研究者データベース
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研究者情報
マスター
アカウント（マスター）
氏名
戸田 泰則(トダ ヤスノリ), トダ ヤスノリ
所属（マスター）
所属（マスター）
独自項目
syllabus
- 2021, 光物性特論, Optical Properties of Materials, 修士課程, 工学院, 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
- 2021, 光物性特論, Optical Properties of Materials, 博士後期課程, 工学院, 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
- 2021, 応用数学Ⅰ, Applied Mathematics I, 学士課程, 工学部, 常微分方程式 ラプラス変換 フーリエ解析 偏微分方程式
- 2021, 応用数学演習Ⅰ, Exercise in Applied Mathematics I, 学士課程, 工学部, 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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プロフィール情報
学位
プロフィール情報
姓
戸田, トダ名
泰則, ヤスノリID各種
201201080963636706
対象リソース
業績リスト
研究キーワード
研究分野
経歴
- 2012年04月 - 現在 北海道大学 工学(系)研究科(研究院) 教授
- 1999年 - 2011年 北海道大学 工学(系)研究科(研究院) 准教授
- 1996年04月 - 1999年08月 東京大学 生産技術研究所 学振特別研究員、助手
学歴
- 1993年04月 - 1996年03月 東京工業大学 大学院総合理工学研究科
- 1991年04月 - 1993年03月 東京工業大学 大学院総合理工学研究科
- 1987年04月 - 1991年03月 東京工業大学 工学部 有機材料工学科
委員歴
受賞
論文
- T. Akiba, Y. Toda, S. Tsuchiya, M. Oda, T. Kurosawa, D. Mihailovic, T. MerteljPhysical Review B 109 1 2024年01月09日
- Satoshi Tsuchiya, Masato Katsumi, Ryuhei Oka, Toshio Naito, Yasunori TodaCondensed Matter 8 4 88 - 88 2023年10月10日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 TodaJournal of the Physical Society of Japan 92 9 094703/1 - 7 2023年09月15日 [査読有り]
- Yasunori Toda, Satoshi Tsuchiya, Keisaku Yamane, Ryuji Morita, Migaku Oda, Tohru Kurosawa, Tomaz Mertelj, Dragan MihailovicOptics Express 31 11 17537 - 17537 2023年05月10日 [査読有り]
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. TodaPhysical Review Research 5 1 2023年01月19日 [査読有り][通常論文]
- Asami Honda, Keisaku Yamane, Kohei Iwasa, Kazuhiko Oka, Yasunori Toda, Ryuji MoritaScientific reports 12 1 18674 - 18674 2022年11月04日
- Asami Honda, Keisaku Yamane, Kohei Iwasa, Kazuhiko Oka, Yasunori Toda, Ryuji MoritaScientific reports 12 1 14991 - 14991 2022年09月02日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, Ryota Tamemoto, Keisaku Yamane, Masato Suzuki, Yasunori Toda, Takashige Omatsu, Ryuji MoritaOptics InfoBase Conference Papers 2022年We successfully demonstrated ultrafast snapshot imaging based on time-to-frequency mapping method with four frames and ∼ 5.5 ns time interval (i.e. sub-GHz framerate) by using our newly-developed recirculation system without ultrafast image sensor.
- Asami Honda, Ryota Tamemoto, Keisaku Yamane, Masato Suzuki, Yasunori Toda, Takashige Omatsu, Ryuji Morita2022 Conference on Lasers and Electro-Optics Pacific Rim, CLEO-PR 2022 - Proceedings 2022年We successfully demonstrated ultrafast snapshot imaging based on time-to-frequency mapping method with four frames and ∼ 5.5 ns time interval (i.e. sub-GHz framerate) by using our newly-developed recirculation system without ultrafast image sensor.
- Y. Toda, S. Tsuchiya, K. Yamane, R. Morita, M. Oda, T. Mertelj, D. MihailovicProceedings of the 2022 Conference on Lasers and Electro-Optics Pacific Rim 2022年Recently, topological lightwaves, represented by optical vortices, have become easily generated and are expected to have many novel applications. In this work, we report on coherent quench spectroscopy of a cuprate superconductor using optical vortex pulses to reveal the spatially modulated superconducting (SC) state. The transient response after quenching by the optical vortex pulse shows the spatially localized SC response that remains unquenched around the dark spot of the optical vortex. The spatially resolved imaging of the optical vortex-induced SC improves the spatial resolution in a similar principle to super-resolution microscopy.
- Asami Honda, Keisaku Yamane, Masato Suzuki, Jin Kawaguchi, Yasunori Toda, Takashige Omatsu, Ryuji Morita2022 Conference on Lasers and Electro-Optics, CLEO 2022 - Proceedings 2022年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. TodaEurophysics Letters 136 5 57001 - 57001 2021年12月01日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. MerteljPhysical Review B 104 11 2021年09月24日 [査読有り]
- Y. Toda, S. Tsuchiya, M. Oda, T. Kurosawa, S. Katsumata, M. Naseska, T. Mertelj, D. MihailovicPhysical Review B 104 9 2021年09月07日 [査読有り]
- Satoshi Tsuchiya, Tomaz Mertelj, Dragan Mihailovic, Jun-ichi Yamada, Yasunori TodaJournal of Superconductivity and Novel Magnetism 33 8 2299 - 2303 2020年08月 [査読有り]
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 TodaJournal of the Physical Society of Japan 89 6 064712 - 064712 2020年06月15日 [査読有り]
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 TodaJOURNAL OF SUPERCONDUCTIVITY AND NOVEL MAGNETISM 33 2427 - 2433 2020年03月 [査読有り][通常論文]
The electronic state of the organic single crystal kappa - [(d-BEDT-TTF)(0.5) (h-BEDT-TTF)(0.5)](2) Cu[N(CN)(2)]Br, in which similar to 50 percent of the BEDT-TTF molecules are substituted by fully deuterated molecules (d-BEDT-TTF), has been characterized by resistance and polarized time-resolved spectroscopy measurements. We found that while the resistance shows no sign of a superconducting transition, the polarized time-resolved spectroscopy reveals a slow decay dynamics associated with the superconducting phase at a low temperature. The result suggests phase separation between the insulating and superconducting phases in the vicinity of the first-order Mott transition. In addition, we found that this slow component shows a steep increase below similar to 18 K, suggesting a fluctuating superconductivity in the conducting domains. Furthermore, the component persists up to at least similar to 50 K, which is almost consistent with the onset temperature of the anomalous magnetic-field-dependent signals observed by the previous Nernst effect measurements (Nam et al. Sci. Rep.3:3390, 2013) of the alloy kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]Cl0.27Br0.73. While interpretation of these anomalies is still debatable, there exists a possibility of the onset of superconducting paring at a very high temperature. - Laguerre-Gaussian vortex mode generation from astigmatic semiconductor microcavityK. Nakagawa, K. Yamane, R. Morita, Y. TodaAppl. Phys. Express 13 042001/1 - 5 2020年03月 [査読有り][通常論文]
- Rin Sasaki, Keisaku Yamane, Kazuhiko Oka, Yasunori Toda, Takashige Omatsu, Ryuji MoritaProceedings of SPIE - The International Society for Optical Engineering 11522 2020年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 YasunoriJOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 88 7 2019年07月15日 [査読有り][通常論文]
- Masato Suzuki, Keisaku Yamane, Kazuhiko Oka, Yasunori Toda, Ryuji MoritaSCIENTIFIC REPORTS 9 2019年07月 [査読有り][通常論文]
We demonstrate a comprehensive quantitative analysis of vector beam states (VBSs) by using a vector field reconstruction (VFR) technique integrating interferometry and imaging polarimetry, where the analysis is given by a cylindrically polarized Laguerre-Gaussian (LG) mode expansion of VBSs. From test examples of cylindrically polarized LG mode beams, we obtain the complex amplitude distributions of VBSs and perform their quantitative evaluations both in radial and azimuthal directions. The results show that we generated (l, p) = (1, 0) LG radially polarized state with a high purity of 98%. We also argue that the cylindrically polarized LG modal decomposition is meaningful for the detail discussion of experimental results, such as analyses of mode purities and mode contaminations. Thus the VFR technique is significant for analyses of polarization structured beams generated by lasers and converters. - Keisaku Yamane, Kohei Iwasa, Rin Sasaki, Kazuhiko Oka, Yasunori Toda, Ryuji Morita2019 Conference on Lasers and Electro-Optics Europe and European Quantum Electronics Conference, CLEO/Europe-EQEC 2019 2019年06月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 Morita2019 Conference on Lasers and Electro-Optics Europe and European Quantum Electronics Conference, CLEO/Europe-EQEC 2019 2019年06月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 MoritaProceedings of SPIE - The International Society for Optical Engineering 11141 2019年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 MoritaOptics InfoBase Conference Papers Part F140-CLEO_Europe 2019 2019年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. AwajiProceedings of SPIE - The International Society for Optical Engineering 11141 2019年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 MoritaOptics InfoBase Conference Papers Part F140-CLEO_Europe 2019 2019年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 YasunoriEPL 122 6 2018年06月 [査読有り][通常論文]
- Masato Suzuki, Keisaku Yamane, Moritsugu Sakamoto, Kazuhiko Oka, Yasunori Toda, Ryuji MoritaOPTICS EXPRESS 26 3 2584 - 2598 2018年02月 [査読有り][通常論文]
We proposed and constructed a system to realize broadband generation of arbitrary axisymmetrically polarized (AP) pulses with spatial complex amplitude modulation. This system employs the combination of a spatial light modulator in the 4-f configuration (4-f SLM), and a space variant wave plate as a common path interferometer. The 4-f SLM and the common path interferometer offer compensation for spatial dispersion with respect to wavelength and stability to perturbation, respectively. We experimentally demonstrated the various AP pulses generation by applying modulations of fundamental and higher-order Laguerre-Gauss modes, whose radial indices were, respectively, p = 0 and 1, with high purity, which showed that we were able to generate arbitral AP pulses with spatial complex amplitude modulation. This technique is expected to be applied in both classical and quantum communications with higher-order modes. (c) 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement - Keisaku Yamane, Yuho Watanabe, Isamu Wakisaka, Kazuhiko Oka, Yasunori Toda, Ryuji MoritaOptics InfoBase Conference Papers Part F114-FIO 2018 2018年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 YasunoriCOMPLEX LIGHT AND OPTICAL FORCES XII 10549 2018年 [査読有り][通常論文]
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. MihailovicPHYSICAL REVIEW B 96 18 2017年11月 [査読有り][通常論文]
A systematic temperature-dependent study of the femtosecond optical superconducting (SC) state destruction and recovery in Bi2Sr2CaCu2O8+delta cuprate superconductor by means of the all-optical polarization-sensitive multipulse spectroscopy is presented. At low temperatures and a partial SC state suppression, an anisotropic SC-gap recovery time scale is suggested by the data. The SC state destruction and recovery dynamics are compared to the recent TR-ARPES-inferred SC-gap dynamics and a qualitative agreement is found. Using a phenomenological response function, the experimental data are also compared to time-dependent Ginzburg-Landau model simulations. - Y. Toda, H. Mochizuki, S. Tsuchiya, T. Kurosawa, M. Oda, T. Mertelj, D. MihailovicJournal of Superconductivity and Novel Magnetism 31 3 1 - 4 2017年10月11日 [査読有り][通常論文]
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. TodaPHYSICAL REVIEW B 96 13 2017年10月 [査読有り][通常論文]
In this work, polarized pump-probe spectroscopy was carried out to investigate the effects of a structural ordering of molecules on photoinduced phase separation (PIPS) in the organic conductors k-(BEDT-TTF)(2)X [X = Cu[N(CN)(2)]Br (k-Br) and Cu(NCS)(2) (k-NCS)]. We found that the anisotropic response for the probe polarization appeared at around T-g, where the glasslike structural transition occurs. The anisotropy can be a result of a transient destruction of the local ordering of molecules, indicating a connection between the glasslike transition and PIPS. Moreover, we found that the PIPS response gradually develops with decreasing temperature in k-Br, whereas it steeply increases in k-NCS. This qualitative difference suggests that the structural ordering caused by a PIPS is more crucial in k-NCS than in k-Br. - Y. Toda, A. Moriya, K. Yamane, R. Morita, K. Shigematsu, Y. AwajiAPPLIED PHYSICS LETTERS 111 10 2017年09月 [査読有り][通常論文]
The single orbital angular momentum mode emission from a commercially available broad-area vertical cavity surface emitting laser (VCSEL) is realized with an optical feedback technique using a spatial light modulator, where a computer generated hologram (CGH) is optimized by taking into account the mode properties of the external cavity VCSEL. The flexibility with which the chirality of the orbital angular momentum mode can be selected is also demonstrated simply by changing the chirality of the CGH. The technique is very simple and easily applicable to other VCSELs without processing. (C) 2017 Author(s). - Keisaku Yamane, Kohei Iwasa, Kohei Kakizawa, Kazuhiko Oka, Yasunori Toda, Ryuji Morita2017 CONFERENCE ON LASERS AND ELECTRO-OPTICS EUROPE & EUROPEAN QUANTUM ELECTRONICS CONFERENCE (CLEO/EUROPE-EQEC) 2017年 [査読有り][通常論文]
- Y. Toda, H. Moriya, K. Shigematsu, K. Yamane, R. Morita, Y. AwajiOPTICAL MANIPULATION CONFERENCE 10252 2017年 [査読有り][通常論文]
Single angular momentum (OAM) mode emissions from a vertical cavity surface emitting laser (VCSEL) were demonstrated by an external optical feedback using computer generated holograms, which are optimized on the OAM modal gain of the free-running VCSEL. Side-mode suppression ratio of more than 23 dB was achieved for the OAM modes with l = +/- 1. - Keisaku Yamane, Kohei Iwasa, Kohei Kakizawa, Kazuhiko Oka, Yasunori Toda, Ryuji MoritaOPTICAL MANIPULATION CONFERENCE 10252 2017年 [査読有り][通常論文]
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 TodaAPPLIED PHYSICS EXPRESS 9 12 2016年12月 [査読有り][通常論文]
By using radially polarized pulses for excitation in degenerate four-wave mixing (DFWM) spectroscopy, we realize highly sensitive snap-shot measurement of optical anisotropies in semiconductor films. Because the radially polarized pulses exhibit spatially varying linear polarizations, the optical anisotropies in samples can be evaluated from the spatial distributions of DFWM signals without rotating the polarization of the excitation pulses. We measure the excitons in GaN layers and evaluate the energies of uniaxial strain and the spin-exchange interaction constant. Our results prove both the accuracy and sensitivity of the snap-shot measurement. (C) 2016 The Japan Society of Applied Physics - K. Nakagawa, S. Tsuchiya, J. Yamada, Y. TodaJOURNAL OF SUPERCONDUCTIVITY AND NOVEL MAGNETISM 29 12 3065 - 3069 2016年12月 [査読有り][通常論文]
We investigated photo-excited carrier relaxation dynamics in the strongly correlated organic superconductors kappa-(BEDT-TTF)(2)Cu(NCS)(2) and kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]Br, using different polarizations of pump and probe pulses. Below the glasslike transition temperature (T (g)) anisotropic responses for probe polarization were observed in both compounds. Decomposing the data into anisotropic and isotropic components, we found the anisotropic component shows no pump polarization dependence, meaning that dissipative excitation process was dominant for the anisotropic carrier relaxation. This behavior indicates that the appearance of anisotropic responses can be associated with spatial symmetry breaking due to structural change of BEDT-TTF molecules. - Satoshi Tsuchiya, Koichi Nakagawa, Jun-ichi Yamada, Yasunori TodaJOURNAL OF SUPERCONDUCTIVITY AND NOVEL MAGNETISM 29 12 3071 - 3074 2016年12月 [査読有り][通常論文]
Photo-induced carrier relaxation dynamics have been investigated for the organic superconductor kappa-(BEDT-TTF)(2)Cu(NCS)(2) with different probe polarization under 1.3 kbar at low temperatures. We successfully observed the isotropic and anisotropic responses for the probe polarization, which were found to appear at 56 K. By comparing the responses with and without applying pressure, we found that those were slightly changed, indicating that the application of pressure has effect on the carrier relaxation dynamics. - Masato Suzuki, Keisaku Yamane, Kazuhiko Oka, Yasunori Toda, Ryuji MoritaPhysical Review A - Atomic, Molecular, and Optical Physics 94 2016年10月26日 [査読無し][通常論文]
© 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 MoritaPHYSICAL REVIEW A 94 4 2016年10月 [査読有り][通常論文]
We derived the Berry connection of vector vortex states (VVSs) from the "true" Hamiltonian obtained through the Maxwell-Schrodinger equation for an inhomogeneous anisotropic (IA) medium, and we experimentally demonstrated measurement of the corresponding Pancharatnam-Berry (PB) geometrical phase of VVSs. The PB phase (PBP) of VVSs can be divided into two phases: homogeneous and inhomogeneous PBPs. Homogeneous and inhomogeneous PBPs are related to the conventional PBP and the spatially dependent geometric phase given by an IA medium such as a polarization converter, respectively. We theoretically detected that inhomogeneous PBP accumulation originates from the gauge dependence of the index of the hybrid-order Poincare sphere, which provides an alternate method for understanding optical spin-orbital angular momentum conversion. The homogeneous PBP, which is explicitly observed, has implications for quantum-statemanipulation and information processing. - Satoshi Tsuchiya, Yohei Kino, Koichi Nakagawa, Daisuke Nakagawa, Jun-ichi Yamada, Yasunori TodaREVIEW OF SCIENTIFIC INSTRUMENTS 87 4 2016年04月 [査読有り][通常論文]
To perform the femtosecond pump-probe spectroscopy under high pressure and low temperature, we constructed a measurement system with a piston cylinder type pressure cell installing an optical fiber bundle. The applied pressure was achieved to 6 kbar and the cell was cooled down to 15 K. Several demonstrations revealed that broadening and change of polarization of pulse (duration of similar to 120 fs) owing to the dispersions in the fiber bundle are much small indicating that those have little influence on the measurement of carrier relaxation dynamics. In the measurements of kappa-(BEDT-TTF)(2)Cu(NCS)(2) under 1.3 kbar at 43 K, we have successfully detected the polarization anisotropy of the carrier relaxation dynamics and estimated the decay time in the same way as the normal measurement. (C) 2016 AIP Publishing LLC. - K. Shigematsu, K. Yamane, R. Morita, Y. TodaPhysical Review B - Condensed Matter and Materials Physics 93 2016年01月15日 [査読無し][通常論文]
© 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. - 内古閑 隆太郎, 重松 恭平, 山根 啓作, 森田 隆二, 戸田 泰則日本物理学会講演概要集 71 1414 - 1414 一般社団法人 日本物理学会 2016年
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。
- 戸田 泰則, 黒澤 徹, 小田 研日本物理学会誌 71 12 830 - 835 一般社団法人 日本物理学会 2016年 [査読無し][通常論文]
<p>超伝導は物理学における最も魅力的な現象の一つであり，カマリング・オネスによる発見から今日に至るまで多くの研究者を魅了してきた．なかでも液体窒素温度をはるかに上回る転移温度を示す銅酸化物は，いわゆる高温超伝導体のプロトタイプとして数多くの研究がなされ，今も新たな知見や問題提起が次々と報告されている．</p><p>超伝導状態では電子間に有効な引力が作用して電子対（クーパー対）が形成され，複数の電子対が位相をそろえて量子凝縮する．凝縮が起こると，フェルミ面近傍の電子スペクトルには，対演算子の期待値（ギャップ関数）によって特徴づけられるエネルギーギャップが開く．Bardeen–Cooper–Schrieffer (BCS) の理論によって説明される従来型超伝導では，格子振動（フォノン）を介した引力で形成される電子対がフェルミ面上に等方的なs波対称性を持つギャップを形成する．これに対して電子間に強い斥力の働く銅酸化物高温超伝導体のギャップは，有効な引力を獲得するために異方的なギャップ関数を持つと考えられており，フェルミ面上にギャップの節（ノード）を持つd波対称性を示す．また超伝導転移温度(<i>T</i><sub>c</sub>)以上でフェルミ面の一部に擬ギャップと呼ばれるギャップ構造が形成されることが知られており，特に反強磁性絶縁相に近いアンダードープ(UD)試料では<i>T</i><sub>c</sub>よりはるかに高温から観測される．擬ギャップの存在は銅酸化物高温超伝導体の最大の特徴であり，古くから精力的な研究がなされているが，超伝導の前駆現象から超伝導と共存/ 競合する秩序相など様々な解釈があり，結論を見ていない．また物質や測定手法に応じて多様な性質を示すため，ギャップの起源や形成メカニズムを決定するには，多面的かつ系統的な物性観測が必要とされている．</p><p>このように多彩な顔を持つ擬ギャップおよび超伝導ギャップと両者の関係性について，従来の実空間や運動量空間の観測手法に加え，超短光パルスを使った時間領域からのアプローチが行われるようになってきた．本稿では物性観測に広く使われる時間分解ポンププローブ分光をベースに，この手法が，これまで観測できなかった高温超伝導体のバルク性質を反映した対称性解析，および秩序形成の相関スケール解析へと拡張可能であることを示す．本稿で扱うポンププローブ分光では，ポンプ光を用いてギャップを形成する電子または正孔（キャリア）を瞬時的に破壊し，高エネルギー状態へと励起する．キャリアのエネルギー緩和に伴って生成される非平衡な準粒子やフォノンは（超伝導・擬ギャップに対応する）ギャップ関数の変化を反映し，ポンプ光に対して遅延時間を持つプローブ光の反射率変化として測定される．励起に使用する光子エネルギーはギャップエネルギーよりもはるかに大きいが，結晶内部への侵入長を大きく取れるため，バルク性質を反映する．拡張されたポンププローブ分光は，それぞれ超伝導ギャップおよび擬ギャップに起因する準粒子応答を一意に決定可能であり，超伝導前駆現象を反映する揺らぎ状態を含め，超伝導ギャップと擬ギャップの個性および両者の関係性をダイナミクスの観点から明らかにする．偏光応答に着目した対称性解析では，擬ギャップの形成温度<i>T </i><sup>*</sup>以下で自発的な回転対称性破れが生じることが示される．他方，秩序形成ダイナミクスからは，擬ギャップが長距離相関のない局在状態を反映することが明らかとなる．</p> - K. Shigematsu, K. Yamane, R. Morita, Y. TodaPHYSICAL REVIEW B 93 4 2016年01月 [査読有り][通常論文]
The coherent dynamics of the exciton center-of-mass motion in bulk GaN are studied using degenerate four-wave-mixing (FWM) spectroscopy with Laguerre-Gaussian (LG) mode pulses. We evaluate the exciton orbital angular momentum (OAM) dynamics from the degree of OAM, which is derived from the distributions of OAM (topological charge) of the FWM signals. When excitons are excited with two single-mode LG pulses, the exciton OAM decay time significantly exceeds the exciton dephasing time, which can be attributed to high uniformity of the exciton dephasing in our bulk sample because the decoherence of the exciton OAM is governed by the angular variation in the exciton dephasing. We also analyze the topological charge (l) dependence of the OAM decay using a multiple-mode LG pump pulse, which allows us to simultaneously observe the dynamics of the exciton OAM for different l values under the same excitation conditions. The OAM decay times of the l = 1 component are usually longer than those of the l = 0 component. The l-dependent OAM decay is supported by a phenomenological model which takes into account the local nonuniformity of the exciton dephasing. - Masato Suzuki, Keisaku Yamane, Kazuhiko Oka, Yasunori Toda, Ryuji MoritaSCIENTIFIC REPORTS 5 2015年12月 [査読無し][通常論文]
Cylindrically polarized (CP) modes are laser beam modes which have rotational symmetry of the polarization distribution around the beam axis. Considerable attention has been paid to CP modes for their various applications. In this paper, by using the extended Stokes parameters and the degree of polarization defined for the spatial distribution (DOP-SD), we fully-quantitatively characterize the spectrally-resolved polarization states of arbitrary CP (axisymmetrically polarized and higher-order cylindrically polarized) broadband pulses generated by coherent beam combining. All the generated pulse states were fully-quantitatively analyzed for the first time and proved to have high symmetry (DOP-SD greater than or similar to 0.95) and low spectral dependence of polarization states. Moreover, we show the DOP-SD, which cannot be defined by the conventional higher-order and hybrid Stokes parameters, enables us to make a quantitative evaluation of small degradation of rotational symmetry of polarization distribution. This quantitative characterization with high precision is significant for applications of precise material processing, quantum information processing, magneto-optical storage and nonlinear spectroscopic polarimetry. - K. Watanabe, S. Tsuchiya, Y. Toda, T. Kurosawa, M. Oda, T. Mertelj, I. Madan, V. V. Kabanov, D. MihailovicINTERNATIONAL JOURNAL OF MODERN PHYSICS B 29 25-26 2015年10月 [査読有り][通常論文]
The dynamics of quasi-particle (QP) excitations with different symmetry is investigated in the superconducting (SC) and pseudogap (PG) states of the high-temperature superconductor Bi2Sr2CaCu2O8+d (Bi2212) using a polarization-sensitive optical pump-probe measurement. The observation of distinct anisotropies for SC excitations and for PG excitations by the probe beam polarization and absence of any dependence on the pump beam polarization evidence the existence of a spontaneous spatial symmetry breaking in the PG state. - S. Tsuchiya, Y. Sugawara, S. Tanda, Y. TodaJOURNAL OF OPTICS 17 8 2015年08月 [査読有り][通常論文]
Photo-induced quasi-particle (QP) relaxation dynamics with different symmetries have been investigated for the multiple charge-density-wave (CDW) compound DyTe3 by using ultrafast polarized pump-probe spectroscopy. By performing symmetry analysis, the QP dynamics with isotropic A(1g) and anisotropic B-2g symmetry were found to show unique anomalies at the first and second CDW transitions. Both the temperature dependence and pump fluence dependence indicate that the B-2g response is very sensitive to the underlying lattice deformation, which provides critical insight into the multiple CDW formations. - I. Madan, T. Kurosawa, Y. Toda, M. Oda, T. Mertelj, D. MihailovicNATURE COMMUNICATIONS 6 2015年04月 [査読有り][通常論文]
A 'pseudogap' was introduced by Mott to describe a state of matter that has a minimum in the density of states at the Fermi level, deep enough for states to become localized. It can arise either from Coulomb repulsion between electrons, and/or incipient charge or spin order. Here we employ ultrafast spectroscopy to study dynamical properties of the normal to pseudogap state transition in the prototype high-temperature superconductor Bi2Sr2CaCu2O8+delta. We perform a systematic temperature and doping dependence study of the pseudogap photodestruction and recovery in coherent quench experiments, revealing marked absence of critical behaviour of the elementary excitations, which implies an absence of collective electronic ordering beyond a few coherence lengths on short timescales. The data imply ultrafast carrier localization into a textured polaronic state arising from a competing Coulomb interaction and lattice strain, enhanced by a Fermi surface instability. - Masato Suzuki, Keisaku Yamane, Kazuhiko Oka, Yasunori Toda, Ryuji MoritaOPTICAL REVIEW 22 1 179 - 183 2015年02月 [査読有り][通常論文]
The extended Stokes parameters for arbitrary cylindrically polarized beams are newly introduced to evaluate their quality. A set of the parameters, expressing a cylindrically polarized state, permits the definition of the degree of polarization that indicates the purity of the spatial symmetry of polarization of a light beam. In addition, the Pancharatnam-Berry phase related to the cylindrically polarized states is described by the new Stokes parameters. - 戸田 泰則日本物理学会講演概要集 70 1210 - 1211 一般社団法人日本物理学会 2015年
- 荒巻 光利, 吉村 信次, 森崎 友宏, 寺坂 健一郎, 戸田 泰則, 田中 雅慶日本物理学会講演概要集 70 875 - 875 一般社団法人日本物理学会 2015年
- Masato Suzuki, Keisaku Yamane, Kazuhiko Oka, Yasunori Toda, Ryuji Morita2015 CONFERENCE ON LASERS AND ELECTRO-OPTICS (CLEO) 2015年 [査読有り][通常論文]
By using the extended Stokes parameters and the degree of polarization defined for the spatial distribution (DOP-SD), we fully quantitatively characterize the polarization states of arbitrary (DOP-SD> 0.96) axisymmetrically-polarized femtosecond pulses generated by coherent beam combining. (C) 2014 Optical Society of America - TODA Yasunori, SHIGEMATSU KyohheiProc Hiroshima Int Symp Synchrotron Radiat 19th 116 - 120 2015年 [査読無し][通常論文]
- S. Ono, Y. Toda, J. OnoePhysical Review B 90 155435 2014年11月 [査読有り][通常論文]
- Shota Ono, Yasunori Toda, Jun OnoePHYSICAL REVIEW B 90 15 155435.1-155435.5 2014年10月 [査読有り][通常論文]
The electron-phonon coupling constant (lambda) of carbon nanotubes (lambda = 0.006) is much smaller than that of alkali-metal-doped C-60 crystals (lambda = 0.6). This difference may be due to the shape of the pi-electron conjugated system: the former has a flat, whereas the latter has a sphere. In order to confirm the shape effects in lambda, we have examined the magnitude of lambda for a one-dimensional (1D) uneven peanut-shaped C-60 polymer that has an intermediate shape of the pi-electron conjugated system between a carbon nanotube and a C-60 system, using femtosecond (fs) time-resolved pump-probe spectroscopy, because it can be expected to have an intermediate value of lambda between them. Theoretical analysis of fs-transient refractivity obtained experimentally found the magnitude of lambda of the 1D C-60 polymer film to be 0.02 as our expectation. This indicates that the shape of the pi-electron conjugated system affects the magnitude of lambda for nanocarbon allotropes significantly. - Y. Toda, F. Kawanokami, T. Kurosawa, M. Oda, I. Madan, T. Mertelj, V. V. Kabanov, D. MihailovicPHYSICAL REVIEW B 90 9 2014年09月 [査読無し][通常論文]
The quasiparticle (QP) dynamics with different symmetry is investigated in the superconducting (SC) and normal state of the high-temperature superconductor Bi2Sr2CaCu2O8+delta (Bi2212) using optical pump-probe experiments with different light polarizations at different doping levels. The observation of distinct selection rules for SC excitations present in A(1g) and B-1g symmetries, and for the pseudogap (PG) excitations present in A(1g) and B-2g symmetries by the probe, and absence of any dependence on the pump beam polarization leads to the unequivocal conclusion of the existence of a spontaneous spatial symmetry breaking in the PG state not limited to the sample surface. - Coherent dynamics of uniaxially-strained GaN excitons excited by cylindrically polarized pulsesK. Shigematsu, S. Morimoto, M. Suzuki, K. Yamane, R. Morita, Y. TodaOptics InfoBase Conference Papers 2014年07月21日
- Eigenmode expansion of optical vortices based on electric-field reconstruction in spatial domainKeisaku Yamane, Shota Nakajima, Masato Suzuki, Yasunori Toda, Ryuji MoritaOptics InfoBase Conference Papers 2014年07月21日
- Masato Suzuki, Keisaku Yamane, Kazuhiko Oka, Yasunori Toda, Ryuji MoritaOPTICS EXPRESS 22 14 16903 - 16915 2014年07月 [査読無し][通常論文]
Nonlinear propagation of focused axisymmetrically-polarized ultrashort optical pulses along the optic axis in a uniaxial crystal is investigated experimentally and theoretically. The energy transfer between an azimuthally-polarized pulse and a radially-polarized pulse is observed. To analyze the nonlinear propagation, a general paraxial equation with a third-order nonlinearity for axisymmetrically-polarized pulses in a uniaxial crystal is derived and the extended Stokes parameters (ESPs) based on cylindrical coordinates are newly-introduced. The simulation results by using this equation, providing the calculated ESPs, well explain our experimental observations: 1) the energy transfer is attributed to the four-wave-mixing effect, reflecting the overlapping between the axisymmetrically polarized modes, 2) the variations of the polarization defined from the ESPs are clarified to be affected by the self-and the cross-phase modulations, which make the effective propagation length long or short. (C) 2014 Optical Society of America - I. Madan, T. Kurosawa, Y. Toda, M. Oda, T. Mertelj, P. Kusar, D. MihailovicSCIENTIFIC REPORTS 4 2014年07月 [査読無し][通常論文]
In classical superconductors an energy gap and phase coherence appear simultaneously with pairing at the transition to the superconducting state. In high-temperature superconductors, the possibility that pairing and phase coherence are distinct and independent processes has led to intense experimental search of their separate manifestations. Using femtosecond spectroscopy methods we now show that it is possible to clearly separate fluctuation dynamics of the superconducting pairing amplitude from the phase relaxation above the critical transition temperature. Empirically establishing a close correspondence between the superfluid density measured by THz spectroscopy and superconducting optical pump-probe response over a wide region of temperature, we find that in differently doped Bi2Sr2CaCu2O8+delta crystals the pairing gap amplitude monotonically extends well beyond T-c, while the phase coherence shows a pronounced power-law divergence as T -> T-c, thus showing that phase coherence and gap formation are distinct processes which occur on different timescales. - Keisaku Yamane, Zhili Yang, Yasunori Toda, Ryuji MoritaNEW JOURNAL OF PHYSICS 16 May 053020 2014年05月 [査読有り][通常論文]
We propose a high-precision method for measuring the orbital angular momentum (OAM) spectrum of ultra-broadband optical-vortex (OV) pulses from fork-like interferograms between OV pulses and a reference plane-wave pulse. It is based on spatial reconstruction of the electric fields of the pulses to be measured from the frequency-resolved interference pattern. Our method is demonstrated experimentally by obtaining the OAM spectra for different spectral components of the OV pulses, enabling us to characterize the frequency dispersion of the topological charge of the OAM spectrum by a simple experimental setup. Retrieval is carried out in quasi-real time, allowing us to investigate OAM spectra dynamically. Furthermore, we determine the relative phases (including the sign) of the topological-charge-resolved electric-field amplitudes, which are significant for evaluating OVs or OV pulses with arbitrarily superposed modes. - C. Gadermaier, V. V. Kabanov, A. S. Alexandrov, L. Stojchevska, T. Mertelj, C. Manzoni, G. Cerullo, N. D. Zhigadlo, J. Karpinski, Y. Q. Cai, X. Yao, Y. Toda, M. Oda, S. Sugai, D. MihailovicPHYSICAL REVIEW X 4 1 011056 2014年03月 [査読有り][通常論文]
We use femtosecond optical spectroscopy to systematically measure the primary energy relaxation rate Gamma(1) of photoexcited carriers in cuprate and pnictide superconductors. We find that Gamma(1) increases monotonically with increased negative strain in the crystallographic a axis. Generally, the Bardeen-Shockley deformation potential theorem and, specifically, pressure-induced Raman shifts reported in the literature suggest that increased negative strain enhances electron-phonon coupling, which implies that the observed direct correspondence between a and Gamma(1) is consistent with the canonical assignment of Gamma(1) to the electron-phonon interaction. The well-known nonmonotonic dependence of the superconducting critical temperature T-c on the a-axis strain is also reflected in a systematic dependence T-c on Gamma(1), with a distinct maximum at intermediate values (similar to 16 ps(-1) at room temperature). The empirical nonmonotonic systematic variation of T-c with the strength of the electron-phonon interaction provides us with unique insight into the role of electron-phonon interaction in relation to the mechanism of high-T-c superconductivity as a crossover phenomenon. - Keisaku Yamane, Yasuyuki Shioda, Masato Suzuki, Yasunori Toda, Ryuji MoritaOptics InfoBase Conference Papers 2014年We demonstrated the generation of ultra-broadband optical pulses with axiallysymmetric polarization, on the basis of a coherent beam combining technique. The high polarization purity (~95 %) ranging from 750 to 880 nm was obtained. © 2014 Optical Society of America.
- Kyohhei Shigematsu, Keisaku Yamane, Ryuji Morita, Yasunori TodaOptics InfoBase Conference Papers 2014年The spatiotemporal coherence of excitons is investigated by degenerate four-wavemixing using optical vortices with multiple orbital angular momentum (OAM). The OAMresolved signal indicates that the topological phase with large OAM is robust against spatial decoherence. © 2014 OSA.
- Keisaku Yamane, Asami Honda, Yasunori Toda, Ryuji MoritaOptics InfoBase Conference Papers 2014年We demonstrated the generation of over 1-mJ intense optical-vortex pulses of which topological charges were programmably controlled by computer-generated holograms. The pulse duration was characterized to be 27 fs by two-dimensional spectral shearing interferometry. © 2014 OSA.
- Kyohhei Shigematsu, Keisaku Yamane, Ryuji Morita, Yasunori Toda2014 CONFERENCE ON LASERS AND ELECTRO-OPTICS (CLEO) 2014年 [査読有り][通常論文]
The spatiotemporal coherence of excitons is investigated by degenerate four-wavemixing using optical vortices with multiple orbital angular momentum (OAM). The OAM-resolved signal indicates that the topological phase with large OAM is robust against spatial decoherence. - Keisaku Yamane, Yasuyuki Shioda, Masato Suzuki, Yasunori Toda, Ryuji Morita2014 CONFERENCE ON LASERS AND ELECTRO-OPTICS (CLEO) 2014年 [査読有り][通常論文]
We demonstrated the generation of ultra-broadband optical pulses with axially-symmetric polarization, on the basis of a coherent beam combining technique. The high polarization purity (similar to 95 %) ranging from 750 to 880 nm was obtained. (C) 2014 Optical Society of America - Ryuji Morita, Keisaku Yamane, Zhili Yang, Yasunori TodaCOMPLEX LIGHT AND OPTICAL FORCES VIII 8999 2014年 [査読無し][通常論文]
Subsequently to the previous generation of ultrashort and ultra-broadband optical-vortex (OV) pulses (pulse energy: several tens of mu J) in few-cycle regime, we demonstrate high-power OV pulse generation with topological charge flexibility by employing a 4-f OV converter and pre- and main-amplifications. Our configuration overcomes the low-throughput drawback of the vortex converter, simultaneously compensating for the angular dispersion. It also gives flexibility of OAM- or topological-charge control. Thus, we succeed in generation of mJ-class ultra-broadband OV pulses (similar to 790-similar to 820 nm) with a programmably-controlled topological charge. Moreover, we experimentally exhibit a high-precision method for measuring frequency-resolved OAM spectrum of femtosecond ultra-broadband OV pulses on the basis of electric-field reconstruction in the spatial domain. In addition, we present the generation of ultrashort pulses with axially-symmetric polarization by coherent beam combining of OVs. - Y. Toda, T. Mertelj, P. Kusar, T. Kurosawa, M. Oda, M. Ido, D. MihailovicPHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS 493 112 - 113 2013年10月 [査読有り][通常論文]
We report nonequilibrium quasiparticle (QP) dynamics in underdoped Bi2212 crystals using ultrafast optical spectroscopy, which allows to analyze the dynamics associated with the superconducting (SC) and psuedogap (PG) QPs independently. In the saturation condition of the SC component, where the SC condensate is fully destroyed within the photoexcited volume, we found a delay of the SC state recovery associated with a transient normal state. The delay increases linearly with increasing the pump fluence. The QP dynamics also shows a contribution of the PG component, whose magnitude at the start of the SC state recovery was almost constant at various pump fluences, suggesting a critical level of PG order before the SC condensate can recover. (C) 2013 Elsevier B.V. All rights reserved. - Yasunori Toda, Kyohhei Shigematsu, Akira Hasegawa, Satoru AdachiJAPANESE JOURNAL OF APPLIED PHYSICS 52 8 08JL03 2013年08月 [査読有り][通常論文]
We report on the four-wave mixing (FWM) spectroscopy of excitons in tensile-strained GaN films. Three excitonic resonances (A-, B-, and C-excitons) were clearly observed with energy separations of Delta(AB) similar to 4.7 meV and Delta(BC) similar to 14.5 meV. Owing to tensile strain, these values are much smaller than those of free-standing GaN, allowing the simultaneous excitations of A-B and B-C excitons pairs. The time evolutions of the FWM in different polarization configurations resolved the differences in the simultaneous excitonic transitions; the excitation of A-B excitons showed an oscillation with a critical pi-phase shift in the orthogonal polarization configuration as expected in the quantum beat, and the oscillation of B-C excitons pair is independent of the polarization configuration, indicating polarization interference. The polarization-dependent FWM spectra of A-B excitons revealed peaks associated with biexciton transitions with binding energies of E-AA = 4.6 meV, E-AB = 2.1 meV, and E-BB = 1.7 meV. (C) 2013 The Japan Society of Applied Physics - Kyohhei Shigematsu, Yasunori Toda, Keisaku Yamane, Ryuji MoritaJAPANESE JOURNAL OF APPLIED PHYSICS 52 8 08JL08 2013年08月 [査読有り][通常論文]
We report on the coherent dynamics of the photoexcited orbital angular momentum (OAM) states of GaN excitons. The measurements were performed by a pump-probe four-wave mixing (FWM) using optical vortex (OV) pulses, where the pump OV pulse transfers its OAM to the center of mass momentum of excitons and the conversion of OAM via FWM with the probe pulse is detected. Especially, we performed analysis of the signal using an OAM-resolved spectroscopy, which allows us to evaluate the dephasing dynamics of excitons in time and space. The results indicate that near perfect conversion is achieved using the OV pulses with nonzero OAM and its spatial coherence remains high during the dephasing time, clearly indicating that the excitons keep the OAM transferred by the pump OV. In contrast, the component associated with the imperfect conversion becomes significant when one of the incident pulses intentionally includes an OAM-free component. The effect is clearly confirmed by the FWM excited by the pump with multiple OAM. (C) 2013 The Japan Society of Applied Physics - Shota Ono, Hiroyuki Shima, Yasunori TodaTrans. Mater. Res. Soc. Jpn. 38 2 283 - 286 The Materials Research Society of Japan 2013年06月 [査読有り][通常論文]
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. TodaJournal of Superconductivity and Novel Magnetism 26 5 1679 - 1682 2013年 [査読有り][通常論文]
The temperature-dependent time-resolved reflectance spectra of the organic superconductors (OSs) κ- and λ- (BETS)2MCl4 (M = Fe, Ga) and κ-(ET)2Cu[N(CN)2]Br are reported. The π electrons of the organic molecules (BETS or ET) are responsible for the electrical conduction in these salts, while only the Fe-containing salts have localized spins on the d electrons in the Fe atoms (S = 5/2) with π–d interaction. Analysis of the spectra provided the relaxation time τ of the carriers, as well as information concerning the number of carriers at the Fermi energy N(EF). The carrier dynamics were classified in terms of high-TC and low-TC OSs. In these salts, the results indicate that (1) the electron-phonon interaction is important in the occurrence and stability of the superconducting phase, and (2) electron correlation can be related to the enhancement of TC. - Y. Toda, K. Shigematsu, K. Yamane, R. MoritaOpt. Commun. 308 147 - 151 2013年 [査読有り][通常論文]
The orbital angular momentum (OAM) resolved spectroscopy is being applied to various research fields associated with optical vortices or Laguerre-Gaussian (LG) beams, in which the key technique is to convert LG beams to a fundamental Gaussian (HG≡HG00) beam. In this work, we propose a method to realize efficient mode conversion by using spiral phase modulations. Numerical simulations and experiments are carried out to compare our proposal with the standard technique using rotational phase modulations. The results reveal that our proposed method improves the uniformity of the conversion efficiency across various OAM, allowing us to obtain precise OAM spectra. © 2013 Elsevier B.V. - Masato Suzuki, Keisaku Yamane, Yasunori Toda, Ryuji Morita2013 CONFERENCE ON LASERS AND ELECTRO-OPTICS PACIFIC RIM (CLEO-PR) 2013年 [査読有り][通常論文]
We investigate nonlinear propagation of ultrashort optical pulses with radially- and azimuthally-polarized modes in a uniaxial crystal. The energy exchange between the modes, ascribed to nonlinear four-wave-mixing effect with Gouy-phase shifts, is discussed. - Masato Suzuki, Keisaku Yamane, Yasunori Toda, Ryuji Morita2013 CONFERENCE ON LASERS AND ELECTRO-OPTICS EUROPE AND INTERNATIONAL QUANTUM ELECTRONICS CONFERENCE (CLEO EUROPE/IQEC) 2013年 [査読有り][通常論文]
- Keisaku Yamane, Asami Honda, Kyohhei Shigematsu, Yasunori Toda, Ryuji Morita2013 CONFERENCE ON LASERS AND ELECTRO-OPTICS PACIFIC RIM (CLEO-PR) 2013年 [査読有り][通常論文]
We demonstrate the optical parametric amplification of optical-vortex pulses with the programmable topological-charge control. Combination of optical-parametric amplification and 4-f configuration overcomes the low-throughput drawback of the vortex converter, simultaneously compensating for the angular dispersion. - Frequency-resolved Orbital Angular Momentum Spectrum Measurement of Ultra-broadband Optical VorticesZhili Yang, Keisaku Yamane, Yasunori Toda, Ryuji Morita2013 CONFERENCE ON LASERS AND ELECTRO-OPTICS PACIFIC RIM (CLEO-PR) 2013年 [査読有り][通常論文]
We experimentally exhibit a high-precision method for measuring frequency-resolved orbital angular momentum (OAM) spectrum of femtosecond ultra-broadband optical-vortex pulses from fork-like interferograms. This method enables us to evaluate topological-charge dispersion of ultra-broadband optical-vortex pulses. - Zhili Yang, Keisaku Yamane, Yasunori Toda, Ryuji Morita2013 CONFERENCE ON LASERS AND ELECTRO-OPTICS (CLEO) 2013年 [査読有り][通常論文]
We experimentally demonstrate a simple and high-precision method for measuring orbital angular momentum (OAM) spectra of femtosecond ultra-broadband optical-vortex pulses from fork-like interferograms. This method enables quasi-real-time OAM measurement for ultra-short or ultra-broadband optical vortices. - Keisaku Yamane, Yasunori Toda, Ryuji Morita2013 CONFERENCE ON LASERS AND ELECTRO-OPTICS (CLEO) 2013年 [査読有り][通常論文]
We demonstrate the amplification of ultrashort optical-vortex pulses through the optical parametric process, where optical-vortex seed pulses with computer-controlled topological charges are employed. In this scheme, the low-throughput drawback of the vortex converter is overcome. (C) 2013 Optical Society of America - T. Naito, S. Matsuo, T. Inabe, Y. TodaJournal of Superconductivity and Novel Magnetism 26 5 1679 - 1682 2013年 [査読無し][通常論文]
The temperature-dependent time-resolved reflectance spectra of the organic superconductors (OSs) κ- and λ- (BETS)2MCl4 (M = Fe, Ga) and κ-(ET)2Cu[N(CN)2]Br are reported. The π electrons of the organic molecules (BETS or ET) are responsible for the electrical conduction in these salts, while only the Fe-containing salts have localized spins on the d electrons in the Fe atoms (S = 5/2) with π–d interaction. Analysis of the spectra provided the relaxation time τ of the carriers, as well as information concerning the number of carriers at the Fermi energy N(EF). The carrier dynamics were classified in terms of high-TC and low-TC OSs. In these salts, the results indicate that (1) the electron-phonon interaction is important in the occurrence and stability of the superconducting phase, and (2) electron correlation can be related to the enhancement of TC. - J. Onoe, T. Ito, H. Shima, Y. Toda, H. Yoshioka, S. KimuraEurophysics Letters 98 2 27001 2012年09月27日 [査読有り][通常論文]
Einstein first applied Riemannian geometry to develop the general theory of relativity almost one hundred years ago and succeeded in understanding astronomical-scale phenomena such as the straining of time-space by a gravitational field. 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. We report here the first observation of Riemannian geometrical effects on the electronic properties of materials such as Tomononaga-Luttinger liquids, which were previously theoretically predicted by our group. We present in situ high-resolution ultraviolet photoemission spectra of a one-dimensional metallic C60 polymer with an uneven periodic peanut-shaped structure. © Europhysics Letters Association, 2012. - Shota Ono, Hiroyuki Shima, Yasunori TodaPHYSICAL REVIEW B 86 10 104512 2012年09月 [査読有り][通常論文]
We develop a theory to describe the energy relaxation of photoexcited carriers in low-temperature ordered states with a band-gap opening. Carrier relaxation time tau near and below transition temperature T-c is formulated by examining the contributions from different carrier-phonon scatterings to the relaxation rate. Transverse acoustic phonon modes are found to play a crucial role in carrier relaxation; their heat capacity determines the tau divergence near T-c. Remarkable agreements with the theory and experimental data on two different materials which exhibit contrasting tau(T) behaviors are also demonstrated. - Keisaku Yamane, Yasunori Toda, Ryuji MoritaOPTICS EXPRESS 20 17 18986 - 18993 2012年08月 [査読有り][通常論文]
We generated a 2.3-cycle, 5.9-fs, 56-mu J ultrashort optical-vortex pulse (ranging from similar to 650 to similar to 950 nm) in few-cycle regime, by optical parametric amplification. It was performed even by using passive elements (a pair of prisms and chirped mirrors) for chirp compensation. Spectrally-resolved interferograms and intensity profiles showed that the obtained pulses have no spatial or topological-charge dispersion during the amplification process. To the best of our knowledge, it is the first generation of optical-vortex pulses in few-cycle regime. They can be powerful tools for ultrabroadband and/or ultrafast spectroscopy and experiments of high-intensity field physics. (C) 2012 Optical Society of America - C. Gadermaier, V. V. Kabanov, A. S. Alexandrov, L. Stojchevska, T. Mertelj, C. Manzoni, G. Cerullo, N. D. Zhigadlo, J. Karpinski, Y. Q. Cai, X. Yao, Y. Toda, M. Oda, S. Sugai, D. Mihailovic2012年05月 [査読有り][通常論文]
The origin of high critical temperature (Tc) superconductivity is still
remarkably elusive. To gain insight into the high- Tc mechanism, we need
experiments which identify the parameters that determine Tc and link them to
the interaction(s) that establish the superconducting state. Here we show that
for pnictides, cuprates, and bismuthates Tc depends systematically on the
primary electron energy relaxation rate 1/tau_1. We find that 1/tau_1 is a
direct experimental measure of the strength of the electron-phonon interaction
(EPI) and correlates with structural parameters, in particular the l... - Shota Ono, Hiroyuki Shima, Yasunori Toda2012年02月 [査読有り][通常論文]
We develop a theory to describe energy relaxation of photo-excited carriers
in low-temperature ordered states with band gap opening and formulate carrier
relaxation time $\tau$ near and below transition temperature $T_{\mathrm{c } }$
by quantifying contributions from different carrier-phonon scatterings to the
relaxation rate. The theory explains anomalous experimental observations of
$\tau$ in gapped systems. Transverse acoustic (TA) phonon modes play a crucial
role in carrier relaxation; their heat capacity determines $\tau$-divergence
near $T_{\mathrm{c } }$. The theory is validated by fitti... - Shota Ono, Hiroyuki Shima, Yasunori TodaPhys. Rev. B 86 10 104512 2012年 [査読有り][通常論文]
- Jun Onoe, Takahiro Ito, Shin-Ichi Kimura, Hiroyuki Shima, Yasunori Toda, Hideo YoshiokaFULLERENES NANOTUBES AND CARBON NANOSTRUCTURES 20 1 1 - 16 2012年 [査読有り][通常論文]
When a C-60 film is irradiated with a 3 kV electron-beam in an ultrahigh vacuum (base pressure: 10(-7) Pa), C-60 molecules are coalesced to form a one-dimensional (1D) uneven peanut-shaped C-60 polymer. From a topological viewpoint, since the polymer has both positive and negative Gaussian curvatures, it can be regarded as a new allotrope of nanocarbons different from graphene, fullerenes, and nanotubes. We will present our recent results of the electronic, optical, and electron-transport properties of the 1D uneven peanut-shaped C-60 polymer and discuss its novel electronic properties on the basis of quantum mechanics in Riemannian space. - Toshio Naito, Saori Matsuo, Tamotsu Inabe, Yasunori TodaJOURNAL OF PHYSICAL CHEMISTRY C 116 3 2588 - 2593 2012年01月 [査読有り][通常論文]
Understanding how electrons behave, i.e., carrier dynamics, in superconductors is indispensable for understanding the mechanism of superconductivity. Recently we have reported the carrier dynamics of kappa- and lambda-(BETS)(2)MCl4 (BETS = bis(ethylenedithio)tetraselenafulvalene, M = Ga, Fe) based on ultrafast spectroscopy, but the interpretation of the results remains an open question. In this paper we interpreted the results with the aid of newly measured magnetic susceptibility, X-ray single crystal structural analysis, and band calculation. Observation of coherent phonons only in the lambda-type salts indicated that e-ph interaction should be characteristically strong in the A-type salts. By comparison of the observed carrier dynamics with a two-temperature model, the temperature-dependence of carrier dynamics is consistently explained by the different strengths of the e-ph interaction between the lambda- and the kappa-type salts. The difference in strength of the e-ph interactions is related to their crystal structures. In conclusion, their carrier dynamics is consistently interpreted and classified by their crystal structures. - Toshio Naito, Saori Matsuo, Tamotsu Inabe, Yasunori TodaJOURNAL OF PHYSICAL CHEMISTRY C 116 3 2588 - 2593 2012年01月 [査読有り][通常論文]
Understanding how electrons behave, i.e., carrier dynamics, in superconductors is indispensable for understanding the mechanism of superconductivity. Recently we have reported the carrier dynamics of kappa- and lambda-(BETS)(2)MCl4 (BETS = bis(ethylenedithio)tetraselenafulvalene, M = Ga, Fe) based on ultrafast spectroscopy, but the interpretation of the results remains an open question. In this paper we interpreted the results with the aid of newly measured magnetic susceptibility, X-ray single crystal structural analysis, and band calculation. Observation of coherent phonons only in the lambda-type salts indicated that e-ph interaction should be characteristically strong in the A-type salts. By comparison of the observed carrier dynamics with a two-temperature model, the temperature-dependence of carrier dynamics is consistently explained by the different strengths of the e-ph interaction between the lambda- and the kappa-type salts. The difference in strength of the e-ph interactions is related to their crystal structures. In conclusion, their carrier dynamics is consistently interpreted and classified by their crystal structures. - J. Onoe, T. Ito, S. Kimura, H. Shima, Y. Toda, H. YoshiokaFullerene, Nanotubes, and Carbon Nanostructures 20 1 1-16 2012年 [査読有り][招待有り]
- 森田 隆二, 戸田 泰則, 尾松 孝茂表面科学 : hyomen kagaku = Journal of the Surface Science Society of Japan 32 12 748 - 754 The Surface Science Society of Japan 2011年12月10日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.
- Quasiparticle relaxation dynamics in underdoped Bi2Sr2CaCu2O8+δ by two-color pump-probe spectroscopyY. Toda, T. Mertelj, P. Kusar, T. Kurosawa, M. Oda, M. Ido, D. MihailovicPhys. Rev. B 84 17 174516 2011年11月 [査読有り][通常論文]
We investigate the relaxation dynamics of photoexcited quasiparticles (QPs) in underdoped Bi2Sr2CaCu2O8+delta (T-c = 78 K). By changing the excitation energy and polarization of the probe beam, two different types of relaxation dynamics, associated with superconducting (SC) and pseudogap (PG) QPs, are quantitatively analyzed independently. From the temperature dependencies, we obtained the SC gap, Delta(SC)(0) = 24 meV, using BCS-type temperature-dependent gap and the pseudogap, Delta(PG) = 41 meV. The pump fluence (F) dependence of the SC-dominated transients shows a contribution of the PG component above the saturation condition of the SC component (F-th = 16 mu J/cm(2)), where Cooper pairs with long-range order are fully destroyed within the photoexcited volume. Assuming a temperature-independent PG decay time, we successfully isolate the native SC transient even above F-th by subtracting the PG response from the original data. In the saturation regime, the exponential decay (recovery of SC) is fast (tau(SC) similar to 2-3 ps), suggesting an efficient nonequilibrium phonon relaxation in this compound. We also find a flat-top response preceding the exponential decay at F > F-th, which appears as a delay of SC recovery in the original data. This response is visible over the whole temperature range below T-c and its duration increases with increasing F. The response is attributable to a photoinduced SC to non-SC phase transition arising from excitation by the nonthermal QPs and/or high-frequency phonons. The consistently near-constant magnitude of the PG response at the start of the SC state recovery from the non-SC phase suggests a correlation between the SC and PG QPs. - Y. Toda, T. Mertelj, T. Naito, D. MihailovicPhys. Rev. Lett. 107 22 227002 2011年11月 [査読有り][通常論文]
We investigate the relaxation dynamics of nonequilibrium carriers in organic conductors kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]X (X = Br and Cl) using ultrafast time-resolved optical spectroscopy. The dynamics for both salts show similar temperature dependences, which is well characterized by the carrier relaxation across the pseudogap (PG) of the magnitude Delta(PG) approximate to 16 meV for Br salt and 7.0 meV for Cl salt. On the other hand, only the Br salt shows an abrupt increase of the decay time at low temperature, indicating an additional decay component associated with the superconducting (SC) gap below T-c. The fluence dependent dynamics at low temperature evidences the superposition of the SC component onto the PG component. These results indicate a metallic-insulating phase separation in the Br salt triggered by photoexcited nonequilibrium carriers. - L. Stojchevska, P. Kusar, T. Mertelj, V. V. Kabanov, Y. Toda, X. Yao, D. MihailovicPHYSICAL REVIEW B 84 18 180507 2011年11月 [査読有り][通常論文]
The processes leading to nonthermal condensate vaporization and charge-density-wave (CDW) melting with femtosecond laser pulses is systematically investigated in different materials. We find that vaporization is relatively slow (tau(v)1 ps) and inefficient in superconductors, exhibiting a strong systematic dependence of the vaporization energy U(v) on T(c). In contrast, melting of CDW order proceeds rapidly (tau(m) = 50-200 fs) and more efficiently. A quantitative model describing the observed systematic behavior in superconductors is proposed based on a phonon-mediated quasiparticle (QP) bottleneck mechanism. In contrast, Fermi-surface disruption by hot QPs is proposed to be responsible for CDW state melting. - Y. Toda, T. Mertelj, P. Kusar, T. Kurosawa, M. Oda, M. Ido, D. MihailovicPHYSICAL REVIEW B 84 17 174516 2011年11月 [査読有り][通常論文]
We investigate the relaxation dynamics of photoexcited quasiparticles (QPs) in underdoped Bi2Sr2CaCu2O8+delta (T-c = 78 K). By changing the excitation energy and polarization of the probe beam, two different types of relaxation dynamics, associated with superconducting (SC) and pseudogap (PG) QPs, are quantitatively analyzed independently. From the temperature dependencies, we obtained the SC gap, Delta(SC)(0) = 24 meV, using BCS-type temperature-dependent gap and the pseudogap, Delta(PG) = 41 meV. The pump fluence (F) dependence of the SC-dominated transients shows a contribution of the PG component above the saturation condition of the SC component (F-th = 16 mu J/cm(2)), where Cooper pairs with long-range order are fully destroyed within the photoexcited volume. Assuming a temperature-independent PG decay time, we successfully isolate the native SC transient even above F-th by subtracting the PG response from the original data. In the saturation regime, the exponential decay (recovery of SC) is fast (tau(SC) similar to 2-3 ps), suggesting an efficient nonequilibrium phonon relaxation in this compound. We also find a flat-top response preceding the exponential decay at F > F-th, which appears as a delay of SC recovery in the original data. This response is visible over the whole temperature range below T-c and its duration increases with increasing F. The response is attributable to a photoinduced SC to non-SC phase transition arising from excitation by the nonthermal QPs and/or high-frequency phonons. The consistently near-constant magnitude of the PG response at the start of the SC state recovery from the non-SC phase suggests a correlation between the SC and PG QPs. - Sou Ryuzaki, Toshihiro Kai, Yasunori Toda, Satoru Adachi, Jun OnoeJOURNAL OF PHYSICS D-APPLIED PHYSICS 44 14 145103 2011年04月 [査読有り][通常論文]
We have examined the structural effects of zinc-octaethylporphyrin [Zn(OEP)] films used as a donor on the external quantum efficiency (EQE) of organic heterojunction photovoltaic (OPV) cells [ITO/Zn(OEP)/C-60/Al], and investigated what exactly causes the improvement of EQE. When the structure of the Zn(OEP) films changed from amorphous to crystalline, the maximum EQE increased from 36% to 42%, which is greater than that of around 35% for previously reported OPV cells using buffer materials (Peumans and Forrest 2001 Appl. Phys. Lett. 79 126). The crystallization of Zn(OEP) films is found to increase the number of inter-molecular charge-transfer (IMCT) excitons and to enlarge the mobility of carriers and IMCT excitons, thus significantly improving the EQE of the photoabsorption band under illumination due to the IMCT excitons. - Masato Suzuki, Yasunori Toda, Ryuji Morita2011 Conference on Lasers and Electro-Optics Europe and 12th European Quantum Electronics Conference, CLEO EUROPE/EQEC 2011 2011年 [査読有り][通常論文]
Having a helical wavefront, a Laguerre-Gaussian (LG) mode or optical vortex is one of the modes of paraxial solutions to wave equation. The optical vortex has unique properties. The beam has a phase singularity on its center, which shows a dark part on its center of intensity profile, and carries an orbital angular momentum of light well defined by the topological charge ℓ[1]. These characteristics recently attracted much attention. For more versatile applications, high power optical-vortex pulses are desired. Typically, optical vortices are generated by using a spiral plate [2], a spatial light modulator (SLM) [3], a photonic-crystal axially-symmetric polarizer/waveplate [4] or a uniaxial crystal [5]. Among them, the most flexible method is by SLM. However, this method using SLM is power-limited owing to its damage threshold. Hence, as an alternative to generate high power optical-vortex pulses, we choose the method using a uniaxial crystal, which is rather flexible. In this scheme, the nonlinear effects have not been clarified in detail. In the present paper, we investigate nonlinear effects on optical-vortex pulse generation through the spin-orbit interaction and its propagation in an anisotropic crystal. © 2011 IEEE. - K. Watanabe, S. Tsuchiya, Y. Toda, T. Kurosawa, M. Oda, T. Mertelj, I. Madan, V. V. Kabanov, D. MihailovicInt. J. Mod. Phys. B 29, 1542031(1-7) (2015) 29 1542031 2011年 [査読有り][通常論文]
- J. Onoe, A. Takashima, Y. TodaAPPLIED PHYSICS LETTERS 97 24 241911 2010年12月 [査読有り][通常論文]
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. TandaPHYSICAL REVIEW LETTERS 105 17 176401-1 - 176401-4 2010年10月 [査読有り][通常論文]
We discovered the chirality of charge-density waves (CDW) in 1T-TiSe(2) by using STM and time-domain optical polarimetry. We found that the CDW intensity becomes Ia(1):Ia(2):Ia(3) 1:0.7 +/- 0.1:0.5 +/- 0.1, where Ia(i) (i = 1, 2, 3) is the amplitude of the tunneling current contributed by the CDWs. There were two states, in which the three intensity peaks of the CDW decrease clockwise and anticlockwise. The chirality in CDW results in the threefold symmetry breaking. Macroscopically, twofold symmetry was indeed observed in optical measurement. We propose the new generalized CDW chirality H(CDW) equivalent to q(1) . (q(2) x q(3)), where q(i) are the CDW q vectors, which is independent of the symmetry of components. The nonzero H(CDW)-the triple-q vectors do not exist in an identical plane in the reciprocal space-should induce a real-space chirality in CDW system. - J. Ishioka, Y. H. Liu, K. Shimatake, T. Kurosawa, K. Ichimura, Y. Toda, M. Oda, S. TandaPHYSICAL REVIEW LETTERS 105 17 2010年10月 [査読有り][通常論文]
We discovered the chirality of charge-density waves (CDW) in 1T-TiSe(2) by using STM and time-domain optical polarimetry. We found that the CDW intensity becomes Ia(1):Ia(2):Ia(3) 1:0.7 +/- 0.1:0.5 +/- 0.1, where Ia(i) (i = 1, 2, 3) is the amplitude of the tunneling current contributed by the CDWs. There were two states, in which the three intensity peaks of the CDW decrease clockwise and anticlockwise. The chirality in CDW results in the threefold symmetry breaking. Macroscopically, twofold symmetry was indeed observed in optical measurement. We propose the new generalized CDW chirality H(CDW) equivalent to q(1) . (q(2) x q(3)), where q(i) are the CDW q vectors, which is independent of the symmetry of components. The nonzero H(CDW)-the triple-q vectors do not exist in an identical plane in the reciprocal space-should induce a real-space chirality in CDW system. - Y. Toda, S. Honda, R. MoritaOPTICS EXPRESS 18 17 17796 - 17804 2010年08月 [査読有り][通常論文]
We study the dynamics of a paired optical vortex (OV) generated by second-harmonic generation (SHG) using sub-picosecond pulses. By changing the position of a thin nonlinear crystal along the propagation direction, we observe a rotation of two vortices with changing separation distance. The dynamics is well explained by SHG with a beam walk-off, which introduces a contamination of zero-order Laguerre-Gaussian beam (LG(0)) together with topological charge doubling. The quantitative analysis indicates that the rotation angle of the OVs manifests the Gouy phase while the splitting provides the walk-off angle of the crystal. We also show that the subtraction of LG(0) is realized by the superposition of LG(0) with an anti-balanced phase in the pump. (C) 2010 Optical Society of America - Y. Toda, J. Sekiya, Y. H. Liu, N. Momono, M. Oda, M. IdoJOURNAL OF SUPERCONDUCTIVITY AND NOVEL MAGNETISM 23 5 871 - 872 2010年07月 [査読有り][通常論文]
We investigate nonequilibrium quasiparticle dynamics measured by ultrafast optical spectroscopy on underdoped Bi2212 crystals, which provide direct evidence that superconducting (SC) and pseudogap (PG) quasiparticies coexist below T. We verify that the ratio of signals from SC and PG quasiparticles depends on both excitation energy and polarization of the probe beam due to the anisotropy of the probe transition matrix elements and the interband transition probability. Based on this property, we successfully separate the SC or PG component and precisely evaluate the temperature dependence of them across T-c. - T. Naito, M. Fukusaku, S. Matsuo, K. Tajima, T. Inabe, Y. TodaPHYSICA B-CONDENSED MATTER 405 11 S269 - S272 2010年06月 [査読有り][通常論文]
In order to compare the carrier dynamics in lambda-(BETS)(2)MCl4 (M=Ga Fe) and kappa-(ET)(2)X (X=Cu(NCS)(2) Cu[N(CN)(2)]Br Cu[N(CN)(2)]Cl) their polarized reflectance spectra were measured by a pump-probe method Polarization-dependences were compared among the different materials at varying temperatures The result indicated that each salt had anisotropy in the conduction planes corresponding to its crystallographic axes at all the temperatures of measurements (C) 2009 Elsevier B V All rights reserved - J. Ishioka, Y. H. Liu, K. Shimatake, T. Kurosawa, K. Ichimura, Y. Toda, M. Oda, S. TandaPHYSICA B-CONDENSED MATTER 405 11 S214 - S216 2010年06月 [査読有り][通常論文]
We report the chirality of charge density waves (COW) in 1T-TiSe2 by using scanning tunneling microscopy (STM) measurements We found that the COW intensity becomes Ia(1) Ia(2) Ia(3) = 1 07 +/- 0 1 0 5 +/- 0 1 where Ia(1) (I = 1 2 3) is the amplitude of the tunneling current contributed by the CDWs We found two states in which the three intensity peaks of the CDW decrease clockwise and anticlockwise when we index each nesting vector in order of intensity in the Fourier transformation of the STM images We found that this difference arises from COW stacking along the c-axis at intervals of 2c(0)/3 This chirality is attributed to the helical stacking such as cholesteric liquid crystals (C) 2010 Elsevier B V All rights reserved - T. Naito, S. Matsuo, T. Inabe, Y. TodaJOURNAL OF PHYSICS-CONDENSED MATTER 22 18 185701-1 - 185701-7 2010年05月 [査読有り][通常論文]
In order to compare the carrier dynamics and reveal the origin of differences in their electrical behaviour, time-resolved reflectivity changes were measured on single crystals of the title compounds. The observed relaxation behaviour explained well their electrical behaviour, and indicated that different conduction mechanisms dominated kappa-and lambda-type BETS salts, irrespective of local spins, at about 8-100 K; the electrical behaviour of the kappa-type salts is governed by relaxation times of carriers and/or density of states at the Fermi levels, while that of the lambda-type salts is dominated by the latter. - Yoshinari Awaji, Naoya Wada, Yasunori Toda2010 23rd Annual Meeting of the IEEE Photonics Society, PHOTINICS 2010 551 - 552 2010年 [査読有り][通常論文]
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. - 足立智, 戸田泰則J. Vac. Soc. Jpn. 53 6 387 - 392 The Vacuum Society of Japan 2010年 [査読有り][招待有り]
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. TodaAppl. Phys. Lett. 97 241911 2010年 [査読有り][通常論文]
- Yu Tokizane, Katsuhiro Shimatake, Yasunori Toda, Kazuhiko Oka, Masakatsu Tsubota, Satoshi Tanda, Ryuji MoritaOPTICS EXPRESS 17 26 24198 - 24207 2009年12月 [査読有り][通常論文]
We evaluate the quasi-one-dimensional (1D) electron dynamics in a NbSe3 ring crystal using polarization vortex pulses with various azimuthal distributions. The single particle relaxation component reveals a large anisotropy on the crystal, indicating that the electrons in the ring maintain their 1D character. The results also suggest that the polarization vortex evaluates the global polarization property of the closed-loop electron that plays an important role in the quantum correlation phenomena such as the Aharonov-Bohm effect. (C) 2009 Optical Society of America - Tetsuro Ishiguro, Yasunori Toda, Satoru AdachiJOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 78 10 174018-1 - 4 2009年10月 [査読有り][通常論文]
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. TawaraOPTICS EXPRESS 17 22 20567 - 20574 2009年10月 [査読有り][通常論文]
We demonstrate the coherent transfer of optical orbital angular momentum (OAM) to the center of mass momentum of excitons in semiconductor GaN using a four-wave mixing (FWM) process. When we apply the optical vortex (OV) as an excitation pulse, the diffracted FWM signal exhibits phase singularities that satisfy the OAM conservation law, which remain clear within the exciton dephasing time (similar to 1ps). We also demonstrate the arbitrary control of the topological charge in the output signal by changing the OAM of the input pulse. The results provide a way of controlling the optical OAM through carriers in solids. Moreover, the time evolution of the FWM with OAM leads to the study of the closed-loop carrier coherence in materials. (C) 2009 Optical Society of America - Y. Toda, R. Onozaki, M. Tsubota, K. Inagaki, S. TandaPHYSICAL REVIEW B 80 12 121103 2009年09月 [査読有り][通常論文]
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 MoritaELECTRICAL ENGINEERING IN JAPAN 167 4 39 - 46 2009年06月 [査読有り][通常論文]
Topology has become one of the key concepts allowing one to understand the intrinsic, qualitative properties of phenomena throughout various scientific fields. To date, this concept has been extended to the field of material science and technology. On the other hand, we can now utilize the spatially controlled light defined by the topology (so-called "optical vortices") in order to characterize the topological properties of materials. In particular, optical vortices in femtosecond pulses will be invaluable for advanced topological spectroscopy. In this work, the authors created femtosecond optical vortices using a spatial light modulator. Their spatiotemporal properties were evaluated using interferogram and correlation measurements. 0 2009 Wiley Periodicals, Inc. Electr Eng Jpn, 167(4): 39-46, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20791 - Yu Tokizane, Katsuhiro Shimatake, Yasunori Toda, Kazuhiko Oka, Masakatsu Tsubota, Satoshi Tanda, Ryuji MoritaThe European Conference on Lasers and Electro-Optics (CLEO/Europe) 2009 2009年06月 [査読有り][通常論文]
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. HoshinoPHYSICA STATUS SOLIDI C - CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 6, NO 1 6 1 42 - + 2009年 [査読有り][通常論文]
Dephasing dynamics of exciton fine structure (EFS) caused by long and short range exchange interactions in GaN was investigated by using a four-wave-mixing (FWM) spectroscopy. The type of exchange interactions was selected by samples (uniaxially strained c-plane GaN films and a thick bulk GaN) and their geometries in optical excitation. Since each component of EFS is highly polarized, the polarization-dependent FWM spectra successfully resolve the dephasing dynamics in the individual EFS resonances. The results show a difference in dephasing time, suggesting a contribution of anisotropic exchange interaction (AEI). (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim - T. Ishiguro, Y. Toda, S. Adachi, S. F. ChichibuPHYSICA STATUS SOLIDI C: CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 6, SUPPL 2 6 S719 - S722 2009年 [査読有り][通常論文]
We report on the dephasing dynamics of exciton fine structure (EFS) in a free standing GaN. The EFS was observed and studied by utilizing a four-wave-mixing (FWM) spectroscopy. In the cleavage plane of wurtzite GaN, we can excite the B-exciton fine structure, whose z-polarized state is energetically well separated from the other in-plane polarized exciton states. Due to the large spin exchange interaction in GaN, an energy shift of in-plane polarized fine structure was also observed. Since the each component of EFS is highly polarized, the polarization-dependent FWM successfully resolves their dephasing dynamics. The results show a significant difference in the dephasing time between EFS, suggesting that anisotropic exchange interaction is effective as reported in our previous work using a GaN film on alpha-plane sapphire. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim - Y. Toda, K. Yamaguchi, T. Ishiguro, K. Hoshino, K. Tadatomo, S. AdachiPHYSICA STATUS SOLIDI C: CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 6, SUPPL 2 6 S684 - S686 2009年 [査読有り][通常論文]
Spatial imaging of spectrally-resolved four-wave mixing (SR FWM) was performed on a GaN film grown on c-sapphire substrate. The image obtained from the exciton resonances at zero time delay clearly showed some highly localized reductions in their intensities whose spatial profiles can be mapped out with a spatial resolution of similar to 20 m. The image obtained from the evaluation of the quantum beats indicates these reductions correspond to the phase changes between two exciton transitions, reflecting the spatial properties of exciton-exciton interactions. The line-localization suggests a contribution of line-defect to the anomalous interactions. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim - Y. Toda, K. Shimatake, S. Tanda, J. OnoeULTRAFAST PHENOMENA IN SEMICONDUCTORS AND NANOSTRUCTURE MATERIALS XIII 7214 2009年 [査読無し][通常論文]
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. IdoPHYSICAL REVIEW LETTERS 101 13 137003 2008年09月 [査読有り][通常論文]
We report the ultrafast optical response of quasiparticles (QPs) in both the pseudogap ( PG) and superconducting (SC) states of an underdoped Bi2Sr2CaCu2O8+y (Bi2212) single crystal measured with the time-resolved pump-probe technique. At a probe energy hw(pr) = 1.55 eV, it is found that the reflectivity change Delta R/R changes its sign at exactly T-c, which allows the direct separation of the charge dynamics of PG and SC QPs. Further systematic investigations indicate that the transient signals associated with PG and SC QPs depend on the probe beam energy and polarization. By tuning them below T-c, two distinct components can be detected simultaneously, providing evidence for the coexistence of PG and SC QPs. - Toshio Naito, Yukari Yamada, Tamotsu Inabe, Yasunori TodaJOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 77 6 064709-1 - 064709-6 2008年06月 [査読有り][通常論文]
The K-type organic superconductors have unique physical properties, many of which are yet to be understood. Most of their physical properties are closely related to carrier dynamics, which can be directly observed by ultrafast optical measurements in their nonequilibrium states. Time-resolved temperature-dependent reflectivity changes were measured on single crystals of an isostructural pair of organic charge transfer salts kappa-(ET)(2)Cu[N(CN)(2)]X, which are ambient- (X = Br) or high-pressure (X = Cl) superconductors. The latter compound in its ground state is a Mott insulator under ambient pressure. The obtained temperature dependences of relaxation times for both salts indicated the existence of an intermediate electronic state between those of metals and insulators at all measurement temperatures. Such an unusual electronic state coexisted with the superconducting and Mott insulating states in X = Br and X = Cl, respectively. In addition, the results for X = Br revealed the formation of a temperature-dependent well-defined gap in the superconducting transition, while those for X = Cl suggested that the Mott transition in this salt does not depend on electron-phonon interactions, consistent with the theory that the Mott transition should originate from electron correlation. - Y. Toda, T. Ishiguro, S. Adachi, K. Hoshino, K. TadatomoPHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS 245 5 878 - 880 2008年05月 [査読有り][通常論文]
We have previously demonstrated that a spectrally-resolved four-wave mixing highlights the anisotropically polarized excitons in the uniaxially strained GaN films [1], where the enhancement of sensitivity for an uniaxial strain, was achieved by nonlinear response of the exciton polarization. In this work, we have applied this technique to the GaN films on various substrates including isotropic ones, and have examined the sensitivity of the technique. The results show distinct polarizations and energy variations originating from the uniaxial strain, the magnitude of which depends on the sample and its position: The minimum changes of the polarized FWM intensity and exchange energy splittings correspond to a uniaxial strain of 5.0 x 10(-5), which currently gives a lower resolution limit of this technique. (C) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. - Y. Toda, S. Ryuzaki, J. OnoeAPPLIED PHYSICS LETTERS 92 9 094102 2008年03月 [査読有り][通常論文]
Time-resolved temperature-dependent transmission changes were measured for both pristine C-60 and electron-beam-irradiated C-60 polymer films using an optical pump-probe technique. Only the signals obtained for the C-60 polymer show a temperature-dependent slow decay, which appears in the low temperature region below similar to 60 K. This slow decay component exhibits a monotonic increase in both relaxation time and amplitude with decreasing temperature, providing evidence of gap formation associated with ordering fluctuations. (c) 2008 American Institute of Physics. - T. Ishiguro, Y. Toda, S. Adachi, K. Hoshino, Y. ArakawaPHYSICA STATUS SOLIDI C - CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 5, NO 6 5 6 2114 - + 2008年 [査読有り][通常論文]
Dephasing dynamics of excitons in a GaN film grown on a-plane sapphire were studied by using the spectrally-resolved four-wave-mixing (SR-FWM) spectroscopy. Due to the anisotropic exchange interaction originates from the anisotropic strain field of the substrate, the degenerate exciton states with the total angular moment +/- 1 mix and split into their fine structures. Since each component of the exciton fine structured (EFS) is highly polarized, the polarization-dependent SR-FWM spectra successfully resolve the dephasing dynamics in the individual EFS resonances. The result of each EFS shows a significantly different dephasing time, suggesting that the anisotropic dephasing contribution occurs in each EFS. - T. Ishiguro, Y. Toda, S. Adachi, S. F. ChichibuULTRAFAST PHENOMENA IN SEMICONDUCTORS AND NANOSTRUCTURE MATERIALS XII 6892 2008年 [査読無し][通常論文]
We perform degenerate four-wave-mixing (FWM) studies of GaN excitons especially for an understanding of the strain-fields in the heteroepitaxial films. The shifts of exciton energies and their beating oscillation variations highlight the biaxial strain, allowing for a precise determination of the strain parameters. The uniaxial strain field can be characterized by the polarization dependence of FWM, which shows distinct polarizations and energy variations depending on the sample and its position. The minimum changes of the polarized FWM intensity and exchange energy splittings correspond to a uniaxial strain of 5.0 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. TandaPHYSICAL REVIEW B 75 11 115120 2007年03月 [査読有り][通常論文]
We report a study of the ultrafast single particle dynamics in quasi-one-dimensional NbSe3 whisker crystals using a two-color pump-probe measurement. A selective enhancement of the relaxation dynamics connected with one of the two charge-density-wave (CDW) phases was realized by tuning the probe energy to around 1.5 eV, in which the temperature dependence across the corresponding phase transition shows a pronounced anomaly. A similar selective excitation was possible in the coherent CDW oscillation. - Tatsuyuki Onozaki, Yasunori Toda, Satoshi Tanda, Ryuji MoritaJAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS BRIEF COMMUNICATIONS & REVIEW PAPERS 46 2 870 - 872 2007年02月 [査読有り][通常論文]
The coherent control of a charge-density-wave (CDW) collective mode in 1T-TaS2 was performed using a femosecond pulse pair. Selective enhancement and suppression of the oscillation were achieved by optimal pulse separation. The ratio of enhancement to Suppression and pulse-separation dependence of the amplitude show a good correspondence with theoretical evaluation in the pulse-separation time range -0.5 to 1 ps. - K. Yamaguchi, Y. Toda, T. Ishiguro, S. Adachi, K. Hoshin, K. TadatomoPHYSICA STATUS SOLIDI C - CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 4 NO 7 2007 4 7 2752 - + 2007年 [査読有り][通常論文]
Quantum beats in the time-resolved four-wave mixing (FWM) signals arising from GaN excitons are observed. Two different types of beats are investigated: beats between the A-exciton (X-A) and B-exciton (X-B) states; beats between the XA and A-biexciton ((XXA)) states. The former exhibits oscillations along all directions in the delay- (T) and real-time (t) plane while the latter exhibits no beat in the direction parallel to the diagonal axis (T = t). The results are well reproduced by a model calculation, showing good homogeneity of the sample. - T. Ishiguro, Y. Toda, S. Adachi, K. Hazu, T. Sota, S. F. ChichibuPHYSICA STATUS SOLIDI C - CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 4 NO 7 2007 4 7 2776 - + 2007年 [査読有り][通常論文]
An optical coherent manipulation of excitonic polarizations in hexagonal bulk GaN is achieved by means of an interferometric technique combined with spectrally-resolved degenerate four-wave-mixing (FWM). Initial phase of the FWM quantum beats between A and B excitonic polarizations are arbitrarily controlled, in which simultaneous increase (decrease) of the beats in both polarizations are observed. In addition, 80% selective amplification and 40% suppression of one of the excitonic polarizations are realized. - T. Ishiguro, Y. Toda, S. AdachiAPPLIED PHYSICS LETTERS 90 1 11904 2007年01月 [査読有り][通常論文]
The authors studied the exciton spin relaxation of bulk GaN by creating spin polarization gratings using degenerate four-wave mixing spectroscopy. The spectrally resolved analysis achieved with this technique facilitated the direct evaluation of spin polarizations in the individual excitons (A and B excitons). The spin polarizations for each exciton decay very quickly (tau(s)similar to 1 ps) at low temperatures. Moreover the tau(s) is faster than the dephasing time T-2 throughout the measured temperature range, suggesting the existence of fast intrinsic spin relaxation processes, which can be attributed to a large exchange constant characterized in GaN. - 戸田 泰則, 長岡 耕志, 島竹 克大, 森田 隆二電気学会論文誌Ｃ（電子・情報・システム部門誌） 127 9 4 - 1313 2007年 [査読無し][通常論文]
Topology has become one of the key concepts allowing one to understand the intrinsic, qualitative properties of phenomena throughout various scientific fields. To date, this concept has been extended to the field of material science and technology. On the other hand, we can now utilize the spatially-controlled light defined by the topology (so called "optical vortices") in order to characterize the topological properties of materials. 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 ArakawaPHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS 243 7 1560 - 1563 2006年06月 [査読有り][通常論文]
We have carried out time-integrated and spectrally-resolved four-wave mixing (FWM) measurements to study the effects of built-in strain within GaN films on their exciton structures and on their coherent dynamics. Precise values for exciton energies, linewidths, and oscillation periods of quantum beats were presented. Remarkable phase shifts of the quantum beats were observed in the GaN films on a-sapphire, suggesting the exciton-exciton interaction caused by the reduction of the crystal symmetry. - T Ishiguro, Y Toda, S Adachi, M Arita, Y ArakawaPHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS 243 7 1564 - 1567 2006年06月 [査読有り][通常論文]
We have studied exciton spin relaxation in strained GaN epilayers on various sapphire substrates using spin-dependent transient reflectivity and three-pulse four-wave mixing measurements. Strikingly fast exciton spin relaxation (< 1 ps) at 10 K was observed in the lowest exciton level (A-exciton) both in bulk and GaN layer on c-sapphire. In contrast, the transient signals in the GaN layer on alpha-sapphire show the break down of the spin polarization caused by the exchange interaction. - Satoru Adachi, Yasunori Toda, Tetsuro IshiguroPHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS 243 7 1568 - 1571 2006年06月 [査読有り][通常論文]
Biexcitons of GaN under uniaxial strain have been investigated by spectrally-resolved four-wave mixing (FWM) technique. Two kinds of biexcitons were observed clearly and the binding energies were obtained. Since the uniaxial strain changes the exciton structure, the biexciton binding is modified and biexciton has also fine structure, which was revealed in the FWM spectra that were obtained by rotating the linearly polarized excitation against the direction of uniaxial strain. - K Shimatake, Y Toda, S TandaPHYSICAL REVIEW B 73 15 153403 2006年04月 [査読有り][通常論文]
A comparison of the single-particle (SP) dynamics of whisker and ring NbSe3 crystals provides new insight into the phase transition properties of quasi-one-dimensional charge density wave (CDW) systems. In the incommensurate CDW phase, SP relaxation triggered by an ultrafast laser pulse reflects the formation of collective states, and reveals the divergence of the relaxation time when approaching a transition temperature. The degree of divergence is less pronounced in rings than in whiskers, suggesting a loss of phase coherence in ring crystals characterized by a closed-loop topology. - K. Shimatake, Y. Toda, S. TandaConference on Lasers and Electro-Optics and 2006 Quantum Electronics and Laser Science Conference, CLEO/QELS 2006 2006年 [査読有り][通常論文]
Ultrafast electron dynamics in quasi-one-dimensional NbSe3 conductors was investigated by means of two-color pump-probe reflectivity measurements. The results clearly demonstrate the resonant probing of the charge-density-wave phases with different transition properties. © 2005 Optical Society of America. - Satoru Adachi, Yasunori Toda, Tetsuro Ishiguro, Katsuyuki Hoshino, Yasuhiko ArakawaPHYSICA STATUS SOLIDI C - CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 3, NO 6 3 6 1595 - 1598 2006年 [査読無し][通常論文]
Exciton fine structures induced by the uniaxial strain have been revealed in gallium nitride grown on an a-plane sapphire substrate by using the spectrally-resolved four-wave mixing technique. Sensitivity on the exciton polarization is enhanced by the nonlinearities of the four wave mixing signals, and therefore the safety alternative tool for mapping of the uniaxial strain field and the resultant anisotropic exchange can be offered. Such a nonlinear spectroscopic measurement gives a kind of crystalline analysis achieved in X-ray diffraction spectroscopy as well as information of the exciton fine structures including their temporal dynamics. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. - T Minami, Y Toda, K Shimatake, S TandaJOURNAL DE PHYSIQUE IV 131 121 - 122 2005年12月 [査読有り][通常論文]
We have investigated ultrafast carrier dynamics in a quasi-one-dimensional metal m-TaS3 by using pump-probe reflectivity measurements. The temperature dependence of transient signal reveals two critical changes at T-p = 240K and T-c similar to 100K. Below T-c a new low-frequency oscillation mode appears and shows a pronounced softening depending oil the temperature, indicating a collective excitation connected with the commensurability of CDW phase. This soft-mode exhibits remarkable polarization anisotropy with respect to the chain direction. - K Shimatake, Y Toda, T Minami, S TandaJOURNAL DE PHYSIQUE IV 131 295 - 298 2005年12月 [査読有り][通常論文]
Influence of the crystal topology on the Peierls transition was investigated by means of ultrafast optical pump-probe measurements. The comparison of the exponential single particle decay between the whisker and ring NbSe3 compounds reveals several pronounced differences around a phase transition attire (T-p). A discrepancy from BCS-like temperature dependence of the gap was observed in the ring, suggesting a loss of phase coherence between adjacent chains in the closed-loop topology. - Y Toda, S Adachi, Y Abe, K Hoshino, Y ArakawaPHYSICAL REVIEW B 71 19 195315 2005年05月 [査読有り][通常論文]
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. - Spectrally-resolved nonlinear spectroscopy of in-plane anistropy in uniaxally-starined GaN epilayersS Adachi, Y TodaPHYSICA STATUS SOLIDI C - CONFERENCES AND CRITICAL REVIEWS, VOL. 2 , NO 11 2 11 3936 - 3940 2005年 [査読有り][通常論文]
Spectrally-resolved four-wave mixing technique has revealed the internal structures of excitons in uniaxially strained gallium nitride films. Nonlinear response of four-wave mixing on the oscillator strength highlights the polarized excitons, and therefore allows us to map out the uniaxial strain field and the resultant anisotropic exchange splitting. Such a measurement gives a kind of crystalline analysis achieved in X-ray diffraction spectroscopy as well as information of the exciton fine structures including their temporal dynamics. - Y Abe, Y Toda, K Hoshino, Y ArakawaJAPANESE JOURNAL OF APPLIED PHYSICS PART 2-LETTERS & EXPRESS LETTERS 44 16-19 L535 - L538 2005年 [査読有り][通常論文]
Two-photon absorption (TPA) imaging, based on a transient pump-probe measurement, has been demonstrated for characterizing the nonlinear optical properties of GaN films in three-dimensions. The use of near-infrared excitation light demonstrated the capability to probe changes in TPA inside the Structure, which is closely associated with fluctuations in defect states. The TPA coefficient at 1.55 eV was evaluated to be 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 TodaPhysica Status Solidi C: Conferences 2 11 3936 - 3940 2005年 [査読有り][通常論文]
Spectrally-resolved four-wave mixing technique has revealed the internal structures of excitons in uniaxially strained gallium nitride films. Nonlinear response of four-wave mixing on the oscillator strength highlights the polarized excitons, and therefore allows us to map out the uniaxial strain field and the resultant anisotropic exchange splitting. Such a measurement gives a kind of crystalline analysis achieved in X-ray diffraction spectroscopy as well as information of the exciton fine structures including their temporal dynamics. © 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. - K Itoh, Y Toda, R Morita, M YamashitaJAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS SHORT NOTES & REVIEW PAPERS 43 9A 6448 - 6451 2004年09月 [査読有り][通常論文]
By using Raman-induced Kerr-effect spectroscopy, two-pulse excitations with different polarization combinations have been demonstrated for the selective coherent excitation of molecular vibrations in liquid CCl4. Anti correlated selective excitations as well as the suppressions of the two asymmetric vibrational motions were observed in the opposite configuration of polarization between the two-pump pulses. The measurements show that we can manipulate molecular motions by controlling the optical phase of sequential pulses through timing and polarization. - Y. Toda, K. Tateishi, S. TandaPhysical Review B - Condensed Matter and Materials Physics 70 3 1 - 33106 2004年07月 [査読有り][通常論文]
We have investigated the coherent dynamics of the collective motions of phonons in the commensurate phase of quasi-two-dimensional 1T-TaS2 using femtosecond laser pulses. One of the coherent phonons increased in intensity as a function of time, this behavior being strongly correlated with the decay of charge-density-wave oscillations as well as the slow decay component in single-particle relaxation. - Y Toda, K Tateishi, S TandaPHYSICAL REVIEW B 70 3 2004年07月 [査読有り][通常論文]
We have investigated the coherent dynamics of the collective motions of phonons in the commensurate phase of quasi-two-dimensional 1T-TaS2 using femtosecond laser pulses. One of the coherent phonons increased in intensity as a function of time, this behavior being strongly correlated with the decay of charge-density-wave oscillations as well as the slow decay component in single-particle relaxation. - Y Toda, T Nakaoka, R Morita, M Yamashita, T Inoue, Y ArakawaPHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES 21 2-4 180 - 183 2004年03月 [査読有り][通常論文]
We carried out optical selective excitation of individual self-assembled quantum dots by using phase-modulated pulses. Based on scattered photoluminescence excitation resonances in individual QDs, the excitation pulses modulated in the spectral region allows for addressing individual ground states emission. The photoluminescence spectra including several QDs showed intensity changes according to both the modulation energies and phases. The results also suggested the individual control of selective QDs even in collective excitation. (C) 2003 Elsevier B.V. All rights reserved. - Y Toda, T Matsubara, R Morita, M Yamashita, K Hoshino, T Someya, Y ArakawaAPPLIED PHYSICS LETTERS 82 26 4714 - 4716 2003年06月 [査読有り][通常論文]
Optical nonlinearity in the yellow luminescence (YL) band of GaN was investigated using thick bulk samples. Transient pump-probe measurements revealed strong transmission changes due to two-photon absorption (TPA) even at the middle of the YL band. The TPA coefficient evaluated reaches similar to5 cm/GW at about 1.3 eV, which was as large as the mid-gap resonance. The TPA spectrum clearly showed that the observed large nonlinearity originated from the YL band. On the basis of efficient TPA in the YL band, relaxation processes in the multiphoton-induced photoluminescence excitation spectrum were also investigated. (C) 2003 American Institute of Physics. - Y Toda, T Matsubara, R Morita, K Hoshino, T Someya, Y Arakawa5TH INTERNATIONAL CONFERENCE ON NITRIDE SEMICONDUCTORS (ICNS-5), PROCEEDINGS 0 7 2627 - 2630 2003年 [査読有り][通常論文]
Transient transmission changes in the yellow luminescence (YL) band of GaN were investigated using excitation far smaller than the bandgap. At lower excitation power, time-resolved measurements revealed efficient transmission changes due to two-photon absorption (TPA), whose spectrum clearly indicates the contribution of the YL band. Upon increasing the excitation power, the transient signal showed an additional offset signal. The excitation power dependences of the offset suggested a two-step absorption process in conjunction with TPA in the YL band. The excitation energy dependence also supported this conclusion. (C) 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. - T Inoue, Y Toda, K Hoshino, T Someya, Y Arakawa5TH INTERNATIONAL CONFERENCE ON NITRIDE SEMICONDUCTORS (ICNS-5), PROCEEDINGS 0 7 2428 - 2431 2003年 [査読有り][通常論文]
Temperature-dependent Raman spectroscopy was used to analyze the effect of residual strain on optical phonons E-2 (high) and A(1) (LO) in high-quality GaN films grown on C-plane 6H-SiC, A- and C-plane sapphire substrates. By comparison with a free-standing GaN, we found that shifts in the Raman spectrum were consistent with compressive and tensile strains. Experimental results indicated that differences in the thermal expansion coefficients between GaN film and substrate have an effect on these frequency shifts. We also compared frequency shifts of the A(1) (LO) mode with those of the E-2 (high) mode and concluded that the strain-induced frequency shift of the A(1) (LO) mode was smaller than that of the E-2 (high) mode. The strain-tensor-dependences of each phonon mode are also discussed. (C) Wiley-VCH Verlag GmbH & Co. KgaA, Weinheim - Y. Toda, T. Inoue, T. Nakaoka, S. Ishida, Y. ArakawaPacific Rim Conference on Lasers and Electro-Optics, CLEO - Technical Digest 2 448 2003年 [査読有り][通常論文]
Phonon-assisted transitions in single self-assembled InGaAs quantum dots are investigated by photoluminescence (PL) and PL excitation (PLE) spectroscopy. Under nearly resonant conditions of the excitonic ground state, a broad sideband is observed in both PL and PLE. The adiabatic description with the localized acoustic phonons well explains the observed spectra. - Y. Toda, T. Matsubara, R. Morita, K. Hoshino, T. Someya, Y. ArakawaPhysica Status Solidi C: Conferences 7 2627 - 2630 2003年 [査読有り][通常論文]
Transient transmission changes in the yellow luminescence (YL) band of GaN were investigated using excitation far smaller than the bandgap. At lower excitation power, time-resolved measurements revealed efficient transmission changes due to two-photon absorption (TPA), whose spectrum clearly indicates the contribution of the YL band. Upon increasing the excitation power, the transient signal showed an additional offset signal. The excitation power dependences of the offset suggested a two-step absorption process in conjunction with TPA in the YL band. The excitation energy dependence also supported this conclusion. © 2003 WILEY-VCH Verlag GmbH & Co. KGaA. - T. Inoue, Y. Toda, K. Hoshino, T. Someya, Y. ArakawaPhysica Status Solidi C: Conferences 7 2428 - 2431 2003年 [査読有り][通常論文]
Temperature-dependent Raman spectroscopy was used to analyze the effect of residual strain on optical phonons E2 (high) and A1 (LO) in high-quality GaN films grown on C-plane 6H-SiC, A- and C-plane sapphire substrates. By comparison with a free-standing GaN, we found that shifts in the Raman spectrum were consistent with compressive and tensile strains. Experimental results indicated that differences in the thermal expansion coefficients between GaN film and substrate have an effect on these frequency shifts. We also compared frequency shifts of the A1 (LO) mode with those of the E2 (high) mode and concluded that the strain-induced frequency shift of the A1 (LO) mode was smaller than that of the E2 (high) mode. The strain-tensor-dependences of each phonon mode are also discussed. © 2003 WILEY-VCH Verlag GmbH & Co. KGaA. - Y Toda, S Kako, M Nishioka, Y ArakawaJAPANESE JOURNAL OF APPLIED PHYSICS PART 2-LETTERS & EXPRESS LETTERS 41 12B L1464 - L1466 2002年12月 [査読有り][通常論文]
Photoluminescence excitation resonances in the multiphonon energy region of single self-assembled quantum dots are investigated. Several multiphonon resonances exhibit dip structures under the saturation condition, whereas the corresponding biexciton resonances have a sharp linewidth. The observed dip structures and inversely correlated biexciton resonances suggest that the photoluminescence excitation resonances include line broadening consisting of several relaxation pathways with different relaxation rates. - S. Kako, T. Sugimoto, Y. Toda, S. Ishida, M. Nishioka, Y. ArakawaPhysica E: Low-Dimensional Systems and Nanostructures 13 2-4 151 - 154 2002年 [査読有り][通常論文]
We have observed dip structures in photoluminescence excitation (PLE) spectra of a highly excited single InGaAs/GaAs self-assembled quantum dot. Some of the exciton PLE peaks lying in continuum states turn into the dips with increasing excitation power. The most remarkable feature is that the biexciton-PLE shows peaks at the excitation energies where the exciton PLE shows the dip structures. Using a model based on the excitation power dependence of the photoluminescence intensity, we discuss the origin of the dip structures. © 2002 Published by Elsevier Science B.V. - Y. Toda, M. Takahashi, T. Sugimoto, M. Nishioka, S. Ishida, Y. ArakawaTechnical Digest - Summaries of Papers Presented at the Quantum Electronics and Laser Science Conference, QELS 2001 20 - 21 2001年 [査読有り][通常論文]
Summary form only given. The study of quantum mechanical interactions between excited carriers in the semiconductor quantum dots (QDs) provides better understanding of zero-dimensional (0D) systems. One of the remarkable features of QDs is their atomic like density of states (DOS). Recently, we performed single dot spectroscopy on InGaAs self-assembled quantum dots (SAQDs) grown by Stranski-Krastnow mode, and found that their photoluminescence excitation (PLE) spectra exhibit 2D-like continuum states as well as discrete DOS structures. Because SAQDs are connected with 2D wetting layer (WL), it is supposed that the observed continuum originate from a crossover from 0D to 2D nature in their DOS. On the other hand, excited carriers in a single discrete state within a continuum show asymmetric resonance in their optical spectrum. This asymmetry is produced by interference between the optical transitions of a discrete energy state with continuum, the so-called Fano interference. In this paper, we carried out PLE spectroscopy with high excitation power and observed asymmetric resonances due to the existence of continuum states. - Y Toda, O Moriwaki, M Nishioka, Y ArakawaPHYSICA E 8 4 328 - 332 2000年12月 [査読有り][通常論文]
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 ArakawaAPPLIED PHYSICS LETTERS 76 26 3887 - 3889 2000年06月 [査読有り][通常論文]
Using near-field optical microscopy, we have performed coherent excitation spectroscopy of self-assembled quantum dots (SAQDs). A pair of coherent pulses with a time delay between them allows measurement of the temporal coherence of the carrier wave function in single quantum dots. The observed decoherence time is about 15 ps and is well explained by resonant Raman scattering of phonons. Furthermore, quantum beats originating from the superposition of two closely spaced coherent states have been observed. This opens up possibilities of quantum mechanical control of the carrier wave function in SAQDs. (C) 2000 American Institute of Physics. [S0003-6951(00)01426-1]. - Y Toda, Y ArakawaIEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS 6 3 528 - 533 2000年05月 [査読有り][招待有り]
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 ArakawaMICROELECTRONIC ENGINEERING 47 1-4 111 - 113 1999年06月 [査読有り][通常論文]
We have investigated carrier relaxations in single InAs/GaAs self-assembled quantum dots using near-field optical spectroscopy. Polarized PL spectroscopy of Zeeman splitting peaks with an external magnetic field allows us to investigate spin relaxation in the excited states. The excitation energy dependence of the polarization of each spin component shows suppression of spin flipping during the relaxation of carriers inside the quantum dots. The result also suggests different spin relaxation processes between in the quantum dots and in a wetting layer. - Y Toda, O Moriwaki, M Nishioka, Y ArakawaPHYSICAL REVIEW LETTERS 82 20 4114 - 4117 1999年05月 [査読有り][通常論文]
Comparison of near-field and far-field photoluminescence excitation (PLE) spectra gives new insight into the carrier relaxation process in InGaAs/GaAs self-assembled quantum dots. The near-field PLE spectra of single quantum dots clearly show 2D-like continuum states and a number of sharp lines, between a large zero-absorption region due to the quasi-0D density of states and the 2D wetting layer absorption edge. The results reveal an efficient intradot relaxation mechanism, proceeding as follows: The carriers can relax easily within continuum states, and make transitions to the excitonic ground state by resonant emission of localized phonons. - Y Toda, S Shinomori, K Suzuki, Y ArakawaPHYSICAL REVIEW B 58 16 R10147 - R10150 1998年10月 [査読有り][通常論文]
We have investigated the spin relaxation of carriers in single InAs/GaAs self-assembled quantum dots using near-field optical spectroscopy. Polarized photoluminescence spectroscopy of Zeeman split peaks in an external magnetic field allows us to investigate spin relaxation in the excited states. The excitation energy dependence of the polarization of each spin component shows the suppression of spin relaxation to be quenched at higher relaxation energies. This effect suggests a change in relaxation processes in the self-assembled dot systems. [S0163-1829(98)51740-7]. - Y Toda, S Shinomori, K Suzuki, Y ArakawaAPPLIED PHYSICS LETTERS 73 4 517 - 519 1998年07月 [査読有り][通常論文]
We report observations of Zeeman spin splitting in photoluminescence from single InAs/GaAs self-assembled quantum dots (SAQDs) fabricated by Stranski-Krastanow growth. A low-temperature near-field optical microscope incorporating a superconducting magnet enabled us to resolve the luminescence from individual SAQDs. We have measured Zeeman split components from single SAQDs in magnetic fields up to 10 T. Circularly-polarized photoluminescence resolved carriers of different spin polarity. Dependence of the splitting on the QD structures was also investigated. (C) 1998 American Institute of Physics. - Y Toda, S Shinomori, T Arakawa, Y ArakawaPHYSICA E 2 1-4 987 - 990 1998年07月 [査読有り][通常論文]
Using near-field optical microscopy we present experimental results on spatially resolved PL spectra of GaAs quantum wire (QWR) structures with a lateral width of 15 nm fabricated by selective growth method. The geometries of the samples make it possible to observe the luminescence from a single QWR structure. The typical fluctuations of the PL peak energy from different QWRs is estimated to be about 5 meV. To reduce the diffusion of carriers from barrier regions, PL spectra are observed by selective excitation of the QWR. A PL linewidth of 5 meV, which is about ten times narrower than that of the conventional PL measurement, is observed. (C) 1998 Elsevier Science B.V. All rights reserved. - Y Toda, S Shinomori, K Suzuki, Y ArakawaSOLID-STATE ELECTRONICS 42 7-8 1083 - 1086 1998年07月 [査読有り][通常論文]
We discuss the realization of a low-temperature near-held optical microscope incorporating a superconducting magnet. Experimental results on spatially resolved photoluminescence (PL) spectra of a single quantum dot (QD) fabricated by Stranski-Krastanow growth demonstrate the performance of this apparatus. The luminescence linewidth was estimated to be less than 0.4 meV -the limit of our spectrometer. The PL due to individual QDs was spatially and spectrally resolved. PL peaks from excited states of single QDs were identified. Small diamagnetic energy shifts (< 100 mu eV@10 T) were observed for the single QDs. (C) 1998 Published by Elsevier Science Ltd. All rights reserved. - Y. Toda, S. Shinomori, K. Suzuki, Y. ArakawaPhysical Review B 58 16 R10147 - R10150 1998年 [査読有り][通常論文]
- Y. Toda, S. Shinomori, K. Suzuki, Y. ArakawaPhysical Review B - Condensed Matter and Materials Physics 58 16 R10147 - R10150 1998年 [査読有り][通常論文]
We have investigated the spin relaxation of carriers in single InAs/GaAs self-assembled quantum dots using near-field optical spectroscopy. Polarized photoluminescence spectroscopy of Zeeman split peaks in an external magnetic field allows us to investigate spin relaxation in the excited states. The excitation energy dependence of the polarization of each spin component shows the suppression of spin relaxation to be quenched at higher relaxation energies. This effect suggests a change in relaxation processes in the self-assembled dot systems. © 1998 The American Physical Society. - Y Arakawa, Y Toda, S ShinomoriPHYSICS AND SIMULATION OF OPTOELECTRONIC DEVICES VI, PTS 1 AND 2 3283 785 - 792 1998年 [査読有り][通常論文]
We report photoluminescence (PL) studies of zero-dimensional excitonic confinement in InAs/GaAs self-assembled quantum dots (SAQDs) fabricated by Stranski-Krastanow growth. A low-temperature near-field optical microscope is employed to identify individual PL peaks due to single SAQDs. Highly spatial and spectral evaluation of each PL peak identify the Zeeman spin splitting with the magnetic field up to 10 T. The comparison of different confinement structures was also discussed. - Y Toda, M Kourogi, M Ohtsu, Y Nagamune, Y ArakawaAPPLIED PHYSICS LETTERS 69 6 827 - 829 1996年08月 [査読有り][通常論文]
We present experimental result on spatially and spectrally resolved imaging of GaAs quantum dot (QD) structures using a near-field optical microscope. Three photoluminescence (PL) peaks which originate from QD, quantum well (QW), and bulk regions were observed at liquid-He temperature. Carriers were observed to diffuse effectively from the AlGaAs barriers to the QD and QW regions. The region of the intension carrier capture manifests itself by emitting a sharp PL peak, and is estimated to be about 300 nm. (C) 1996 American Institute of Physics. - Y TODA, M OHTSUIEEE PHOTONICS TECHNOLOGY LETTERS 7 1 84 - 86 1995年01月 [査読有り][通常論文]
We propose a high-resolution diagnostics technique for optical waveguides using a photon-scanning tunneling microscope (P-STM). This technique has the advantageous capabilities of nondestructive measurement and informative analysis of guided mode. The capability of this technique was evaluated by measuring the characteristics of an LiTaO3 waveguide. Scattered light spot of 500-nm diameter caused by defects was measured, and a normalized width was estimated to be 3.7 mu m. Moreover, we obtained the power dispersion in Y-branch waveguides, which were in good agreement with the theoretical plots calculated by the beam propagation method (BPM). - Y TODA, T ENAMI, M OUTSUJAPANESE JOURNAL OF APPLIED PHYSICS PART 2-LETTERS & EXPRESS LETTERS 32 9A L1233 - L1235 1993年09月 [査読有り][通常論文]
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 OHTSUAPPLIED PHYSICS LETTERS 61 16 1886 - 1888 1992年10月 [査読有り][通常論文]
Both second harmonic and sum-frequency generations by using 1.5 mum diode lasers in a type II phase matched potassium titanyl phosphate are demonstrated for the first time. A maximum second harmonic output power of 1.0 muW is obtained with the use of an external builtup cavity, and a 0.23 muW sum-frequency output power at 0.54 mum is obtained by using 1.5 and 0.82 mum diode lasers.
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- 土屋聡, MERTELJ Tomaz, MIHAILOVIC Dragan, 山田順一, 戸田泰則 日本物理学会講演概要集(CD-ROM) 74 (1) 2019年
- 中川紘一, 佐藤貴裕, 土屋聡, 山田順一, 戸田泰則 日本物理学会講演概要集(CD-ROM) 74 (2) 2019年
- 土屋聡, 桑江良太, 児玉匠, 戸田泰則, 中村祐介, 栗原芽来美, 山本貴, 内藤俊雄 日本物理学会講演概要集(CD-ROM) 74 (2) 2019年
- 中川紘一, 土屋聡, 谷口弘三, 戸田泰則 日本物理学会講演概要集(CD-ROM) 74 (2) 2019年
- 土屋聡, 中川大輔, 戸田泰則 高圧討論会講演要旨集 59th 163 2018年11月13日 [査読無し][通常論文]
- 脇坂勇武, 渡辺悠歩, 山根啓作, 岡和彦, 戸田泰則, 森田隆二 Optics & Photonics Japan講演予稿集(CD-ROM) 2018 ROMBUNNO.2aC5 2018年10月24日 [査読無し][通常論文]
- 土屋聡, 中川紘一, 谷口弘三, 戸田泰則 日本物理学会講演概要集(CD-ROM) 73 (2) ROMBUNNO.11aPS‐63 2018年09月21日 [査読無し][通常論文]
- 土屋聡, 中川大輔, 戸田泰則 日本物理学会講演概要集(CD-ROM) 73 (2) ROMBUNNO.11aPS‐62 2018年09月21日 [査読無し][通常論文]
- 佐々木凜, 岩佐康平, 山根啓作, 岡和彦, 戸田泰則, 森田隆二 応用物理学会秋季学術講演会講演予稿集(CD-ROM) 79th ROMBUNNO.20a‐211A‐8 2018年09月05日 [査読無し][通常論文]
- 渡辺悠歩, 山根啓作, 岡和彦, 戸田泰則, 森田隆二 応用物理学会秋季学術講演会講演予稿集(CD-ROM) 79th ROMBUNNO.20a‐211A‐7 2018年09月05日 [査読無し][通常論文]
- 山根啓作, 脇坂勇武, 渡辺悠歩, 岡和彦, 戸田泰則, 森田隆二 電子情報通信学会技術研究報告 118 (129(LQE2018 20-35)) 1‐4 2018年07月05日 [査読無し][通常論文]
- 中川紘一, 土屋聡, 谷口弘三, 戸田泰則 日本物理学会講演概要集(CD-ROM) 73 (1) ROMBUNNO.25aB404‐2 2018年03月23日 [査読無し][通常論文]
- 中川大輔, 土屋聡, 戸田泰則 応用物理学会北海道支部/日本光学会北海道支部合同学術講演会講演予稿集 53rd-14th 41 2018年01月06日 [査読無し][通常論文]
- 中川紘一, 土屋聡, 谷口弘三, 戸田泰則 応用物理学会北海道支部/日本光学会北海道支部合同学術講演会講演予稿集 53rd-14th 43 2018年01月06日 [査読無し][通常論文]
- 佐藤貴裕, 土屋聡, 山田順一, 戸田泰則 応用物理学会北海道支部/日本光学会北海道支部合同学術講演会講演予稿集 53rd-14th 42 2018年01月06日 [査読無し][通常論文]
- 住吉祥伍, 山根啓作, 戸田泰則, 森田隆二 応用物理学会北海道支部/日本光学会北海道支部合同学術講演会講演予稿集 53rd-14th 62 2018年01月06日 [査読無し][通常論文]
- 坪谷春輝, 司馬哲裕, 山根啓作, 戸田泰則, 森田隆二 応用物理学会北海道支部/日本光学会北海道支部合同学術講演会講演予稿集 53rd-14th 61 2018年01月06日 [査読無し][通常論文]
- 内古閑隆太郎, 山根啓作, 森田隆二, 戸田泰則 応用物理学会北海道支部/日本光学会北海道支部合同学術講演会講演予稿集 53rd-14th 60 2018年01月06日 [査読無し][通常論文]
- 山根啓作, 渡辺悠歩, 岡和彦, 戸田泰則, 森田隆二 応用物理学会春季学術講演会講演予稿集(CD-ROM) 65th 2018年
- 戸田泰則 光アライアンス 28 (9) 11‐15 -15 2017年09月01日 [査読無し][通常論文]
- 望月洋人, 戸田泰則, 土屋聡, 山根啓作, 森田隆二, 黒沢徹, 小田研 応用物理学会秋季学術講演会講演予稿集(CD-ROM) 78th ROMBUNNO.6a‐S45‐5 2017年08月25日 [査読無し][通常論文]
- 森谷淳史, 戸田泰則, 山根啓作, 森田隆二, 重松恭平, 淡路祥成 応用物理学会秋季学術講演会講演予稿集(CD-ROM) 78th ROMBUNNO.6p‐S45‐2 2017年08月25日 [査読無し][通常論文]
- 岩佐康平, 柿澤康平, 山根啓作, 岡和彦, 戸田泰則, 森田隆二 応用物理学会秋季学術講演会講演予稿集(CD-ROM) 78th ROMBUNNO.6p‐S45‐4 2017年08月25日 [査読無し][通常論文]
- 柿澤康平, 岩佐康平, 山根啓作, 岡和彦, 戸田泰則, 森田隆二 応用物理学会春季学術講演会講演予稿集(CD-ROM) 64th ROMBUNNO.16p‐311‐8 2017年03月01日 [査読無し][通常論文]
- 山根啓作, 戸田泰則, 森田隆二 応用物理学会春季学術講演会講演予稿集(CD-ROM) 64th ROMBUNNO.16p‐311‐7 2017年03月01日 [査読無し][通常論文]
- 戸田泰則, 黒澤徹, 小田研 日本物理学会誌 71 (12) 830‐835 -835 2016年12月05日 [査読無し][通常論文]
超伝導は物理学における最も魅力的な現象の一つであり，カマリング・オネスによる発見から今日に至るまで多くの研究者を魅了してきた．なかでも液体窒素温度をはるかに上回る転移温度を示す銅酸化物は，いわゆる高温超伝導体のプロトタイプとして数多くの研究がなされ，今も新たな知見や問題提起が次々と報告されている．
超伝導状態では電子間に有効な引力が作用して電子対（クーパー対）が形成され，複数の電子対が位相をそろえて量子凝縮する．凝縮が起こると，フェルミ面近傍の電子スペクトルには，対演算子の期待値（ギャップ関数）によって特徴づけられるエネルギーギャップが開く．Bardeen–Cooper–Schrieffer (BCS) の理論によって説明される従来型超伝導では，格子振動（フォノン）を介した引力で形成される電子対がフェルミ面上に等方的なs波対称性を持つギャップを形成する．これに対して電子間に強い斥力の働く銅酸化物高温超伝導体のギャップは，有効な引力を獲得するために異方的なギャップ関数を持つと考えられており，フェルミ面上にギャップの節（ノード）を持つd波対称性を示す．また超伝導転移温度(Tc)以上でフェルミ面の一部に擬ギャップと呼ばれるギャップ構造が形成されることが知られており，特に反強磁性絶縁相に近いアンダードープ(UD)試料ではTcよりはるかに高温から観測される．擬ギャップの存在は銅酸化物高温超伝導体の最大の特徴であり，古くから精力的な研究がなされているが，超伝導の前駆現象から超伝導と共存/ 競合する秩序相など様々な解釈があり，結論を見ていない．また物質や測定手法に応じて多様な性質を示すため，ギャップの起源や形成メカニズムを決定するには，多面的かつ系統的な物性観測が必要とされている．
このように多彩な顔を持つ擬ギャップおよび超伝導ギャップと両者の関係性について，従来の実空間や運動量空間の観測手法に加え，超短光パルスを使った時間領域からのアプローチが行われるようになってきた．本稿では物性観測に広く使われる時間分解ポンププローブ分光をベースに，この手法が，これまで観測できなかった高温超伝導体のバルク性質を反映した対称性解析，および秩序形成の相関スケール解析へと拡張可能であることを示す．本稿で扱うポンププローブ分光では，ポンプ光を用いてギャップを形成する電子または正孔（キャリア）を瞬時的に破壊し，高エネルギー状態へと励起する．キャリアのエネルギー緩和に伴って生成される非平衡な準粒子やフォノンは（超伝導・擬ギャップに対応する）ギャップ関数の変化を反映し，ポンプ光に対して遅延時間を持つプローブ光の反射率変化として測定される．励起に使用する光子エネルギーはギャップエネルギーよりもはるかに大きいが，結晶内部への侵入長を大きく取れるため，バルク性質を反映する．拡張されたポンププローブ分光は，それぞれ超伝導ギャップおよび擬ギャップに起因する準粒子応答を一意に決定可能であり，超伝導前駆現象を反映する揺らぎ状態を含め，超伝導ギャップと擬ギャップの個性および両者の関係性をダイナミクスの観点から明らかにする．偏光応答に着目した対称性解析では，擬ギャップの形成温度T *以下で自発的な回転対称性破れが生じることが示される．他方，秩序形成ダイナミクスからは，擬ギャップが長距離相関のない局在状態を反映することが明らかとなる．
- 重松恭平, 鈴木雅人, 山根啓作, 森田隆二, 戸田泰則 応用物理学会春季学術講演会講演予稿集(CD-ROM) 63rd ROMBUNNO.20P-P3-1 2016年03月03日 [査読無し][通常論文]
- 鈴木雅人, 山根啓作, 岡和彦, 戸田泰則, 森田隆二 応用物理学会春季学術講演会講演予稿集(CD-ROM) 63rd ROMBUNNO.21P-S622-20 2016年03月03日 [査読無し][通常論文]
- Masato Suzuki, Keisaku Yamane, Kazuhiko Oka, Yasunori Toda, Ryuji Morita 2016年02月08日 [査読無し][通常論文]
In a recent letter, Milione et al. reported the Pancharatnam-Berry phase
(PBP) on a higher-order Poincar\'e sphere (HOPS) [Phys. Rev. Lett, 108, 190401
(2012)]. They used two spin-orbit converters (SOCs) to make a trajectory along
the HOPS. However, the SOCs actually do not make a path on the HOPS. We briefly
discuss a way of acquiring a PBP made by SOCs. - 西坂拓馬, 重松恭平, 由利洋樹, 山根啓作, 山根啓作, 森田隆二, 森田隆二, 淡路祥成, 戸田泰則, 戸田泰則 応用物理学会北海道支部・日本光学会北海道地区合同学術講演会講演予稿集 51st-12th 35 2016年01月09日 [査読無し][通常論文]
- 里水里, 山根啓作, 山根啓作, 戸田泰則, 戸田泰則, 森田隆二, 森田隆二 応用物理学会北海道支部・日本光学会北海道地区合同学術講演会講演予稿集 51st-12th 61 2016年01月09日 [査読無し][通常論文]
- 中川紘一, 土屋聡, 山田順一, 戸田泰則 応用物理学会北海道支部/日本光学会北海道支部合同学術講演会講演予稿集 51st-12th 2016年
- 渡邉健太, 土屋聡, 戸田泰則, 板垣征一朗, 黒澤徹, 小田研 応用物理学会北海道支部/日本光学会北海道支部合同学術講演会講演予稿集 51st-12th 2016年
- 西坂拓馬, 重松恭平, 山根啓作, 山根啓作, 森田隆二, 森田隆二, 淡路祥成, 戸田泰則, 戸田泰則 応用物理学会春季学術講演会講演予稿集(CD-ROM) 63rd 2016年
- 柿澤康平, 山根啓作, 岡和彦, 戸田泰則, 森田隆二 応用物理学会秋季学術講演会講演予稿集(CD-ROM) 77th 2016年
- 角田恵一郎, 山根啓作, 山根啓作, 戸田泰則, 戸田泰則, 森田隆二, 森田隆二 応用物理学会北海道支部/日本光学会北海道支部合同学術講演会講演予稿集 51st-12th 2016年
- Kyohhei Shigematsu, Masato Suzuki, Keisaku Yamane, Ryuji Morita, Yasunori Toda 2016 CONFERENCE ON LASERS AND ELECTRO-OPTICS (CLEO) 2016年 [査読有り][通常論文]
We performed four-wave-mixing (FWM) spectroscopy using radially polarized pulses on uniaxially strained GaN excitons. The FWM dynamics show a deformation of the polarization distributions attributable to the rotational symmetry breaking in the exciton coherent dynamics. - 土屋 聡, 児玉 匠, 中村 祐介, 山本 貴, 内藤 俊雄, 戸田 泰則 日本物理学会講演概要集 71 (0) 1595 -1595 2016年 [査読無し][通常論文]
<p>有機超伝導体{\kappa}-(ET)2Cu[N(CN)2]X（X=Cl,Br,I）はアニオンの原子Xを化学置換することで、結晶構造を変化させることなく、電子物性に対する圧力効果を系統的に調査できる系として注目さている。しかし、X=Iの試料に関して作製の困難さから測定例が非常に少なく、その物性は未解明の部分が多い。今回X=Iの試料に対してフェムト秒光パルスを用いた時間分解分光行い、非平衡準粒子緩和ダイナミクスを調べた。</p> - 中川 紘一, 土屋 聡, 山田 順一, 戸田 泰則 日本物理学会講演概要集 71 (0) 1596 -1596 2016年 [査読無し][通常論文]
<p>有機超伝導体!LaTeX$\kappa$-(BEDT-TTF)_2_Xではアニオン層Xに化学置換を施すことで電子相関を制御可能である。本研究では電子相関の異なるX=Cu[N(CN)_2_]Br及びCu(NCS)_2_に対しエネルギーギャップの検出が可能な偏光時間分解分光を実施した。その結果、両試料において擬ギャップ応答と考えられる空間対称性破れを検出することに成功した。さらに超伝導応答が、電子相関がより大きいX=Cu[N(CN)_2_]Brの試料では転移温度以上においても検出され、超伝導発現における電子相関の重要性を示唆する結果が得られた。</p> - 鈴木雅人, 山根啓作, 山根啓作, 中嶌奨太, 坂本盛嗣, 岡和彦, 戸田泰則, 戸田泰則, 森田隆二, 森田隆二 応用物理学会秋季学術講演会講演予稿集(CD-ROM) 76th ROMBUNNO.14A-2G-3 2015年08月31日 [査読無し][通常論文]
- 中嶌奨太, 山根啓作, 山根啓作, 鈴木雅人, 戸田泰則, 戸田泰則, 森田隆二, 森田隆二 応用物理学会秋季学術講演会講演予稿集(CD-ROM) 76th ROMBUNNO.14A-2G-2 2015年08月31日 [査読無し][通常論文]
- 重松恭平, 森本祥平, 鈴木雅人, 山根啓作, 山根啓作, 森田隆二, 森田隆二, 戸田泰則, 戸田泰則 日本物理学会講演概要集(CD-ROM) 70 (1) ROMBUNNO.21PCP-4 -1471 2015年03月24日 [査読無し][通常論文]
- 鈴木雅人, 山根啓作, 岡和彦, 戸田泰則, 森田隆二 応用物理学会春季学術講演会講演予稿集(CD-ROM) 62nd ROMBUNNO.12P-A15-12 2015年02月26日 [査読無し][通常論文]
- 山根啓作, 横瀬尚也, 鈴木雅人, 戸田泰則, 森田隆二 応用物理学会春季学術講演会講演予稿集(CD-ROM) 62nd ROMBUNNO.12P-A15-13 2015年02月26日 [査読無し][通常論文]
- 山根啓作, 中嶌奨太, 鈴木雅人, 戸田泰則, 森田隆二 応用物理学会春季学術講演会講演予稿集(CD-ROM) 62nd ROMBUNNO.12P-A15-14 2015年02月26日 [査読無し][通常論文]
- 土屋聡, 中川紘一, 山田順一, 戸田泰則 日本物理学会講演概要集(CD-ROM) 70 (1) 2015年
- 土屋 聡, 中川 紘一, 山田 順一, 戸田 泰則 日本物理学会講演概要集 70 (0) 1844 -1844 2015年 [査読無し][通常論文]
- 土屋 聡, 木野 洋平, 中川 紘一, 山田 順一, 戸田 泰則 日本物理学会講演概要集 70 (0) 1336 -1336 2015年 [査読無し][通常論文]
- Keisaku Yamane, Keisaku Yamane, Asami Honda, Yasunori Toda, Ryuji Morita Springer Proceedings in Physics 162 809 -812 2015年01月01日 [査読無し][通常論文]
© Springer International Publishing Switzerland 2015.We demonstrated the generation of over 1-mJ intense optical-vortex pulses of which topological charges were programmable controlled by computergenerated holograms. The pulse duration was characterized to be 27 fs by twodimensional spectral shearing interferometry. - 本田亜沙美, 山根啓作, 鈴木雅人, 戸田泰則, 森田隆二 応用物理学会秋季学術講演会講演予稿集(CD-ROM) 75th ROMBUNNO.19P-C1-12 2014年09月01日 [査読無し][通常論文]
- 西坂拓馬, 重松恭平, 由利洋樹, 山根啓作, 森田隆二, 淡路祥成, 戸田泰則 応用物理学会秋季学術講演会講演予稿集(CD-ROM) 75th ROMBUNNO.19P-C1-15 2014年09月01日 [査読無し][通常論文]
- 由利洋樹, 重松恭平, 西坂拓馬, 山根啓作, 森田隆二, 淡路祥成, 戸田泰則 応用物理学会秋季学術講演会講演予稿集(CD-ROM) 75th ROMBUNNO.19P-C1-16 2014年09月01日 [査読無し][通常論文]
- 鈴木雅人, 山根啓作, 岡和彦, 戸田泰則, 森田隆二 応用物理学会秋季学術講演会講演予稿集(CD-ROM) 75th ROMBUNNO.19P-C1-14 2014年09月01日 [査読無し][通常論文]
- 横瀬尚也, 鈴木雅人, 山根啓作, 岡和彦, 戸田泰則, 森田隆二 応用物理学会秋季学術講演会講演予稿集(CD-ROM) 75th ROMBUNNO.19P-C1-13 2014年09月01日 [査読無し][通常論文]
- 土屋 聡, 菅原 悠太, 丹田 聡, 戸田 泰則 日本物理学会講演概要集 69 (2) 554 -554 2014年08月22日 [査読無し][通常論文]
- 野並拓矢, 吉田紘行, 岸文香, 黒澤徹, 戸田泰則, 竹山幸作, 小田研, 伊土政幸 日本物理学会講演概要集 69 (2) 421 -421 2014年08月22日 [査読無し][通常論文]
- Masato Suzuki, Keisaku Yamane, Kazuhiko Oka, Yasunori Toda, Ryuji Morita Optics InfoBase Conference Papers 2014年07月21日 [査読無し][通常論文]
- 荒巻 光利, 野々部 和樹, 田部井 翔太, 戸田 泰則, 吉村 信次, 森崎 友宏, 後藤 基志, 寺坂 健一郎, 田中 雅慶, 西澤 典彦 日本物理学会講演概要集 69 (1) 281 -281 2014年03月05日
- 山根啓作, 本田亜沙美, 戸田泰則, 森田隆二 応用物理学会春季学術講演会講演予稿集(CD-ROM) 61st ROMBUNNO.18A-F7-1 2014年03月03日 [査読無し][通常論文]
- 鈴木雅人, 山根啓作, 塩田康之, 岡和彦, 戸田泰則, 森田隆二 応用物理学会春季学術講演会講演予稿集(CD-ROM) 61st ROMBUNNO.17A-F6-8 2014年03月03日 [査読無し][通常論文]
- 渡辺健太, 土屋聡, 戸田泰則, 板垣征一朗, 黒澤徹, 小田研 応用物理学会秋季学術講演会講演予稿集(CD-ROM) 75th 2014年
- 塩田康之, 山根啓作, 覚間誠一, 戸田泰則, 森田隆二 応用物理学会秋季学術講演会講演予稿集(CD-ROM) 74th ROMBUNNO.19P-A3-3 2013年08月31日 [査読無し][通常論文]
- 川之上 史篤, 戸田 泰則, 板垣 征一朗, 黒澤 徹, 小田 研, 伊土 政幸 日本物理学会講演概要集 68 (2) 500 -500 2013年08月26日
- 長谷川 明, 戸田 泰則, 足立 智, 森田 隆二, 中山 正昭 日本物理学会講演概要集 68 (2) 698 -698 2013年08月26日
- 重松恭平, 戸田泰則, 山根啓作, 森田隆二 日本物理学会講演概要集 68 (2) 699 -699 2013年08月26日 [査読無し][通常論文]
- 菅原 悠太, 石岡 準也, 戸田 康則, 小田 研, 黒澤 徹, 松浦 徹, 市村 晃一, 丹田 聡 日本物理学会講演概要集 68 (1) 892 -892 2013年03月26日
- 鈴木雅人, 山根啓作, 戸田泰則, 森田隆二 応用物理学会春季学術講演会講演予稿集(CD-ROM) 60th ROMBUNNO.28A-D2-7 2013年03月11日 [査読無し][通常論文]
- 山根啓作, 本田亜沙美, 戸田泰則, 森田隆二 応用物理学会春季学術講演会講演予稿集(CD-ROM) 60th ROMBUNNO.28A-D2-6 2013年03月11日 [査読無し][通常論文]
- 川之上史篤, 土屋聡, 戸田泰則, 板垣征一朗, 黒澤徹, 小田研, 伊土政幸 応用物理学会北海道支部・日本光学会北海道地区合同学術講演会講演予稿集 49th-10th 2013年
- 長谷川明, 戸田泰則, 足立智, 森田隆二, 中山正昭 応用物理学会北海道支部・日本光学会北海道地区合同学術講演会講演予稿集 49th-10th 2013年
- Keisaku Yamane, Zhili Yang, Kyouhei Shigematsu, Yasunori Toda, Ryuji Morita 2013 CONFERENCE ON LASERS AND ELECTRO-OPTICS EUROPE AND INTERNATIONAL QUANTUM ELECTRONICS CONFERENCE (CLEO EUROPE/IQEC) 2013年 [査読有り][通常論文]
- 鈴木雅人, 山根啓作, 戸田泰則, 森田隆二 Optics & Photonics Japan講演予稿集(CD-ROM) 2013 ROMBUNNO.12AE4 2013年 [査読無し][通常論文]
- 本田亜沙美, 山根啓作, 横瀬尚也, 重松恭平, 戸田泰則, 森田隆二 Optics & Photonics Japan講演予稿集(CD-ROM) 2013 ROMBUNNO.12AE3 2013年 [査読無し][通常論文]
- 重松恭平, 戸田泰則, 山根啓作, 森田隆二 応用物理学会学術講演会講演予稿集(CD-ROM) 73rd ROMBUNNO.13A-B2-3 2012年08月27日 [査読無し][通常論文]
- 鈴木雅人, 山根啓作, 戸田泰則, 森田隆二 応用物理学会学術講演会講演予稿集(CD-ROM) 73rd ROMBUNNO.13A-B2-2 2012年08月27日 [査読無し][通常論文]
- 小野 頌太, 島 弘幸, 戸田 泰則 日本物理学会講演概要集 67 (2) 689 -689 2012年08月24日
- 荒巻 光利, 戸田 泰則, 吉村 信次, 寺坂 健一郎, 田中 雅慶 日本物理学会講演概要集 67 (2) 214 -214 2012年08月24日
- 淡路 祥成, 和田 尚也, 戸田 泰則, 林 哲也 電子情報通信学会総合大会講演論文集 2012 (1) 150 2012年03月06日
- 戸田 泰則, Mertelj T., Kusar P., 黒澤 徹, 小田 研, 伊土 政幸, Mihailovic D. 日本物理学会講演概要集 67 (1) 648 -648 2012年03月05日
- 淡路 祥成, 和田 尚也, 戸田 泰則 電子情報通信学会技術研究報告. PN, フォトニックネットワーク 111 (475) 71 -74 2012年03月05日ラゲール・ガウスモードを用いたモード分割多重伝送方式において、マルチコアファイバ伝搬後の軌道角運動量スペクトルを観測し、良好なコントラストで主モード及び陪モードを識別することができた。このように、複数のモード成分が混在する軌道角運動量スペクトル上の任意のコンポーネントをモード変換し、空間フィルタなどを関して零次モードとして弁別することで、通信チャネルが割り当てられた各次数のラゲール・ガウスモードを直接スイッチング可能である。
- 重松恭平, 戸田泰則, 戸田泰則, 森田隆二, 森田隆二 日本物理学会講演概要集 67 (1) 796 -796 2012年03月05日 [査読無し][通常論文]
- 山根啓作, 戸田泰則, 森田隆二 応用物理学関係連合講演会講演予稿集(CD-ROM) 59th ROMBUNNO.18A-E9-9 2012年02月29日 [査読無し][通常論文]
- 鹿島準一郎, 戸田泰則, 森田隆二 応用物理学会北海道支部・日本光学会北海道地区合同学術講演会講演予稿集 47th-8th 2012年
- Keisaku Yamane, Yasunori Toda, Ryuji Morita 2012 CONFERENCE ON LASERS AND ELECTRO-OPTICS (CLEO) 2012年 [査読有り][通常論文]
The generation of 2.3-cycle, 5.9-fs, 56-mu J ultrashort optical-vortex pulses by optical parametric amplification was performed. To the best of our knowledge, it is the first generation of optical vortex pulses in the few-cycle regime. (C) 2011 Optical Society of America - 鈴木雅人, 山根啓作, 戸田泰則, 戸田泰則, 森田隆二, 森田隆二 応用物理学会学術講演会講演予稿集(CD-ROM) 72nd ROMBUNNO.30P-ZH-10 2011年08月16日 [査読無し][通常論文]
- 重松恭平, 戸田泰則, 戸田泰則, 天野竜郎, 森田隆二, 森田隆二 応用物理学会学術講演会講演予稿集(CD-ROM) 72nd ROMBUNNO.30P-ZH-11 2011年08月16日 [査読無し][通常論文]
- 淡路 祥成, 和田 尚也, 戸田 泰則, 林 哲也 電子情報通信学会技術研究報告. OFT, 光ファイバ応用技術 111 (69) 25 -28 2011年05月19日ラゲール・ガウスモードがイメージファイバの複数のコアを通過した際に軌道角運動量スペクトルが保存されることが従来、実験的に確かめられていたが、今回、よりコア数(画素数)の少ない(粗い)7コアファイバを通過した際の軌道角運動量の保存について、実験的に検証したのでこれを報告する。
- 鈴木雅人, 戸田泰則, 森田隆二 応用物理学関係連合講演会講演予稿集(CD-ROM) 58th 2011年
- 石岡 準也, 島竹 克大, 黒澤 徹, 市村 晃一, 戸田 泰則, 小田 研, 丹田 聡 日本物理学会講演概要集 65 (1) 812 -812 2010年03月01日 [査読無し][通常論文]
- 斎藤崇広, 戸田泰則, 森田隆二 Optics & Photonics Japan講演予稿集 2010 2010年
- 時実悠, 岡和彦, 戸田泰則, 森田隆二 応用物理学関係連合講演会講演予稿集(CD-ROM) 57th 2010年
- 本田翔一, 戸田泰則, 森田隆二 応用物理学関係連合講演会講演予稿集(CD-ROM) 57th 2010年
- 本田翔一, 戸田泰則, 淡路祥成, 上野雄鋭, 森田隆二 応用物理学会学術講演会講演予稿集(CD-ROM) 71st 2010年
- 戸田 泰則 表面科学 : hyomen kagaku = Journal of the Surface Science Society of Japan 30 (12) 667 -672 2009年12月10日
- 時実悠, 島竹克大, 戸田泰則, 岡和彦, 丹田聡, 森田隆二 応用物理学関係連合講演会講演予稿集 56th (3) 2009年
- 上野雄鋭, 戸田泰則, 足立智, 森田隆二, 俵毅彦 応用物理学会学術講演会講演予稿集 70th (3) 2009年
- 本田翔一, 上野雄鋭, 森田隆二, 戸田泰則 応用物理学会学術講演会講演予稿集 70th (3) 2009年
- 島竹 克大, 戸田 泰則 應用物理 77 (6) 672 -675 2008年06月10日
- 小野崎龍恒, 小野崎龍恒, 坪田雅功, 丹田聡, 戸田泰則 応用物理学関係連合講演会講演予稿集 55th (3) 1139 2008年03月27日 [査読無し][通常論文]
- 劉 英豪, 戸田 泰則, 島竹 克大, 桃野 直樹, 小田 研, 伊土 政幸, 黒澤 徹 日本物理学会講演概要集 63 (1) 630 -630 2008年02月29日
- 上野雄鋭, 上野雄鋭, 戸田泰則, 戸田泰則, 足立智, 森田隆二, 俵毅彦 応用物理学会学術講演会講演予稿集 69th (3) 2008年
- 島竹 克大, 小野崎 龍之, 戸田 泰則, 丹田 聡 日本物理学会講演概要集 62 (2) 810 -810 2007年08月21日
- 小野崎龍之, 坪田雅功, 丹田聡, 戸田泰則 日本物理学会講演概要集 62 (2) 782 -782 2007年08月21日 [査読無し][通常論文]
- 戸田 泰則, 島竹 克大 マテリアルインテグレ-ション 20 (5) 44 -48 2007年05月
- 戸田 泰則, 島竹 克大, 小野崎 龍之, 丹田 聡 日本物理学会講演概要集 61 (2) 637 -637 2006年08月18日
- 小野崎 龍之, 島竹 克大, 丹田 聡, 戸田 泰則 日本物理学会講演概要集 61 (2) 584 -584 2006年08月18日
- 島竹 克大, 戸田 泰則, 丹田 聡 日本物理学会講演概要集 61 (1) 772 -772 2006年03月04日
- 島竹 克大, 戸田 泰則, 丹田 聡 日本物理学会講演概要集 60 (1) 742 -742 2005年03月04日
- 戸田 泰則, 足立 智, 阿部 泰裕, 星野 勝之, 荒川 泰彦 日本物理学会講演概要集 60 (1) 667 -667 2005年03月04日
- 島竹 克大, 南 丈史, 戸田 泰則, 丹田 聡 日本物理学会講演概要集 59 (2) 699 -699 2004年08月25日
- 戸田泰則, 森田隆二, 丹田聡 応用物理学会学術講演会講演予稿集 65th (3) 2004年
- 戸田 泰則, 立石 幸一, 稲垣 克彦, 市村 晃一, 丹田 聡 日本物理学会講演概要集 58 (2) 646 -646 2003年08月15日 [査読無し][通常論文]
- 伊藤晃一, 立石幸一, 森田隆二, 山下幹雄, 戸田泰則 応用物理学関係連合講演会講演予稿集 50th (3) 2003年
- 松原崇史, 戸田泰則, 森田隆二, 山下幹雄, 星野勝之, 染谷隆夫, 荒川泰彦 応用物理学関係連合講演会講演予稿集 50th (1) 2003年
- 戸田 泰則, 荒川 泰彦 應用物理 71 (6) 714 -715 2002年06月10日
- 松原崇史, 戸田泰則, 森田隆二, 山下幹雄, 星野勝之, 染谷隆夫, 荒川泰彦 応用物理学会学術講演会講演予稿集 63rd (1) 2002年
- 伊藤晃一, 立石幸一, 森田隆二, 山下幹雄, 戸田泰則 応用物理学会学術講演会講演予稿集 63rd (3) 2002年
- 戸田 泰則, 荒川 泰彦 日本物理学会講演概要集 53 (2) 231 -231 1998年09月05日
書籍等出版物
- Laser Spectroscopy Using Topological Light BeamsYasunori Toda (担当:共著)
Springer Series in Progress in Nanophotonics 3 2014年 - 光物性入門戸田泰則 (担当:共著)
朝倉書店 2009年 - ナノオプティクス：ナノスケールの光物性戸田泰則 (担当:共著)
オーム社 2004年 - Optical Characterization of In(Ga)As/GaAs Self-assembled Quantum Dots Using Near-Field Spectroscopy戸田泰則 (担当:共著)
Springer Series in Optical Sciences Volume 86 2003年 - ナノ光工学ハンドブック戸田泰則 (担当:共著)
朝倉書店 2002年
所属学協会
共同研究・競争的資金等の研究課題
- 日本学術振興会：科学研究費助成事業研究期間 : 2022年04月 -2025年03月代表者 : 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 新学術領域研究(研究領域提案型)研究期間 : 2019年06月 -2024年03月代表者 : 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。
- 日本学術振興会：科学研究費助成事業 基盤研究(B)研究期間 : 2020年04月 -2023年03月代表者 : 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。 - 日本学術振興会：科学研究費助成事業 国際共同研究加速基金(国際共同研究強化(B))研究期間 : 2018年10月 -2023年03月代表者 : 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)研究期間 : 2019年06月 -2022年03月代表者 : 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）。
- 日本学術振興会：科学研究費助成事業 基盤研究(B)研究期間 : 2019年04月 -2022年03月代表者 : 戸田 泰則, 小田 研, 土屋 聡本研究では光渦の空間特異性を超伝導に対する光誘起相制御として活用し、従来技術では実現困難な時空間で変調された準安定電子状態を創出するとともに、相制御のもたらす新規物性および新機能を開拓することを目的として研究を進めている。昨年度は研究計画の基盤となる空間特異性に起因する超伝導過渡応答変化をビスマス系高温超伝導体（Bi2212）試料に対して捉えることに成功し、超伝導相制御に対する光渦パルス適用の道筋を示すことができた。以下に研究成果をまとめる。 銅酸化物高温超伝導体Bi2212の超伝導相転移温度（Tc）以上には不完全なギャップ（擬ギャップ）が存在し、特徴的な空間特性が発現する。擬ギャップ構造は反強磁性相互作用の強い低キャリア濃度領域で空間不均一化を伴って発達するが、Bi置換による超伝導面外の秩序攪乱（ディスオーダー）を用いることにより制御可能であることが知られる。特に超伝導ギャップが最大となるオプティマルドープ試料では、Bi置換と過剰酸素制御との併用により、擬ギャップに対する空間不均一化の影響を選択的に調査できる。光渦パルスの空間特異性を相抑圧として活用するため、昨年度はBi置換量の異なるオプティマルドープ試料を準備し、時間分解ポンププローブ分光による過渡応答を系統的に調査した。実験結果からBi置換量に応じた擬ギャップおよび超伝導の超短パルス光誘起過渡応答緩和の温度特性および飽和強度特性の本質的な変化が観測され、空間不均一性にもとづく超伝導と擬ギャップとの相関制御の可能性を示すことができた。すなわち空間特異性にもとづく光誘起相制御への展開が示され、本研究の目指す超伝導相制御に対する光渦パルス適用の道筋を提示できた。
- 日本学術振興会：科学研究費助成事業 基盤研究(C)研究期間 : 2019年04月 -2022年03月代表者 : 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。
- 日本学術振興会：科学研究費助成事業 基盤研究(B)研究期間 : 2016年04月 -2019年03月代表者 : 戸田 泰則, 土屋 聡本研究では、高温超伝導体（Bi2212）に対して光渦パルス励起によって発生するキャリアの軌道角運動量（OAM）ダイナミクス観測を実現し、超伝導物性に関して次のことを明らかにした：(a) 超伝導の電子対もしくは準粒子は光渦のOAMを受け取り、そのカイラリティに依存した過渡反射率応答変化が現れる；(b) 超伝導相回復応答で観測されるクーパー対の初期形成時には特徴的な量子ゆらぎ応答が存在する；(c) OAM応答の温度依存性はゆらぎ応答を反映しないことから、光渦パルス励起は巨視的量子相関に起因するコヒーレントな空間変化を超伝導に誘起している。
- 光の軌道角運動量を用いた巨視的量子相関ダイナミクス探索日本学術振興会：基盤研究(B)研究期間 : 2016年 -2018年代表者 : 戸田泰則
- 日本学術振興会：科学研究費助成事業 挑戦的萌芽研究研究期間 : 2015年04月 -2017年03月代表者 : 戸田 泰則本研究ではビーム断面に偏光の定まらない特異点を有する偏光渦パルスを利用した新規コヒーレント分光（縮退四光波混合分光と呼ばれる非線形レーザー分光を用いた時間分解ポラリメトリ）を開発し、半導体GaN励起子（電子-正孔対）に対して次の項目に関する知見を得ることに成功した：(a）励起子交換相互作用と歪エネルギーの定量的瞬時評価、特に等方性試料に残留する微小な異方性の高感度かつ高精度な評価、(b)励起子の対称性破れを反映した偏光特異点ダイナミクス、(c)スピンおよび擬スピン応答およびスピン相関ダイナミクス
- 日本学術振興会：科学研究費助成事業 基盤研究(B)研究期間 : 2014年04月 -2017年03月代表者 : 森田 隆二, 戸田 泰則モノサイクル域で軌道角運動量可変な高強度超短光渦パルス（パルスエネルギー 1 ｍJ超，パルス幅 27fs）の発生に成功し，また， 2つの直交偏光光渦パルスをコヒーレントビーム結合させることにより，超広帯域偏光渦パルス（波長帯域750-880 nm）の生成に成功した。さらに，超広帯域偏光渦パルスの偏光対称性，空間的偏光純度に関する精密測定法の開発を行い，その有効性を実証した。また，径偏光超短パルスによる半導体のスナップショット偏光分光実験を行い，この手法の高精度・高感度特性を実証した。
- 日本学術振興会：科学研究費助成事業 基盤研究(B)研究期間 : 2013年04月 -2017年03月代表者 : 荒巻 光利, 戸田 泰則, 吉村 信次, 寺坂 健一郎, 伊藤 清一光渦は近軸ヘルムホルツ方程式の円筒座標系の解であり，らせん状の等位相面を持つ．光渦が3次元のらせん状の等位相面を持つため，光渦中の観測者は3次元のドップラー効果を感じる．本研究では，光渦の特異なドップラー効果を利用して，ビームを横切る方向の流れに感度がある分光法を開発した．この光渦分光法により，ビームを横切るガス流を，光渦のドップラー吸収スペクトルおよび飽和吸収分光のLamb dipの横ドップラーシフトとして検出することに成功した． 横ドップラーシフトの位相特異点からの距離に対する依存性は定性的に理論と一致したが，定量的な評価には，横ドップラーシフト検出の更なる精度向上が必要となる．
- 偏光渦パルスを用いた時間分解コヒーレントポラリメトリ日本学術振興会：挑戦的萌芽研究研究期間 : 2015年 -2016年代表者 : 戸田泰則
- 日本学術振興会：科学研究費助成事業 挑戦的萌芽研究研究期間 : 2013年04月 -2015年03月代表者 : 戸田 泰則, 足立 智, 中山 正昭半導体励起子を対象とした時空間四光波混合(FWM)分光を(A)可変制御ラゲールガウス(LG)パルスと(B)高精度軌道角運動量(OAM)分解分光を用いて実現した。励起子OAMダイナミクスから(1)ほぼ完全な励起子重心運動へのOAM変換が可能、(2)OAM変換の不完全性はOAMが0の成分を含むとき顕著になること、(3)バルク試料においてOAM緩和は位相緩和の空間均一性を反映して著しく長くなることを明らかにした。さらにトポロジカルチャージ依存性からロバスト性が示唆された。トポロジカル変換の応用として、面発光レーザーの空間変調光帰還を実施し、横モードに対して高い選択性を持つOAM制御に成功した。
- 日本学術振興会：科学研究費助成事業 基盤研究(B)研究期間 : 2012年04月 -2015年03月代表者 : 戸田 泰則, 小田 研ビスマス系高温超伝導体における超伝導と擬ギャップの光誘起相転移ダイナミクスを超短パルス光を用いて調査し、1)擬ギャップが自発的対称性破れを伴うこと、2)超伝導ギャップ形成と位相コヒーレンスの関係性、3)擬ギャップ形成ダイナミクスが長距離秩序を持たない局在状態を反映すること、4)アンダードープ領域において超伝導秩序形成における擬ギャップの相関が強まることを明らかにした。
- 日本学術振興会：科学研究費助成事業 基盤研究(B)研究期間 : 2011年04月 -2014年03月代表者 : 森田 隆二, 戸田 泰則サイクル域の光渦パルスの発生，ならびに軌道角運動量可変な超mJ高強度・27 fs超短光渦パルス発生に初めて成功し，また超広帯域光渦パルスの高精度軌道角運動量スペクトル準実時間測定を実証した。さらに，光渦を用いたポンプ・プローブ閉ループ時間分解非線型励起分光法を開発し，GaN励起子の軌道角運動量の緩和時間を評価した。その他，スピン角運動量および軌道角運動量を制御した光渦パルスを用いて，ナノサイズの金属螺旋ニードル構造を作製することにも初めて成功した。
- トポロジカル変換を用いた縮退四光波混合分光の多次元化日本学術振興会：挑戦的萌芽研究研究期間 : 2013年 -2014年代表者 : 戸田泰則
- ビスマス系高温超伝導体における光誘起超伝導相転移ダイナミクスの研究日本学術振興会：基盤研究(B)研究期間 : 2012年 -2014年代表者 : 戸田泰則
- 日本学術振興会：科学研究費助成事業 基盤研究(C)研究期間 : 2010年 -2012年代表者 : 小田 研, 戸田 泰則, 伊土 政幸本研究では、銅酸化物高温超伝導体に特徴的な擬ギャップを伴った電子系電荷秩序の起源と高温超伝導との関係を明らかにするために、その代表物質の一つであるBi2212で磁場中STM/STS実験とポンプ・プローブ分光実験を行った。STM/STS実験では、Bi2212の電荷秩序が、電荷とスピンのストライプに由来すること、また、バルクの性質としては揺らいだ状態にあることを示唆する結果が得られた。ポンプ・プローブ分光実験では、擬ギャップ・電荷秩序は、超伝導と単なる競合関係にあるのではなく、超伝導の発現に必要な電子系の性質であることが明らかとなった。これらの結果を考え合わせると、ストライプ秩序の揺らぎが高温超伝導の機構に深く関っている可能性があると結論できる。
- 時空間制御光波を用いた電子相関物質における巨視的量子コヒーレンスの探索研究日本学術振興会：優秀若手研究者海外派遣事業研究期間 : 2010年03月 -2011年01月代表者 : 戸田泰則
- 日本学術振興会：科学研究費助成事業 新学術領域研究(研究課題提案型)研究期間 : 2009年 -2011年代表者 : 尾上 順, 島 弘幸, 戸田 泰則1次元金属ピーナッツ型凹凸周期フラーレンポリマーの伝導電子の状態を光電子分光で調べた結果,リーマン幾何学効果を取り入れた理論予測を見事に再現した。この成果は,1次元電子状態が純粋に凹凸曲面曲率(リーマン幾何学)に影響を受け,凹凸周期曲面上に沿って電子が動いていることを初めて実証したもので,過去の研究ではアインシュタインにより予測された光の重力レンズ効果(曲がった空間を光子が動く)以外に観測例はなく,電子系では調べる限りこれが初めてである。
- 日本学術振興会：科学研究費助成事業 基盤研究(B)研究期間 : 2008年 -2010年代表者 : 森田 隆二, 戸田 泰則表面相互作用の異なる金属基板上での光異性化前後アゾベンゼン単分子について,走査トンネル顕微鏡による観測を行い,表面相互作用の強い基板上では平面構造のトランス体が存在し,この場合,異性化過程において反転機構が支配的であることを見出した。また,光異性化,分子コンフォメーション変化を促す新たな光源として,時間・周波数領域極限位相・振幅制御だけでなく,ビーム内空間位相・偏光の制御を行った光波の高効率発生に成功,それを用いたリング状非線型分光法を確立し,さらには金属との相互作用実験により,光の全角運動量の効果を初めて明らかにした。
- 日本学術振興会：科学研究費助成事業 基盤研究(B)研究期間 : 2005年 -2007年代表者 : 足立 智, 戸田 泰則, 武藤 俊一本研究ではTHz光またはレーザー光を用いて,磁場中におかれた結合量子ドットのゼーマン分離した電子準位間でのスピン転送技術を確立することを目的としている.そのために以下の2つの手法の比較を実験的におこなった;テラヘルツ光を使用する手法と連続準位を介した2光子過程を用いる方法を試み,後者の手法で良好な結果を得た.単一量子ドットの磁場下でゼーマン分裂した2つの励起子準位(スピンアップとスピンダウン)の間を量子ドットの連続準位の裾を中間準位とした系において,各励起子準位からの発光をモニターすることにより,連続状態のコヒーレント励起による励起子スピン状態の制御が可能であることを実証し,論文化した.時間差をつけたダブルパルスによる同軸励起を用いることにより,パルス時間差の関数として励起子スピンの同位相および逆位相での振動の観測に成功したが,逆位相で振動する場合には,発光強度の振動だけでなく,ゼーマン分裂エネルギーの振動も観測された.これはゼーマン分裂した励起子準位の占有確率が逆位相で振動する場合,励起子スピン偏極度の振動となるため量子ドットの核スピンが偏極され,電子が感じる実効的な磁場強度が核磁場により変調されたためと考えられる.この結果により,量子ドットの電子スピンと核スピンのコントロールに,連続状態をラムダ型遷移の中間準位として使用可能であることが初めて示しめされた.また電子スピン-核スピン間の磁気的相互作用を詳細に調べ,核スピン分極の励起強度・励起偏光・外部磁場依存性に双安定領域が現れることをInAlAs単一量子ドットで初めて明らかにした.この成果により外部磁場を完全にキャンセルする核磁場が双安定領域の一端で自動的に形成されていることが判り,電子の実効g因子=0を達成するとともに,新たなg因子制御法を確立した.
- 日本学術振興会：科学研究費助成事業 萌芽研究研究期間 : 2006年 -2006年代表者 : 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。
- 日本学術振興会：科学研究費助成事業 基盤研究(B)研究期間 : 2004年 -2006年代表者 : 戸田 泰則, 足立 智, 荒川 泰彦半導体では歪や欠陥がデバイス性能に直結するため、その影響はこれまで広く研究されてきた。格子欠陥は結晶成長条件や基板、構造に応じて3次元的にスケールの異なる様々な種類が存在する。またヘテロエピタキシャル薄膜で重要となる歪も、結晶内部の局所的な効果をもち、不純物や欠陥とも関連する。したがって歪や欠陥の影響を調べるためには、より詳細な、例えば3次元ナノイスケールメージング等の新しい測定技術が必要とされている。本研究では3次元イメージングを実現するため、非線形光学遷移の利用に着目し、i)バンド端以下の光子エネルギーを利用した2光子吸収(TPA)による3次元イメージングとii)励起子非線形性を用いた歪の高感度検出を実現した。これらの研究成果はGaNだけでなくナノ構造を含む幅広いデバイス物質に応用可能であり、高機能物性評価手法として今後の展開が期待できる。TPAイメージングでは非線形性を利用した高空間分解能をもつ欠陥分布の3次元イメージングに成功した。超短パルスを用いたポンププローブ分光をベースにしているため、デバイス性能評価指数となる二光子吸収係数の情報を取得可能である。またこの吸収係数の分布をもとに、GaN薄膜中では局所的な高密度欠陥準位が存在し、著しく高い吸収係数を示すことを明らかにした。別の縮退FWMをベースとした非線形分光では、高感度歪計測に成功した。電子-正孔対(励起子)の分極回折格子を利用すると、三次の非線形効果により汎用光学評価法と比べて累乗倍の異方性増強が実現される。そのため結晶に内在する不純物や欠陥にもとづく歪の高感度測定が可能となる。複数のGaN試料を用いて装置の性能評価を実施し、分裂エネルギーと強度比から算出される歪と応力を見積もった結果、X線解析装置と同等の歪解析能力を達成していることを明らかにできた。
- 日本学術振興会：科学研究費助成事業 基盤研究(B)研究期間 : 2004年 -2006年代表者 : 森田 隆二, 郷原 一寿, 戸田 泰則1.自己位相変調効果を用いて、1オクターブを超える超広帯域スペクトル(460-1060nm)を有する光パルスを発生し、独自に作製した、変形スペクトル干渉位相電場再生システムおよびフィードバック位相補償システムによるフィードバック位相補償を行うことにより、2.8fs,1.5サイクルのフーリエ変換限界光パルスの発生に成功した。これは、スペクトル位相が決定された可視・赤外領域における世界最短パルスである。 2.生体物質の3次元空間分解観測に適用可能な光第2高調波レーザー顕微鏡を開発し、ラット胎児大脳皮質由来の神経細胞の観察を行った。細胞体の空間反転対称性の破れを反映した信号を観測することにより、核、軸索からの強い信号の観測に成功している。現在、空間分解能は、1.5μm程度である。 3.フェムト秒光パルスレーザー増幅器を光源としたコヒーレント反ストークスラマン散乱レーザー顕微鏡の開発を行った。波長800nmと1040nmの光を用いて、650nmのCARS信号光を検出することにより、タマネギ表皮細胞の2900cm^<-1>のC-H伸縮振動、波長800nmと1100nmの光を用いて、630nmのCARS信号光を検出することにより3400cm^<-1>のN-H伸縮振動を空間分解観測した。その結果を比較することにより、C-Hリッチ、N-Hリッチな領域のマッピングを行った。 4.メチルレッドをインターカレートしたDNAの二重鎖の結合、解離を光第2高調波顕微鏡のようなマクロな測定により観測することを試みた。そのメカニズムは、結合している場合二重鎖は反転対称性を持ち、解離している場合反転対称性が破れ、光第2高調波活性になることによる。現在、信号レベルと雑音レベルが同程度で有意な結果が得られていないが、非線形性の大きなアゾ色素を用いることにより、信号強度の向上を図っている段階である。
- 日本学術振興会：科学研究費助成事業研究期間 : 2002年 -2005年代表者 : 武藤 俊一, 足立 智, 戸田 泰則, 白峰 賢一, 陽 完治量子ドットの電子スピンは将来の大規模量子コンピューティング実現へ向けての有力候補の1つである。報告者らはすでに量子ドットの電子スピンの光支援トンネルとそのクーロン・ブロッケイドを用いた普遍量子ゲートを提案している。この提案の妥当性を検証するための実験的検討を行った。また量子コンピューティングは量子情報光通信との整合性が必須と考えられる。そこで電子スピンと光子の間の量子ビット変換の手法を提案し、その実験的検討を行った。 実験に先立ち、普遍量子ゲートの計算機シミュレーションを行った。この結果、100ps程度のスピンコヒーレンスが有ればCNOTの実験的検証が可能であることが分かった。 次に、電子スピンのコヒーレンスを実験的に確認するため、4波混合によるスピン緩和時間の測定技術を立ち上げた。まず、ポンプ・プローブ法によるスピン緩和測定法が確立している量子井戸の電子スピンについて、スピン回折4波混合を適用し、従来のポンプ・プローブ法と矛盾しない結論を得られることを確認した。次にこの方法をInAlAs/AlGaAs量子ドットに適用、最長で7nsにおよぶ緩和時間を観測した。これは従来、量子ドットに対して便宜的に用いられた非共鳴的観測であるストリーク・カメラによるものよりも数倍長く、4波混合のような共鳴的な観測の必要を明示したものでもある。 量子ビット変換の新しい方法として核磁場による縮退した電子準位とエネルギーの異なる正孔準位の形成を提案した。実際にInAlAs/AlGaAs量子ドットにおいて核磁場の形成実験を行ったところ、量子ビット変換に必要なバンド構造が形成可能であることが分かった。
- 日本学術振興会：科学研究費助成事業研究期間 : 2002年 -2003年代表者 : 武藤 俊一, 白峰 賢一, 戸田 泰則, 足立 智, 竹内 淳15層積層したInAlAs/AlGaAs量子ドットを試作、非共鳴励起下でのスピン緩和測定を試みた。実験は発振波長780nm、パルス幅80fsのチタンサファイアレーザーを励起パルス光源とし、右回り円偏光(σ+)パルスで試料を励起し、その発光のσ+成分と左回り円偏光(σ-)成分を分けてストリークスコープを用いて時間分解発光測定した。分光器の入射スリット手前に1/4波長板と偏光子を置き、発光を一度直線偏光に変換し、偏光子により水平および垂直直線偏光発光成分を分けて測定することで発光の偏光を識別した。試料は、10Kに冷却した。励起エネルギーと検出エネルギーとの差はおよそ100meVである。直線偏光励起での時間変化は単一指数関数で良くフィッティングでき、励起子の寿命(再結合時間)は約1.18nsと見積ることができた。σ+成分とσ-成分との差の減衰はスピン緩和時間をτ_sとするとexp(-2t/τ_s)で与えられる。これから得られたスピン緩和時間τ_sは1.31nsとなり励起子再結合時間と同程度となることが分かった。2次元量子井戸に比べて非常に長いスピン緩和時間となっており、井戸で有効なスピン緩和機構が量子ドットでは消失したと解釈できる。量子ドットが電子スピンのコヒーレント制御に好適な材料であることが確認できた。 次世代のスピントロニクス材料としてワイドギャップ半導体およびポリフェニルアセチレン(PPA)の磁気的性質を調べた。PPAではスピングラス的な磁気的性質に加えて、磁場-磁化(M-H)曲線が閉じないことなど極めて興味深いスピン物性を示すことが明らかとなった。
- 日本学術振興会：科学研究費助成事業研究期間 : 2001年 -2002年代表者 : 足立 智, 田中 克彦, 戸田 泰則本研究は光学フォノンと音響フォノン(超音波)の可視化技術を利用し、その伝播過程を映像化することにより、ドメイン反転前の基板結晶内の欠陥等による散乱の検出やドメイン反転終了後の結晶評価を目的としている。誘電体タンタル酸リチウムLiTaO3を用いて、c面より光を入射し光学フォノン発生・伝播過程のイメージングを行った。数ミリから1センチ程度の直径のレーザーパルスを時間遅れをつけて照射し、フォノン波束による回折光で波束の像を形成できる。フォノンは屈折率のみを変化させている位相物体であるため、光強度にしか反応しない目やCCDカメラ等では見えないが、位相情報を振幅情報に変換する工夫をしてやると発生から伝搬、干渉などの過程をテレビを見るように画像化することができる。さらに考案した特殊な光学配置において、第1パルス対で選択励起したフォノン(ポラリトン)を第2パルスで増幅したり、強制的に消滅させたりする過程をリアルタイムで画像化ことに成功した。すなわち時間遅れを伴った光パルスを用いて、フォノンどうしの干渉および光パルス間の時間差による位相の違いを利用してフォノン制御が可能であることを示したことになる。得られた画像データをフーリエ変換することにより、励起されたフォノンがどの程度の波数をもち、伝播過程でどのように分散・減衰していくかがわかる。実験ではフォノン波束が伝播中に分散(伝播速度の波数依存性)するため、完全に波束を消去することは出来ていないが、第2パルス対に用いるレーザーパルスの形を工夫することにより、励起したフォノンを完全にうち消すような干渉を起こすことが可能であると考えられる。この成果はフォノン分光学に大きく貢献するだけでなく、フォノンのコヒーレント制御に新しい画期的な方向性を与えるものと考えられる。また同じ四光波混合法を用いて、半導体GaNにおいてフォノン散乱、励起子間相互作用に着目して半導体結晶の品質を評価するとともに、その過程で新たな励起子分子を発見した。
- 日本学術振興会：科学研究費助成事業研究期間 : 2000年 -2001年代表者 : 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。
- 日本学術振興会：科学研究費助成事業研究期間 : 2000年 -2001年代表者 : 戸田 泰則本研究の目標は、走査型近接場光学顕微鏡を用いた単一量子ドット(QD)分光を通してQDの新しい物性を探索し、これを時空間に制御された近接場光を使った量子状態制御へと発展させることにある。昨年度は共鳴の位相緩和の測定と、近接共鳴を利用した量子干渉の生成と観測を行った。本年度は共鳴の均一幅に関して、緩和メカニズムの解明を目的とした実験を行った。 自己形成量子ドット(SAQD)の大きな特徴のひとつは、広いエネルギー間隔を持つ離散的なエネルギー構造である。このエネルギー構造は、音響フォノンを介したキャリア緩和を、高次元構造などに比べて著しく減少させる。その結果、量子情報処理への応用に適した長いコヒーレンスを実現することが期待される。他方、SAQDのバンド間エネルギー差は光学フォノンのエネルギーよりも広いため、発光励起スペクトル(PLE)に現れる零吸収域には、出射光に共鳴した光学フォノンラマン線が観測される。このことはSAQDのキャリア緩和に光学フォノン緩和が存在することを示している。すなわちQDにおける次元低下に伴う緩和経路の減少が、キャリアと光学フォノンの相互作用を顕著にする。さらに2LOのエネルギー領域では、光学フォノンと同時に励起準位も存在していると考えられるため、多数の緩和過程が存在する。本年度の研究では、これら2LOの励起共鳴が高速のフォノン緩和過程を含むことを明らかにした。観測される高速フォノン緩和は励起子基底準位発光よりも速いため、低い励起強度で効率的な励起子分子生成を可能にする。この様子は励起強度を変化させるときに励起子共鳴に見られるピークからディップへの変化から確認される。さらにディップは広い線幅を持つピークの中心に現れ、単一QDのPLE共鳴が不均一広がりを含むことが確認された。すなわち高速な光学フォノン緩和経路と結合した緩和経路が存在し、これが単一QDのPLE共鳴に不均一広がりを与えている。したがってフォノンとの相互作用を低減することにより、励起準位の共鳴が得られる。二色励起のスペクトルでは、フォノン共鳴の抑圧と不均一広がりの除去された励起準位に対応する共鳴ピークの存在が明らかとなった。