-and N-H-rich domains in cells were determined. 4.Macroscopic observation of hybridization and dehybridization for methylred-intercalated DNA was performed using second-harmonic laser microscope. While hybridized one has the inversion symmetry, dehybridized one does not. Significant signals were not obtained so far because of noise. Improvement is going on by introducing other azo-dyes with higher optical nonlinearity.
  • Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research
    Date (from‐to) : 2002 -2005 
    Author : MUTO Shunichi, ADACHI Satoru, TODA Yasunori, SHIRAMINE Kenichi, YOH Kanji
     
    An electron spin in a quantum dot is one of important candidates to be used for future large scale quantum computing. We have already proposed a set of universal quantum gates using photon-assisted tunneling and its coulomb blockade. In this project, we have done various experimental studies on the feasibility of this proposal. Also, the quantum computing is required to be connected to the photonic quantum infromation processing. Therefore we proposed a scheme to cunvert a photon qubit and an electron spin qubit and experimentally evaluated this proposal. Before experiments, we did computer simulation of our universal gates and found that the experimental demonstration of the CNOT gateis possible ifthe coheraice of an electron spin is largo-than 100 ps. Then, we developed a measurem ent technology based on the four-wave-mixing FWM to detarmine the electron spin coherence. We applied this to the spin relaxation measurements of electrons in a quantum well for which the pump -probe technique is established. The result of the new method, i.e. spin-diffracted FWM gave the same result as the pump -probe, indicating the validity of the method. We then, applied this to the ensemble of InAlAs/AlGaAs quantum dots and found that the longest coherence of these dots is as long as 7 ns. This value is much longer than those in the conventional method using streak-camera and indicates that the measurements of resonant excitation and dedection such as ours are necessary for precise determination of spin coherence instead of the non-resonant measurements such as those using streak-camers. We also proposed a new qubit conversion scheme using the nuclear field to realize the degenerate electron levels and energy-split hole levels in a quantum dot. By exerimentally realizing the nuclear field by optically orienting the electron spins in an InAlAs/AlGaAs quantum dot, we found that we can realize the band structure for a qubit conversion between a photon and an dectron spin.
  • Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research
    Date (from‐to) : 2002 -2003 
    Author : MUTO Shunichi, SHIRAMINE Kenichi, TODA Yasunori, ADACHI Satoru, TAKEUCHI Atsushi
     
    Self-assembled InAlAs/AlGaAs quantum dots are grown by the Stranski-Krastanow mode of Molecular Beam Epitaxy (MBE). We measured the spin relaxation time of the structure under the non-resonant optical excitation by using 80 fs pulses by the titanium sapphire laser at wavelength of 780 nm. We used the right circular polarized light for the excitation of carrires to obtain fixed electron spins. The luminescence from the quantum dots at 10 K was divided into the right circular and the left circular components and was temporary resolved by the streak-scope. While the lifetime of the exciton was about 1 18 ns, the spin relaxation time determined by the decay of the polarization of luminescence was 1.31 us, which is comparable to the lifetime. The result indicates that the major relaxation mechanisms in the quantum well are not effective in the quantum dots and that the quantum dot is a good material for the coherent control of electron spins. As the spin-electronisc material for the future generations, wide-gap semiconductors and a polymer was studied with emphasis on the magnetic or spintronic properties.
  • Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research
    Date (from‐to) : 2001 -2002 
    Author : ADACHI Satoru, TANAKA Katsuhiko, TODA Yasunori
     
    Im present work, we aimed the estimation of the crystal quality via the detection of the phonon scattering by the defect and so on by using our developed phonon-polariton imaging technique. We have performed the imaging experiment for the LiTaO_3. The phonon generation and monitoring of the propagating polariton wavepackets have been imaged. Illuminating the delayed pulse into the crystal in order to probe the generated phonon-polariton wavepackets, we could construct the image of the wavepackets at the real time within the temporal resolution of the used pulse. The phonon polariotn is really the modulation of the refractive indices and therefore the phase object. Although it is impossible usually to detect the phase of the scattered light since our detection ability is sensitive only to the intensity of the light, the conversion from the phase information to the intensity information can be achieved by using Talbot effect. We have succeeded to capture the interference of the phonon polariotn wavepackets at the real time via the special arrangement. Also we could control the amplitude of the propagating phonon polariton wavepacket within some extent. The obtained results can contribute greatly to the phonon spectroscopy and indicated the new direction of the research. By using the similar four-wave mixing technique, we could estimate the sample quality of the bulk GaN, which is the promising material for the laser diode and so on, via the phonon scattering and exciton-exciton interaction, and found the heterobiexciton. The hetero biexciton consists of the heavy-hole and light-hole excitons and we found the heterobiexciton in GaN for the first time.
  • 日本学術振興会：科学研究費助成事業
    Date (from‐to) : 2000 -2001 
    Author : 大場 良次, 戸田 泰則
     
    本研究の目的は、固体分光計測において高い空間分解能を有する走査型近接場光学顕微鏡(SNOM)の小型化を図り、外部装置との整合性を高め、空間的な多次元化を実現することである。本年度は、昨年度製作したSNOMを用いて、半導体量子ナノ構造の光学特性の取得と、その小型化を含めた多次元計測への応用を目標とした。 半導体量子ナノ構造を調査する目的として、制御性の高い量子ドットを測定対象とした。量子ドットではキャリアの自由度が全方向に制限されるため、ナノ領域の光物性として特異な性質を示すことが知られている。量子ドットを光励起すると、キャリアはまずドットの閉じ込めに起因する離散状態に励起され、発光再結合準位へと緩和し発光する。さらに今回試料として用いている自己形成量子ドットでは、周囲の媒質を反映した多くの局在フォノンモードが存在し、その緩和スペクトルはドットのキャリア閉じ込めと周囲の格子状態を反映することになる。したがって個々のドットの共鳴エネルギーに対して、その発光分布像を得ることにより、その共鳴がどのようなフォノンと結合しているか決定することが可能となる。今回の測定結果では、バルクのフォノンモードと、局在フォノンモードの共鳴エネルギーに対してそれぞれ異なる発光分布像が観測された。このことは自己形成量子ドットにおけるキャリア-フォノン結合状態の違いを意味しており、ヘテロ界面の重要性を明らかにしたものである。したがって空間的な多次元計測を達成すれば、より詳細な物性が明らかになると考えられる。この目的のために、いくつかの装置要素に対して小型化と多機能化を試みた。具体的にはイナーシャルスライダーの小型化、高さ制御ピエゾの小型化、光学機器の低減を達成した。装置の小型化は、機械的な安定性も向上させるため、上記の結果をもとにしたフォノンイメージングへと発展させることが可能となる。
  • Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research
    Date (from‐to) : 2000 -2001 
    Author : TODA Yasunori
     
    The strain-driven self-assembly of semiconductor quantum dots (SAQDs) provides nanometer scale carrier confinement in all three dimensions. One of the fascinating characteristics of SAQDs is their discrete density of states (DOS) with large energy-level spacing, in which acoustic phonon-mediated scattering should be suppressed compared with higher-dimensional structures. As a consequence, excitons in SAQDs are expected to exhibit long coherence time, which is advantageous for application of quantum information processing. In terms of inter-level relaxation, interaction with longitudinal optical (LO) phonons is an important aspect of SAQDs. These phonon-mediated transitions can be confirmed in the single-dot photoluminescence excitation (PLE) spectra, in which the reduction of relaxation paths renders individual LO phonons visible in the ground state emissions. To date several LO phonon-related transitions including bulk GaAs have been clearly observed. In the energy region higher than 1LO, single-dot PLE spectra reveal a number of intense PLE features, recognized as 2LO phonon resonances. As excitonic excited states are also present, several relaxation pathways with qualitatively different processes exist in the 2LO region. In this research, we investigate in detail the 2LO resonances in the single-dot PLE. Several 2LO resonances emerge as dip structures in the peaks under various conditions of excitation power. The observed dip structures suggest line broadening, even in the individual resonace lines. In contrast, the corresponding biexciton resonances still have sharp peaks, indicating that the 2LO resonances consist of several relaxation channels with differing scattering processes. These observations can explain why PLE resonances exhibit short dephasing times even in single SAQDs. Moreover, we found that phonon mediated transitions can be largely removed by two-color excited PLE spectroscopy. The reduction of line broadening indicates that the PLE includes a number of phonon-related transitions with large relaxation windows.

Educational Activities

Teaching Experience

  • Optical Properties of Materials
    開講年度 : 2021
    課程区分 : 修士課程
    開講学部 : 工学院
    キーワード : 線形光学、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、半導体、半導体ナノ構造、励起子
  • Optical Properties of Materials
    開講年度 : 2021
    課程区分 : 博士後期課程
    開講学部 : 工学院
    キーワード : 線形光学、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、半導体、半導体ナノ構造、励起子
  • Applied Mathematics I
    開講年度 : 2021
    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
  • Exercise in Applied Mathematics I
    開講年度 : 2021
    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　　　

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