Details of a Researcher - Watanabe, Shinichi

Papers - Watanabe, Shinichi

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Fluctuations in Spin Dynamics Excited by Pulsed Light

T. Sato, S. Watanabe, M. Matsuo, and T. Kato

Phys. Rev. Lett. 134 ( 10 ) 106702 2025.03

Research paper (scientific journal), Joint Work, Accepted

We theoretically investigate nonequilibrium spin fluctuations in a ferromagnet induced by a light pulse. Using a Lindblad equation consistent with the Landau-Lifshitz-Gilbert equation, we compute the autocorrelation function of magnetization. Our analysis reveals that this function comprises both thermal and nonequilibrium components. To examine the latter in detail, we introduce a Fano factor similar to nonequilibrium current noise in electronic circuits. We demonstrate that this factor encapsulates insights into the transfer of spin units to the environment. Our findings lay the groundwork for nonequilibrium spin noise spectroscopy, offering valuable insights into spin relaxation dynamics.

Ultrafast movies of the photo-induced phonon and magnon propagation using dual frequency-comb technology

D. Nishikawa, K. Maezawa, R. Shibata, S. Fujii, and S. Watanabe

Opt. Lett. 50 ( 6 ) 1929 - 1932 2025.03

Research paper (scientific journal), Joint Work, Last author, Corresponding author, Accepted

Gyro-spintronic material science using vorticity gradient in solids

Y. Nozaki, H. Sukegawa, S. Watanabe, S. Yunoki, T. Horaguchi, H. Nakayama, K. Yamanoi, Z. Wen, C. He, J. Song, T. Ohkubo, S. Mitani, K. Maezawa, D. Nishikawa, S. Fujii, M. Matsuo, J. Fujimoto, and S. Maekawa

Science and Technology of Advanced Materials (STAM) 26 ( 1 ) 2428153 2025.02

Joint Work, Accepted

We present a novel method for generating spin currents using the gyromagnetic effect, a phenomenon discovered over a century ago. This effect, crucial for understanding the origins of magnetism, enables the coupling between various macroscopic rotational motions and electron spins. While higher rotational speeds intensify the effect, conventional mechanical rotations, typically, below 10,000 RPM, produce negligible results comparable to geomagnetic fluctuations, limiting applied research. Our studies demonstrate that spin current generation comparable to that of rare metals can be achieved through atomic rotations induced by GHz-range surface acoustic waves and the rotational motion of conduction electrons in metallic thin films with nanoscale gradient modulation of electrical conductivity. These effects, termed the acoustic gyromagnetic effect and the current-vorticity gyromagnetic effect, are significant in different contexts. The acoustic gyromagnetic effect is notable in high-conductivity materials like aluminum and copper, which are more abundant than conventional spintronics materials with strong spin–orbit interactions (SOIs). Conversely, the current-vorticity gyromagnetic effect requires a large conductivity gradient to produce current vorticity efficiently. This is achieved by using composition gradient structures from highly conductive metals to poorly conductive oxides or semiconductors. Consequently, unlike traditional strong-SOI materials, we can create highly efficient spin current generators with low energy dissipation due to reduced Joule loss.

Architecture for coherent dual-comb spectroscopy and low-noise photonic microwave generation using mechanically actuated soliton microcombs

T. Murakami, K. Wada, S. Kogure, R. Takabayashi, L. Yang, R. Shibata, H. Kumazaki, S. Watanabe, A. Ishizawa, T. Tanabe, and S. Fujii

Opt. Lett. 50 ( 4 ) 1417 - 1420 2025.02

Research paper (scientific journal), Joint Work, Accepted

Integer-locking condition for stable dual-comb interferometry in situations with fluctuating frequency-comb repetition rates.

R. Shibata, S. Fujii, and S. Watanabe

Opt. Express 32 ( 10 ) 17373 - 17387 2024.04

Research paper (scientific journal), Joint Work, Last author, Corresponding author, Accepted

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Spatiotemporal visualization of a surface acoustic wave coupled to magnons across a submillimeter-long sample by pulsed laser interferometry.

K. Maezawa, S. Fujii, K. Yamanoi, Y. Nozaki, and S. Watanabe

Phys. Rev. Applied 21 ( 4 ) 044047 2024.04

Research paper (scientific journal), Joint Work, Last author, Corresponding author, Accepted

Surface acoustic waves (SAWs) coupled to magnons have attracted much attention because they allow for the long-range transport of magnetic information that cannot be achieved by magnon alone. We employed pulsed laser interferometry to visualize the entire spatiotemporal dynamics of a SAW that travels on a nickel (Ni) thin film and is coupled to magnons. It was possible to trace the coupling-induced amplitude reduction and phase shift that occurs as the SAW propagates over a distance of 0.4 mm. The observed changes are consistent with results obtained from conventional radio-frequency transmission measurements, which probe the total SAW absorption due to magnon-phonon coupling. This result verifies that our method can accurately capture the spatiotemporal dynamics of a SAW coupled to magnons across the entire length of the sample. Additionally, we validated our time-resolved profiles by comparing them with theoretical results that take the echo wave due to reflection into account. The impact of the echo wave is significant even when it has propagated over a distance of the order of millimeters. Our imaging results highlight the visualization of the long-range propagation of the SAW coupled to magnons and offer more information about the surface vibration profiles in such devices.

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A two-color dual-comb system for time-resolved measurements of ultrafast magnetization dynamics using triggerless asynchronous optical sampling

(D. Nishikawa, K. Maezawa, S. Fujii, M. Okano, S. Watanabe) 94 063003 2023.06

Research paper (scientific journal), Joint Work, Accepted

Temporal-offset dual-comb vibrometer with picometer axial precision

A. Iwasaki, D. Nishikawa, M. Okano, S. Tateno, K. Yamanoi, Y. Nozaki, S. Watanabe

APL Photonics 7 ( 10 ) 106101 2022.10

Research paper (scientific journal), Joint Work, Last author, Corresponding author, Accepted

We demonstrate a dual-comb vibrometer where the pulses of one frequency-comb
are split into pulse pairs. We introduce a delay between the two pulses of each
pulse pair in front of the sample, and after the corresponding two consecutive
reflections at the vibrating sample surface, the initially introduced delay is
cancelled by a modified Sagnac geometry. The remaining phase difference between
the two pulses corresponds to the change in the axial position of the surface
during the two consecutive reflections. The Sagnac geometry reduces the effect
of phase jitter since both pulses propagate through nearly the same optical
path (in opposite directions), and spurious signals are eliminated by time
gating. We determine the amplitude of a surface vibration on a
surface-acoustic-wave device with an axial precision of 4 pm. This technique
enables highly accurate determination of extremely small displacements.

Triggerless data acquisition in asynchronous optical-sampling terahertz time-domain spectroscopy based on dual-comb system

M. Okano, and S. Watanabe

Opt. Express 30 ( 22 ) 39613 - 39623 2022.10

Research paper (scientific journal), Joint Work, Last author, Accepted

Polarization-sensitive terahertz time-domain spectroscopy system without mechanical moving parts

M. Nakagawa, M. Okano, and S. Watanabe

Opt. Express 30 ( 16 ) 29421 - 29434 2022.07

Research paper (scientific journal), Joint Work, Last author, Corresponding author, Accepted

Strain-induced irreversible change of the conductive network in a rubber/carbon-black composite revealed by polarization-resolved terahertz dielectric spectroscopy

K. Mizuta, M. Okano, T. Morimoto, S. Ata, and S. Watanabe

Appl. Phys. Lett. 121 ( 2 ) 024101 2022.07

Research paper (scientific journal), Joint Work, Last author, Accepted

Ultra-precise determination of thicknesses and refractive indices of optically thick dispersive materials by dual-comb spectroscopy.

Kana A Sumihara, Sho Okubo, Makoto Okano, Hajime Inaba, Shinichi Watanabe

Optics express (Optics Express) 30 ( 2 ) 2734 - 2747 2022.01

Precise measurements of the geometrical thickness of a sample and its refractive index are important for materials science, engineering, and medical diagnosis. Among the possible non-contact evaluation methods, optical interferometric techniques possess the potential of providing superior resolution. However, in the optical frequency region, the ambiguity in the absolute phase-shift makes it difficult to measure these parameters of optically thick dispersive materials with sufficient resolution. Here, we demonstrate that dual frequency-comb spectroscopy can be used to precisely determine the absolute sample-induced phase-shift by analyzing the data smoothness. This method enables simultaneous determination of the geometrical thickness and the refractive index of a planar sample with a precision of five and a half digits. The thickness and the refractive index at 193.414 THz (λ = 1550 nm) of a silicon wafer determined by this method are 0.5204737(19) mm and 3.475625(58), respectively, without any prior knowledge of the refractive index.

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Interferogram-based determination of the absolute mode numbers of optical frequency combs in dual-comb spectroscopy

Fukuda T, Okano M, Watanabe S

Optics Express (Optics Express) 29 ( 14 ) 22214 - 22227 2021.07

Dual-comb spectroscopy (DCS), which uses two optical frequency combs (OFCs), requires an accurate knowledge of the mode number of each comb line to determine spectral features. We demonstrate a fast evaluation method of the absolute mode numbers of both OFCs used in DCS system. By measuring the interval between the peaks in the time-domain interferogram, it is possible to accurately determine the ratio of one OFC repetition frequency (frep) to the difference between the frep values of the two OFCs (∆frep). The absolute mode numbers can then be straightforwardly calculated using this ratio. This method is applicable to a broad range of ∆frep values down to several Hz without any additional instruments. For instance, the minimum required measurement time is estimated to be about 1 s for ∆frep ≈ 5.6 Hz and frep ≈ 60 MHz. The optical frequencies of the absorption lines of acetylene gas obtained by DCS with our method of mode number determination shows good agreement with the data from the HITRAN database.

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Ultra-Precise Complex Refractive Index Measurement Using Dual-Comb Spectroscopy

Sumihara K.A., Okubo S., Okano M., Inaba H., Watanabe S.

2021 Conference on Lasers and Electro-Optics, CLEO 2021 - Proceedings (2021 Conference on Lasers and Electro-Optics, CLEO 2021 - Proceedings) 2021.05

ISSN 9781943580910

Ultra-precise complex refractive index measurement using dual-comb spectroscopy

Sumihara K.A., Okubo S., Okano M., Inaba H., Watanabe S.

Optics InfoBase Conference Papers (Optics InfoBase Conference Papers) 2021

ISSN 9781557528209

Deformation-Induced Structural Changes of Carbon Black Fillers in Rubbers Investigated by Terahertz Polarization Spectroscopy

Mizuta K., Okano M., Morimoto T., Ata S., Watanabe S.

International Conference on Infrared, Millimeter, and Terahertz Waves, IRMMW-THz (International Conference on Infrared, Millimeter, and Terahertz Waves, IRMMW-THz) 2021-August 2021

ISSN 21622027

We investigated the polarization-dependent terahertz optical conductivity of rubber compounds with carbon black (CB) fillers under cyclic deformation to detect deformation-induced changes of the internal CB structure in a contactless manner. It is clearly observed that the conductivity during the unloading is reduced from that during the loading at the same draw ratios. To interpret this hysteresis of the conductivity, we perform theoretical calculation based on the effective medium theory. Comparing with the calculation, we attribute the hysteresis of the conductivity to the reduction of the anisotropy of the CB aggregates, which might result from the deformation-induced rupture of them.

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Coherent Control of Higher-Order Spin Precession Modes in Ferromagnetic Permalloy Thin Films by Double Pulse Excitation

Okano M., Takahashi T., Watanabe S.

Conference Proceedings - Lasers and Electro-Optics Society Annual Meeting-LEOS (Conference Proceedings - Lasers and Electro-Optics Society Annual Meeting-LEOS) 2020-May 2020.05

ISSN 9781943580767

Terahertz time-domain polarimetry (THz-TDP) based on the spinning E-O sampling technique: determination of precision and calibration

K. Xu, E. Bayati, K. Oguchi, S. Watanabe, D. P. Winebrenner, and M. H. Arbab

Opt. Express (Optics Express) 28 ( 9 ) 13482 - 13496 2020.04

Research paper (scientific journal), Joint Work, Accepted

© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement We have developed a terahertz time-domain polarimetry (THz-TDP) system by applying frequency modulation to electro-optic sampling detection in a nonlinear crystal. We characterized the precision of this system in determining the polarization angles to be 1.3° for fixed time delay, and 0.5° for complete time-domain waveform. Furthermore, we calculated the Jones matrix of the optical components used for beam propagation to calibrate the induced systematic error. The advantages of employing this calibration approach are demonstrated on a sapphire crystal investigated at different sample test positions in transmission configuration, and using high resistivity Si, AlN and quartz in reflection geometry. The new THz-TDP technique has the advantage of not using any external polarizers, and therefore is not constrained by their optical performance limitations, such as restricted bandwidths and frequency-dependent extinction ratio. Finally, the THz-TDP technique can be easily implemented on existing time-domain spectroscopy (TDS) systems.

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Evaluation of crystallinity and hydrogen bond formation in stereocomplex poly(lactic acid) films by terahertz time-domain spectroscopy

J. Hirata, N. Kurokawa, M. Okano, A. Hotta, and S. Watanabe

Macromolecules (Macromolecules) 53 ( 16 ) 7171 - 7177 2020

Research paper (scientific journal), Joint Work, Accepted, ISSN 00249297

Copyright © 2020 American Chemical Society. Stereocomplex poly(lactic acid) (sc-PLA) crystallites exhibit higher mechanical strength and thermal stability than PLA homocrystallites. In sc-PLA, a hydrogen bond that governs the physical properties is formed between two enantiomeric PLA chains during crystallization. Therefore, the investigation of this hydrogen bond is essential for a better understanding of the properties of sc-PLA. Here, we report on the terahertz spectroscopy of sc-PLA films with low crystallinities. The dielectric function in the terahertz regime allows us to estimate the crystallinity of sc-PLA because various intra- and intermolecular vibration modes lie in the terahertz frequency range. By comparison with X-ray diffraction measurements, the validity of the crystallinity evaluation based on terahertz spectroscopy is verified. Furthermore, a remarkable red-shift of the peak in the imaginary part of the dielectric function occurs as the crystallinity increases. By comparison with Fourier transform infrared spectroscopy, it is considered that this red-shift is caused by the anharmonic nature of the vibrational potential corresponding to the intermolecular libration mode of the sc-PLA crystallites. The hydrogen bond formation associated with the sc-PLA crystallization increases the anharmonicity of the vibrational potential of sc-PLA, and then the red-shift of the peak occurs. Our findings suggest that structural changes due to the stereocomplexation can be sensitively probed by terahertz spectroscopy.

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Ultrafast coherent control of higher-order spin waves in a nife thin film by double-pulse excitation

M. Okano, T. Takahashi, and S. Watanabe

Applied Physics Letters (Applied Physics Letters) 117 ( 8 ) 082407 - 082407 2020

Research paper (scientific journal), Joint Work, Accepted, ISSN 00036951

© 2020 Author(s). By using a double-pulse excitation scheme, we demonstrate the optical control of higher-order spin waves in a ferromagnetic permalloy thin film. Three spin precession modes are observed in the film under single pulse excitation. Based on a theoretical calculation, these spin precession modes are assigned to the fundamental Kittel mode and the first- and second-order perpendicular standing spin-wave (PSSW) modes. In order to excite the first-order PSSW mode selectively, we use double-pulse excitation at 400 nm. We show that, by choosing an appropriate time interval between the two pump pulses, it is possible to obtain a relatively large amplitude of the first-order PSSW mode while the amplitudes of the other two modes are almost completely suppressed. By analyzing the spin dynamics, it is found that the spin precession motion under the double-pulse excitation condition can be explained by the interference between the spin waves that have been induced by the first and second pulses. Our finding indicates that the selective excitation of a spin precession mode with a high precession frequency in a ferromagnetic metal may be realized by a multiple-pump pulse excitation scheme with more than two pulses.

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Coherent control of higher-order spin precession modes in ferromagnetic permalloy thin films by double pulse excitation

Okano M., Takahashi T., Watanabe S.

Optics InfoBase Conference Papers (Optics InfoBase Conference Papers) Part F182-CLEO-QELS 2020 2020

ISSN 9781557528209

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Cyclic-deformation-induced irreversible changes of terahertz optical conductivity in black rubber composites

Mizuta Keisuke, Okano Makoto, Takahiro Morimoto, Seisuke Ata, Shinichi Watanabe

Meeting Abstracts of the Physical Society of Japan (The Physical Society of Japan) 75.1 ( 0 ) 1421 - 1421 2020

Polarization-sensitive dual-comb spectroscopy with an electro-optic modulator for determination of anisotropic optical responses of materials

K. A. Sumihara, S. Okubo, K. Oguchi, M. Okano, H. Inaba, and S. Watanabe

Opt. Express (Optics Express) 27 ( 24 ) 35141 - 35165 2019.11

Research paper (scientific journal), Joint Work, Accepted

We propose and demonstrate a polarization-sensitive dual-comb spectroscopy (DCS) technique that employs an electro-optic modulator for determining the anisotropic optical responses of materials. This straightforward extension of the typical DCS setup directly provides amplitudes and phases in two mutually orthogonal directions of the electric field of light. Using this method, we determined the optic axis direction and the anisotropy in the complex refractive index of a sample whose optical parameter is well defined. We estimate a birefringence of the sample to be 5.49(55)×10-5 at a comb tooth in the 780 nm region.

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Real-Time Monitoring of Structural Changes in Poly(lactic acid) during Thermal Treatment by High-Speed Terahertz Time-Domain Spectroscopy For Nondestructive Inspection

M. Nakamura, M. Okano, and S. Watanabe

ACS Appl. Polym. Mater. (ACS Applied Polymer Materials) 1 ( 11 ) 3008 - 3016 2019.10

Research paper (scientific journal), Joint Work, Accepted

Thermally induced structural changes in polymeric materials are usually accompanied by a change of vital properties such as mechanical stability. We demonstrate an emerging technique for real-time monitoring of thermally induced structural changes of poly(lactic acid) (PLA). The technique is based on terahertz time-domain spectroscopy (THz-TDS) because spectra of the dielectric function of PLA in the terahertz frequency range reflect macroscopic molecular vibration modes which are strongly linked to the PLA morphology and structure. By comparison with conventional X-ray diffraction data, we confirm a strong correlation between the polymer structure and the spectra of the dielectric function. We achieve real-time monitoring of two essential thermally induced structural changes: the cold crystallization process from amorphous phase to the crystalline α-phase and the glass transition process. The proposed technique is useful for understanding the structural changes in polymeric materials induced by thermal treatments and holds promise for nondestructive inspection of polymeric products in a contactless manner.

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Development of polarization-sensitive dual-comb spectroscopy for anisotropic materials

Sumihara K.A., Okubo S., Oguchi K., Inaba H., Watanabe S.

2019 Conference on Lasers and Electro-Optics Europe and European Quantum Electronics Conference, CLEO/Europe-EQEC 2019 (2019 Conference on Lasers and Electro-Optics Europe and European Quantum Electronics Conference, CLEO/Europe-EQEC 2019) 2019.06

ISSN 9781728104690

© 2019 IEEE. Dual-comb spectroscopy (DCS) is a modern method using two frequency combs with slightly different repetition rates [1,2]. The DCS down-converts a material response in the optical frequency domain to a rf signal that is easy to handle. It provides significant advantages over conventional spectroscopy such as high spectral resolution, broad spectral range, frequency precision, short measurement time, and so on. The DCS has mainly been applied to precise molecular gas spectroscopy [3]. There are several attempts to expand its application to solid state physics. One of the applications is the determination of complex refractive index of materials [4]. As changes in both the amplitude and phase inside the material can be measured by DCS, one can determine both the real and imaginary part of the refractive index without utilizing the Kramers-Kronig relation. In addition, we consider physically interesting materials such as low-dimensional nanostructures and polymers with polarization-dependent complex refractive index (optical anisotropy). For investigating their physical property, it is important to add polarization sensitiveness to the DCS. So far, we have developed a polarization-sensitive (PS) DCS using a rotating compensator polarimetry [5].

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Spatio-temporal imaging of terahertz electric-field vector: Observation of polarization-dependent knife-edge diffraction

K. Suzuki, K. Oguchi, Y. Monnai, M. Okano, and S. Watanabe

Appl. Phys. Express 12 ( 5 ) 052010 2019.05

Research paper (scientific journal), Joint Work, Accepted

Spatio-temporal imaging of terahertz electric-field vectors: observation of polarization-dependent knife-edge diffraction

Suzuki Kenta, Oguchi Kenichi, Monnai Yasuaki, Okano Makoto, Watanabe Shinichi

Appl. Phys. Express (IOP Publishing) 12 ( 5 ) 052010 - 052010 2019.05

Research paper (scientific journal), Joint Work, Accepted, ISSN 18820778

© 2019 The Japan Society of Applied Physics. We develop a terahertz electric-field vector imaging system that uses the rotating polarizer technique. The imaging system is used to investigate the polarization-dependent optical response of a patterned indium tin oxide (ITO) film deposited on a glass substrate. While ITO is transparent for near-infrared probe light, it acts as a metal for terahertz light. Therefore, polarization-dependent diffraction can be observed at the edge of the ITO thin film, and only electric field components perpendicular to the edge boundary can propagate behind the ITO layer. By comparison with a numerical simulation, we reveal the polarization dependence of this so-called knife-edge diffraction.

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Polarization-sensitive electro-optic sampling of elliptically-polarized terahertz pulses: Theoretical description and experimental demonstration

K. Oguchi, M. Okano, and S. Watanabe

Particles (MDPI AG) 2 ( 1 ) 70 - 89 2019.01

Research paper (scientific journal), Joint Work, Accepted

We review our recent works on polarization-sensitive electro-optic (PS-EO) sampling, which is a method that allows us to measure elliptically-polarized terahertz time-domain waveforms without using wire-grid polarizers. Because of the phase mismatch between the employed probe pulse and the elliptically-polarized terahertz pulse that is to be analyzed, the probe pulse senses different terahertz electric-field (E-field) vectors during the propagation inside the EO crystal. To interpret the complex condition inside the EO crystal, we expressed the expected EO signal by “frequency-domain description” instead of relying on the conventional Pockels effect description. Using this approach, we derived two important conclusions: (i) the polarization state of each frequency component can be accurately measured, irrespective of the choice of the EO crystal because the relative amplitude and phase of the E-field of two mutually orthogonal directions are not affected by the phase mismatch; and, (ii) the time-domain waveform of the elliptically-polarized E-field vector can be retrieved by considering the phase mismatch, absorption, and the effect of the probe pulse width. We experimentally confirm the above two conclusions by using different EO crystals that are used for detection. This clarifies the validity of our theoretical analysis based on the frequency-domain description and the usefulness of PS-EO sampling.

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Terahertz Sensing of Anisotropy in Polymeric Materials

WATANABE Shinichi, OKANO Makoto, NAKASAKO Masayoshi

The Review of Laser Engineering (The Laser Society of Japan) 47 ( 1 ) 21 - 26 2019

Doctoral thesis, Joint Work, Accepted, ISSN 0387-0200

We investigated the structural anisotropy of poly(lactic acid) (PLA) film samples by polarizationsensitive(PS) terahertz time-domain spectroscopy (THz-TDS). The dielectric function of the PLA filmbecomes more anisotropic with the sample's increased draw ratio. This behavior is explained by theclassical Lorentz oscillator model that takes into account the orientation of PLA chains along thestretching direction. For further understanding of structural anisotropy in PLA, we performed twodimensional(2D) wide-angle X-ray scattering (WAXS) experiments and found that anisotropy isstrongly correlated to the orientation of the crystalline PLA chains. Our results suggest that PS THz-TDS is more suitable for examining the anisotropy of amorphous PLA chains.

Development of polarization-sensitive dual-comb spectroscopy for anisotropic materials

Sumihara K.A., Okubo S., Oguchi K., Inaba H., Watanabe S.

Optics InfoBase Conference Papers (Optics InfoBase Conference Papers) Part F143-EQEC 2019 2019

ISSN 9781557528209

© 2019 IEEE Dual-comb spectroscopy (DCS) is a modern method using two frequency combs with slightly different repetition rates [1,2]. The DCS down-converts a material response in the optical frequency domain to a rf signal that is easy to handle. It provides significant advantages over conventional spectroscopy such as high spectral resolution, broad spectral range, frequency precision, short measurement time, and so on. The DCS has mainly been applied to precise molecular gas spectroscopy [3]. There are several attempts to expand its application to solid state physics. One of the applications is the determination of complex refractive index of materials [4]. As changes in both the amplitude and phase inside the material can be measured by DCS, one can determine both the real and imaginary part of the refractive index without utilizing the Kramers-Kronig relation. In addition, we consider physically interesting materials such as low-dimensional nanostructures and polymers with polarization-dependent complex refractive index (optical anisotropy). For investigating their physical property, it is important to add polarization sensitiveness to the DCS. So far, we have developed a polarization-sensitive (PS) DCS using a rotating compensator polarimetry [5].

A nondestructive inspection method for black rubber using polarization-sensitive terahertz spectroscopy and its application

WATANABE Shinichi, OKANO Makoto

Oyo Buturi (The Japan Society of Applied Physics) 88 ( 2 ) 91 - 95 2019

Doctoral thesis, Joint Work, ISSN 0369-8009

We introduce polarization-sensitive (PS) terahertz spectroscopy as a novel tool for non-destructive inspection of black rubber. The PS terahertz optical response of black rubber is mostly determined by the concentration and orientation of the carbon black fillers inside the sample. Therefore, PS terahertz spectroscopy allows access to filler information inside the material. In addition, we can quantitatively estimate the internal strain of the sample through the filler-orientation information. Our analysis model based on a mixture of the polymer matrix and fillers can be applied not only to black rubber but also general polymer composite materials. Recently, rubber and polymer composite materials are widely used in our daily life, so our novel nondestructive inspection method is useful to test such materials.

Internal Status of Visibly Opaque Black Rubbers Investigated by Terahertz Polarization Spectroscopy: Fundamentals and Applications

M. Okano, and S. Watanabe

Polymers (Polymers) 11 ( 1 ) 9 2018.12

Research paper (scientific journal), Joint Work, Accepted, ISSN 2073-4360

© 2018 by the authors. We discuss the internal status of rubber composites consisting of an insulating rubber matrix and conductive carbon black (CB) fillers ("black rubber") using polarization-sensitive terahertz time-domain spectroscopy (THz-TDS). The black rubber composites under stretched conditions exhibit a large optical anisotropy or birefringence in the terahertz regime. From systematic studies, it is revealed that the large birefringence of black rubbers is due to the orientation distribution of anisotropically shaped CB aggregates in the rubber matrix and the orientation distribution is strongly linked to the mechanical deformation of the black rubber. A model simulation based on this relation between deformation and reorientation allows conversion of the birefringence (optical) information into strain (mechanical) information. In addition, the spectroscopic information obtained using the THz-TDS technique is useful to evaluate the changes in the internal conductive filler network caused by the mechanical deformation. Our findings demonstrate that the terahertz polarization spectroscopy is a promising nondestructive inspection method for contactless investigation of the

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Impact of carbon black aggregates on the optical properties of black rubber samples in the terahertz frequency range(accepted for publication)

S. Watanabe, M. Fujii, and M. Okano

Advanced Materials Letters (VBRI Press) 10 ( 1 ) 48 - 51 2018.12

Research paper (scientific journal), Joint Work

Terahertz polarization imaging and its applications

S. Watanabe

Photonics (MDPI) 5 ( 4 ) 58 2018.12

© 2018 by the author. This review focuses on several recent research activities regarding precise and fast polarization-sensitive terahertz time-domain spectroscopy systems for imaging purposes, and explains three interesting application examples. Owing to modulation techniques that have recently been developed for the evaluation of the instantaneous terahertz electric-field (E-field) vector, fast and precise terahertz polarization imaging becomes feasible. This terahertz technology enables high-resolution surface topography, precise understanding of the spatial E-field vector distribution of the focused terahertz pulse, and examination of strain-induced birefringence in polymeric materials. These examples constitute a new application area of terahertz photonics with emphasis on both fundamental optics and industrial applications.

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Inspection of internal filler alignment in visibly opaque carbon-black–rubber composites by terahertz polarization spectroscopy in reflection mode

Okano M, Watanabe S

Polym. Test. (Polymer Testing) 72 196 - 201 2018.12

Single Work, ISSN 01429418

© 2018 As the industrial utilization of polymeric materials is increasing owing to recent improvements in material durability and functionality, product safety becomes an important issue. Therefore, various inspection methods for polymeric materials are being developed. In this work, the rapid and non-destructive inspection of the averaged internal orientation of conductive carbon black fillers in rubber composites is demonstrated using a handmade high-speed terahertz polarization spectroscopy system that employs reflection geometry. From the birefringent information obtained from the black rubber sample (with arbitrary stretching condition), the internal orientation of the conductive fillers, which plays an essential role in the properties of the rubber composite, was determined. In addition, since data acquisition with the reflection method is fast, the spatially resolved measurement of the filler orientation could also be performed. The information of the internal orientation of the carbon black fillers allows verification of the condition of the rubber product.

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Optical Response Change of Black Rubbers under Cyclic Deformation Investigated by Terahertz Polarization Spectroscopy

Tsujimoto T, Moriwaki A, Fujii M, Okano M, Watanabe S

International Conference on Infrared, Millimeter, and Terahertz Waves, IRMMW-THz (International Conference on Infrared, Millimeter, and Terahertz Waves, IRMMW-THz) 2018-September 2018.10

Joint Work, ISSN 9781538638095

© 2018 IEEE. We recently demonstrated the nondestructive and contactless inspection of visibly-opaque black rubbers with a conductive carbon fillers, which is required for the rubber industrial community, by terahertz polarization spectroscopy. Using our developed technique, in this work, the terahertz optical and mechanical properties of the black rubbers were simultaneously measured under cyclic deformation. The black rubbers show the hysteresis in the draw ratio dependence of the terahertz birefringence as well as the well-known hysteresis behavior in the stress. The result implies that the terahertz optical response of black rubbers have correlation with their mechanical properties.

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Inspection of internal filler alignment in visibly opaque carbon-black?rubber composites by terahertz polarization spectroscopy in reflection mode

M. Okano, and S. Watanabe

Polymer Testing 72 196 - 201 2018.10

Research paper (scientific journal), Joint Work, Accepted

Ultrafast control of coherent spin precession in ferromagnetic thin films via thermal spin excitation processes induced by two-pulse laser excitation

H. Shibata, M. Okano, and S. Watanabe

Phys. Rev. B (American Physical Society) 97 ( 1 ) 014438 2018.01

Research paper (scientific journal), Joint Work, ISSN 24699950

We investigate the laser-induced spin precession dynamics in ferromagnetic Ni0.8Fe0.2 thin films by using a time-resolved magneto-optical Kerr effect (TR-MOKE) measurement with two pump pulses. By changing the time interval between the pump pulses, the amplitude of the TR-MOKE signal is strongly modulated according to the phase of the TR-MOKE signal at the time when the second pulse arrives. The change in the oscillation amplitude can be explained with the temporal evolution of the spin precession induced by laser heating, as shown with a numerical simulation. We conclude that the ultrafast control of coherent spin precession dynamics is possible by multiple pump pulses via thermal excitation.

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Internal strain imaging of visibly-opaque black rubbers by terahertz polarization spectroscopy

Moriwaki A, Okano M, Watanabe S

Optics InfoBase Conference Papers (Optics InfoBase Conference Papers) Part F113-CLEOPR 2018 2018

Joint Work, ISSN 9781943580453

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Electric-field vector imaging of terahertz surface waves on an indium tin oxide thin film

Suzuki K, Oguchi K, Okano M, Watanabe S

Optics InfoBase Conference Papers (Optics InfoBase Conference Papers) Part F113-CLEOPR 2018 2018

Joint Work, ISSN 9781943580453

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Measurement of elliptically-polarized E-field waveforms in mid infrared range by electro-optic sampling

Oguchi K, Kurihara T, Fischer M, Watanabe S, Brida D

Optics InfoBase Conference Papers (Optics InfoBase Conference Papers) Part F125-JSAP 2018 2018

Joint Work, ISSN 9784863486942

© 2018 OSA - The Optical Society. All rights reserved. Electro-optic sampling (EOS) is a technique widely used to measure electric field waveform in ultra-broadband frequency range from terahertz (THz), mid- (MIR) [1], and near infrared (NIR) [2]. Since EOS signal depends on the relative orientations between probe, THz and crystal axis, the modulation of EOS signal upon crystal rotation provides polarization information of the infrared pulses [3, 4]. This so-called polarization-sensitive (PS-) EOS has an advantage that no additional polarization optics such as wire-grid polarizer is needed to measure polarization. In this talk we extend PS-EOS for the first time to the MIR region, wherein fruitful application possibilities such as vibrational circular dichroism [5] are expected.

Polarization-sensitive electro-optic sampling in mid infrared region

Oguchi Kenichi, Kurihara Takayuki, Fischer Marco, Watanabe Shinichi, Brida Daniele

Meeting Abstracts of the Physical Society of Japan (The Physical Society of Japan) 73 ( 0 ) 990 - 990 2018

Joint Work

Observation of laser induced spin wave propagation in NiFe microstructures using magneto-optical measurement

Arai Y., Shibata H., Okano M., Watanabe S.

Meeting Abstracts of the Physical Society of Japan (The Physical Society of Japan) 73 ( 0 ) 1052 - 1052 2018

Joint Work

Anisotropic percolation conduction in elastomer-carbon black composites investigated by polarization-sensitive terahertz time-domain spectroscopy

M. Okano, M. Fujii, and S. Watanabe

Appl. Phys. Lett. (AMER INST PHYSICS) 111 ( 22 ) 221902 2017.11

Research paper (scientific journal), Joint Work, Accepted, ISSN 0003-6951

We investigated the draw ratio (DR) dependence of the anisotropic dielectric function and conductivity of styrene butadiene rubbers (SBRs) with different carbon black (CB) concentrations by polarization-sensitive terahertz time-domain spectroscopy. From the frequency dependence of the conductivity in the unstretched SBRs ranging from direct current to terahertz frequencies, it is found that the SBR with a CB concentration above 30 wt. % exhibits percolation conductivity. We investigated the spectral shape of the dielectric function and conductivity of the SBR samples below and above the percolation threshold for two representative DRs in the terahertz frequency region. We found that the DR dependence of the spectral shape is well explained by the effective medium approximation, except for the sample with the CB concentration above 30 wt. % under the unstretched condition. The conductivity in that sample remarkably changes in the low terahertz frequency region, which suggests a change in the CB network by deformation. The investigation of the dielectric anisotropy and percolation conductivity using our polarization technique can be applied to a wide range of elastomer composites. Published by AIP Publishing.

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Internal triaxial strain imaging of visibly opaque black rubbers with terahertz polarization spectroscopy

A. Moriwaki, M. Okano, and S. Watanabe

APL Photonics 2 ( 10 ) 106101 2017.09

Research paper (scientific journal), Joint Work, Accepted

Retrieving the undistorted terahertz time-domain electric-field vector from the electro-optic effect

K. Oguchi, M. Okano, and S. Watanabe

J. Opt. Soc. Am. B 34 ( 9 ) 1946 - 1956 2017.08

Research paper (scientific journal), Joint Work, Accepted

Controlled terahertz birefringence in stretched poly(lactic acid) films investigated by terahertz time-domain spectroscopy and wide-angle X-ray scattering

H. Iwasaki, M. Nakamura, N. Komatsubara, M. Okano, M. Nakasako, H. Sato, and S. Watanabe

J. Phys. Chem. B (AMER CHEMICAL SOC) 121 ( 28 ) 6951 - 6957 2017.06

Research paper (scientific journal), Joint Work, Accepted, ISSN 1520-6106

We report a correlation between the dielectric property and structure of stretched poly(lactic acid) (PLA) films, revealed by polarization-sensitive terahertz time domain spectroscopy and two-dimensional (2D) wide-angle X-ray scattering (WAXS). The experiments evidence that the dielectric function of the PLA film becomes more anisotropic with increasing draw ratio (DR). This behavior is explained by a classical Lorentz oscillator model assuming polarization-dependent absorption. The birefringence can be systematically altered from 0 to 0.13 by controlling DR The combination of terahertz spectroscopy and 2D WAXS measurement reveals a clear correlation between the birefringence in the terahertz frequency domain and the degree of orientation of the PLA molecular chains. These findings imply that the birefringence is a result of the orientation of the PLA chains with anisotropic macromolecular vibration modes. Because of a good controllability of the birefringenco, polymer-based materials will provide an attractive materials system for phase retarders in the terahertz frequency range.

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Detailed study of transient anomalous electric field vector focused by parabolic mirror

Shibata Kazunori, Uemoto Mitsuharu, Takai Mayuko, Watanabe Shinichi

Journal of Optics (IOP PUBLISHING LTD) 19 ( 3 ) 2017.03

Research paper (scientific journal), Joint Work, Accepted, ISSN 2040-8978

This paper provides a detailed theoretical analysis of the unexpected transient divergent and rotational distributions of the focused electric field vector reported in Shibata et al (2015 Phys. Rev. A 92 053806). We reveal the physical origin of these distributions. More quantitatively, we derive the semi-analytic expressions and clarify how these distributions depend on the mirror size, offset angle, and the intensity distribution of the incident parallel light. We compare the formulas with numerical calculations and evaluate the area where linearity holds. If the wavelength and the mirror size are sufficiently shorter than the focal length, the radius of the linear area becomes longer than the wavelength. These formulas and evaluations are useful for studies, which require high spatio-temporal resolution.

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Magneto-optic Kerr effect CCD imaging with polarization modulation technique

S. Nakayama, M. Okano, Y. Nozaki, and S. Watanabe

AIP Advances (AMER INST PHYSICS) 7 ( 5 ) 056802 2017.01

Research paper (scientific journal), Joint Work, Accepted, ISSN 2158-3226

We have developed a magneto-optic Kerr effect (MOKE) imaging system with a charge-coupled-device (CCD) camera by using the rotating compensator technique. We chose optimal conditions of the rotation frequency of the compensator with stable rotation along with a CCD camera frame rate that allowed precise control of the exposure timing in order to link with the angle of the compensator. Precise timing management of the CCD exposure enables us to carry out repeated experiments, which greatly improves the signal-to-noise ratio of the longitudinal MOKE signal. We applied the technique to the material characterization of the Ni-81 Fe-19 thin film and its microstructure, and succeeded in evaluating the spatial variation of the complex magneto-optic constant Q of the sample. Because of its attractive advantages such as high-speed and compactness, the present method provides a novel platform for investigating the domain structures in various magnetic materials. (C) 2017 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

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Anisotropic optical response of optically opaque elastomers with conductive fillers as revealed by terahertz polarization spectroscopy

M. Okano and S. Watanabe

Scientific Reports (NATURE PUBLISHING GROUP) 6 39079 2016.12

Research paper (scientific journal), Joint Work, Accepted, ISSN 2045-2322

Elastomers are one of the most important materials in modern society because of the inherent viscoelastic properties due to their cross-linked polymer chains. Their vibration-absorbing and adhesive properties are especially useful and thus utilized in various applications, for example, tires in automobiles and bicycles, seismic dampers in buildings, and seals in a space shuttle. Thus, the nondestructive inspection of their internal states such as the internal deformation is essential in safety. Generally, industrial elastomers include various kinds of additives, such as carbon blacks for reinforcing them. The additives make most of them opaque in a wide spectral range from visible to mid-infrared, resulting in that the nondestructive inspection of the internal deformation is quite difficult. Here, we demonstrate transmission terahertz polarization spectroscopy as a powerful technique for investigating the internal optical anisotropy in optically opaque elastomers with conductive additives, which are transparent only in the terahertz frequency region. The internal deformation can be probed through the polarization changes inside the material due to the anisotropic dielectric response of the conductive additives. Our study about the polarization-dependent terahertz response of elastomers with conductive additives provides novel knowledge for in situ, nondestructive evaluation of their internal deformation.

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Polarization-sensitive dual-comb spectroscopy

K. A. Sumihara, S. Okubo, M. Okano, H. Inaba, and S. Watanabe

Journal of the Optical Society of America B (OPTICAL SOC AMER) 34 ( 1 ) 154 - 159 2016.12

Research paper (scientific journal), Joint Work, Accepted, ISSN 0740-3224

We introduce a method of polarization-sensitive dual-comb spectroscopy. This technique is a combination of dual-comb spectroscopy and rotating compensator polarimetry, in which the rotation frequency of the compensator is optimized to establish the frequency sidebands of each comb tooth. Thus, we can determine the polarization state of each comb tooth by analyzing the amplitudes and phases of the sidebands. For a frequency resolution of 48 MHz and a measurement time of 726.5 s, the precisions of the orientation and the ellipticity angles of the individual comb teeth are similar to 0.01 rad. (C) 2016 Optical Society of America.

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Spatial polarization variation in terahertz electromagnetic wave focused by off-axis parabolic mirror.

WATANABE SHINICHI

Applied Physics Express (IOP PUBLISHING LTD) 9 ( 5 ) 052206 2016.04

Research paper (scientific journal), Joint Work, Accepted, ISSN 1882-0778

We have investigated the spatial distribution of the polarization state of a terahertz electromagnetic wave focused by an off-axis parabolic mirror (OPM) in the focal plane. We employed polarization-resolved terahertz time-domain spectroscopy and found that a steep spatial variation in the polarization state appears slightly distant from the focus when a linearly polarized terahertz wave is focused. The spatial variation includes an abrupt change in the polarization state (states change between circular and linear polarizations) within a wavelength. The observed phenomena are confirmed by numerical calculations and are shown to be intrinsic to the reflection from the OPM. (C) 2016 The Japan Society of Applied Physics

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Polarization-sensitive electro-optic detection of terahertz wave using three different types of crystal symmetry: Toward broadband polarization spectroscopy.

K. Oguchi, H. Iwasaki, M. Okano, and S. Watanabe

Applied Physics Letters (AMER INST PHYSICS) 108 ( 1 ) 011105 2016.01

Research paper (scientific journal), Joint Work, Accepted, ISSN 0003-6951

We investigated polarization-sensitive electro-optic (EO) detection of terahertz (THz) waves by using two uniaxial crystals: a c-cut gallium selenide and a c-cut lithium niobate crystals. We formulated a general frequency-domain description of EO detection by in-plane isotropic EO crystals, which holds regardless of the frequency. Based on this description, the polarization of THz waves can be derived by analyzing EO sampling signals measured with two orthogonal configurations of the in-plane isotropic EO crystals as well as typical (111) zinc-blende EO crystals. In addition, we experimentally demonstrated that the frequency-dependent polarization of THz waves can be reproducibly retrieved using three EO crystals with different crystal symmetries and with different phase matching conditions. Our description provides essential information for practical polarization sensing in the THz frequency range as well as in the mid-infrared range. (C) 2016 AIP Publishing LLC.

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Studies of optical anisotropy of poly lactic acid by terahertz time-domain spectroscopy

Iwasaki Hotsumi, Okano Makoto, Komatsubara Nozomu, Nakamura Madoka, Moriwaki Atsuto, Sato Harumi, Watanabe Shinichi

Meeting Abstracts of the Physical Society of Japan (The Physical Society of Japan) 71 1354 - 1354 2016

テラヘルツ周波数領域には高分子特有の振動モードが存在しており、テラヘルツ光を用いた高分子材料の物性測定に、注目が集まっている。我々は高分子ポリ乳酸薄膜を延伸することで、ポリ乳酸に複屈折が生じることを発見した。また、複屈折の延伸率依存性をテラヘルツ帯の回転検光子型偏光計で調べた。さらにテラヘルツ時間領域分光法を用いて、延伸ポリ乳酸薄膜の複素屈折率が持つ偏光依存性を調べた。

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Birefringent properties of poly-lactic acid at terahertz range

Hotsumi Iwasaki, Makoto Okano, Nozomu Komatsubara, Madoka Nakamura, Atsuto Moriwaki, Harumi Sato, Shinichi Watanabe

2016 41ST INTERNATIONAL CONFERENCE ON INFRARED, MILLIMETER, AND TERAHERTZ WAVES (IRMMW-THZ) (IEEE) 2016

Accepted, ISSN 2162-2027

We report on a drawing-ratio dependence of the refractive index difference between the two optic axes (Delta n) of poly lactic acid films in terahertz frequency range. We found that Delta n can be controlled in a range of 0-0.04 by changing the drawing ratio. The result shows that we can design a terahertz wave plate with arbitral retardation, which will be useful for establishing polymer-based terahertz optical component with low cost and easy implementation in mass production.

Optical anisotropy in elastomer with conductive additives revealed by polarization-sensitive terahertz spectroscopy

Okano Makoto, Watanabe Shinichi

Meeting Abstracts of the Physical Society of Japan (The Physical Society of Japan) 71 1437 - 1437 2016

今回、我々は可視光から赤外光領域に至るまで不透明な導電性添加物を含んだゴムに対してテラヘルツ偏光測定を行い、大きな光学異方性を観測した。誘電関数の異方性はMaxwell-Granett理論によってよく記述され、導電性添加物の誘電関数の異方性がゴム全体の異方性の起源であることを明らかにした。この結果は、テラヘルツ領域における二元混合物の光学応答の好例であり、また実用性も高い重要な成果である。

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21pPSA-58 Quantitative measurement of magneto-optical Kerr effect in NiFe microstructures using a CCD camera

Nakayama S., Okano M., Nozaki Y., Watanabe S.

Meeting Abstracts of the Physical Society of Japan (The Physical Society of Japan (JPS)) 71 1617 - 1617 2016

ISSN 2189-079X

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Intrinsic formation of electromagnetic divergence and rotation by parabolic focusing

K. Shibata, M. Takai, M. Uemoto, and S. Watanabe

Physical Review A (AMER PHYSICAL SOC) 92 ( 5 ) 053806 2015.11

Research paper (scientific journal), Joint Work, Accepted, ISSN 1050-2947

A linearly polarized electromagnetic wave is found to form divergent and rotation fields after focused by an off-axis parabolic mirror. These distributions are generated within a subwavelength scale around the focus and at times when the electric field at the focus vanishes. We theoretically and experimentally show that not only the direction but also the structure of the distributions varies with the incident polarization. In addition, the distributions move in one direction with a phase velocity faster than the speed of light.

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24aCN-8 Effect of an intense Terahertz pulse irradiation on a ferromagnetc disk with a magnetic vortex

Sozawa M., Tachizaki T., Ropagnol Xavier, Razavipour Hadi, Ozaki T., Cooke David, Nozaki Y., Watanabe S.

Meeting Abstracts of the Physical Society of Japan (The Physical Society of Japan (JPS)) 70 1625 - 1625 2015

ISSN 2189-079X

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Polarization detection of terahertz radiation via the electro-optic effect using zinc-blende crystal symmetry

K. Oguchi, N. Yasumatsu, and S. Watanabe

Journal of the Optical Society of America B (OPTICAL SOC AMER) 31 ( 12 ) 3170 - 3180 2014.11

Research paper (scientific journal), Joint Work, Accepted, ISSN 0740-3224

We provide a frequency-domain picture of the electro-optic (EO) detection of elliptically polarized terahertz radiation utilizing the crystal angle dependence of the EO effect using zinc blende crystal symmetry. In contrast to EO detection of a linearly polarized terahertz pulse, the measured EO signal would not be a replica of the terahertz pulse, even though the phase-matching condition is completely fulfilled. However, the rotation of the EO crystal reduces the complexity, and the ellipticity and the angle of rotation of each frequency component of the elliptically polarized terahertz pulse can be determined irrespective of what kind of zinc blende crystals are used for the EO detection. We experimentally show that the ellipticity and angle of rotation are reproducibly obtained by using three different crystals with different thicknesses and compositions. Direct detection of the elliptically polarized terahertz radiation enables us to precisely determine the optical axis and retardation of a quartz crystal using the Poincare sphere representation of the state of polarization. (C) 2014 Optical Society of America

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Video-rate terahertz electric-field vector imaging

M. Takai, M. Takeda, M. Sasaki, T. Tachizaki, N. Yasumatsu, and S. Watanabe

Applied Physics Letters (AMER INST PHYSICS) 105 ( 15 ) 151103 2014.10

Research paper (scientific journal), Joint Work, Accepted, ISSN 0003-6951

We present an experimental setup to dramatically reduce a measurement time for obtaining spatial distributions of terahertz electric-field (E-field) vectors. The method utilizes the electro-optic sampling, and we use a charge-coupled device to detect a spatial distribution of the probe beam polarization rotation by the E-field-induced Pockels effect in a < 110 >-oriented ZnTe crystal. A quick rotation of the ZnTe crystal allows analyzing the terahertz E-field direction at each image position, and the terahertz E-field vector mapping at a fixed position of an optical delay line is achieved within 21 ms. Video-rate mapping of terahertz E-field vectors is likely to be useful for achieving real-time sensing of terahertz vector beams, vector vortices, and surface topography. The method is also useful for a fast polarization analysis of terahertz beams. (C) 2014 AIP Publishing LLC.

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High-speed terahertz time-domain polarimeter based on an electro-optic modulation technique

N. Yasumatsu, A. Kasatani, K. Oguchi, and S. Watanabe

Applied Physics Express (IOP PUBLISHING LTD) 7 ( 9 ) 092401 2014.09

Research paper (scientific journal), Joint Work, Accepted, ISSN 1882-0778

We present a method of developing a high-speed terahertz time-domain polarimeter based on an electro-optic sampling setup, which consists of a rapid polarization analysis of the probe optical beam within 1 ms with an angular resolution of similar to 100 mrad using an electro-optic modulator (EOM). A fast optical delay line enabled us to obtain the vector waveform of the terahertz pulse within 100 ms. The setup does not utilize a manual/mechanical rotational stage, but the use of the polarization modulation technique employing the EOM ensures a high-speed and high-accuracy measurement of the state of polarization of terahertz waves. (C) 2014 The Japan Society of Applied Physics

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Development of precise polarization measurement system using dual-comb spectroscopy

Kana Alyssa Sumihara, Sho Okubo, Makoto Okano, Hajime Inaba, Shinichi Watanabe

Optics InfoBase Conference Papers (OSA - The Optical Society) 3 2014.07

Accepted

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Time-domain picture of the terahertz vector waveform measured by the electro-optic sampling method using the crystal symmetry

Kenichi Oguchi, Naoya Yasumatsu, Shinichi Watanabe

2014 39TH INTERNATIONAL CONFERENCE ON INFRARED, MILLIMETER, AND TERAHERTZ WAVES (IRMMW-THZ) (IEEE) 2014

Accepted, ISSN 2162-2027

回転電気光学結晶法によるテラヘルツ波の電場ベクトル計測技術

WATANABE SHINICHI, Naoya Yasumatsu

テラヘルツテクノロジーフォーラム通信 (テラヘルツテクノロジーフォーラム) 11 ( 2 ) 2-5 2013.12

Research paper (other academic), Joint Work

Robustness of electric field vector sensing of electromagnetic waves by analyzing crystal angle dependence of the electro-optic effect

Yasumatsu Naoya, and WATANABE SHINICHI

The Journal of the Optical Society of America B 30 ( 11 ) 2940-2951 2013.10

Research paper (scientific journal), Joint Work, Accepted

Real-time terahertz time-domain polarization analyzer with 80-MHz repetition-rate femtosecond laser pulses

WATANABE SHINICHI, Naoya Yasumatsu, Kenichi Oguchi, Masatoshi Takeda, Takeshi Suzuki, and Takehiro Tachizaki

Sensors 13 ( 3 ) 3299-3312 2013.03

Research paper (scientific journal), Joint Work, Accepted

テラヘルツ電磁波の偏波情報を用いた超高感度凹凸イメージング手法の開発

渡邉紳一, 安松直弥

検査技術 6-9 2013.03

Research paper (scientific journal), Joint Work

A Real-Time Terahertz Time-Domain Polarization Analyzer with 80-MHz Repetition-Rate Femtosecond Laser Pulses

Shinichi Watanabe, Naoya Yasumatsu, Kenichi Oguchi, Masatoshi Takeda, Takeshi Suzuki, Takehiro Tachizaki

SENSORS (MDPI AG) 13 ( 3 ) 3299 - 3312 2013.03

Accepted, ISSN 1424-8220

We have developed a real-time terahertz time-domain polarization analyzer by using 80-MHz repetition-rate femtosecond laser pulses. Our technique is based on the spinning electro-optic sensor method, which we recently proposed and demonstrated by using a regenerative amplifier laser system; here we improve the detection scheme in order to be able to use it with a femtosecond laser oscillator with laser pulses of a much higher repetition rate. This improvement brings great advantages for realizing broadband, compact and stable real-time terahertz time-domain polarization measurement systems for scientific and industrial applications.

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回転検出結晶法によるテラヘルツ光の高速・精密偏光計測法の開発

渡邉紳一, 安松直弥

固体物理 47 ( 12 ) 737 2012.12

Research paper (scientific journal), Joint Work, Accepted

T-ray topography by time-domain polarimetry

Naoya Yasumatsu, WATANABE SHINICHI

Optics Letters 37 ( 13 ) 2706-2708 2012.06

Research paper (scientific journal), Joint Work, Accepted

Precise real-time polarization measurement of terahertz electromagnetic waves by a spinning electro-optic sensor

Naoya Yasumatsu, and WATANABE SHINICHI

Review of Scientific Instruments (AMER INST PHYSICS) 83 ( 2 ) 023104 2012.02

Research paper (scientific journal), Joint Work, ISSN 0034-6748

We propose and develop a method to quickly and precisely determine the polarization direction of coherent terahertz electromagnetic waves generated by femtosecond laser pulses. The measurement system consists of a conventional terahertz time-domain spectroscopy system with the electro-optic (EO) sampling method, but we add a new functionality in the EO crystal which is continuously rotating with the angular frequency omega. We find a simple yet useful formulation of the EO signal as a function of the crystal orientation, which enables a lock-in-like detection of both the electric-field amplitude and the absolute polarization direction of the terahertz waves with respect to the probe laser pulse polarization direction at the same time. The single measurement finishes around two periods of the crystal rotations (similar to 21 ms), and we experimentally prove that the accuracy of the polarization measurement does not suffer from the long-term amplitude fluctuation of the terahertz pulses. Distribution of the measured polarization directions by repeating the measurements is excellently fitted by a Gaussian distribution function with a standard deviation of sigma = 0.56 degrees. The developed technique is useful for the fast direct determination of the polarization state of the terahertz electromagnetic waves for polarization imaging applications as well as the precise terahertz Faraday or Kerr rotation spectroscopy. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3683570]

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Intense terahertz pulse-induced nonlinear responses in carbon nanotubes

Ryo Shimano, WATANABE SHINICHI, and Ryusuke Matsunaga

Journal of Infrared, Millimeter, and Terahertz Waves 33 ( 8 ) 861-869 - 869 2012

Research paper (scientific journal), Joint Work, Accepted, ISSN 1866-6892

By using intense terahertz(THz) monocycle pulses, nonlinear light-matter interaction in aligned semiconducting single-walled carbon nanotubes(SWNTs) embedded in a polymer film was investigated. THz electric-field-induced ultrafast Stark effect of onedimensional excitons in SWNTs was observed at room temperature, suggesting the potential functionality of SWNTs for high speed electro-optic devices operating at telecom wavelength with a THz bandwidth. When the peak electric field amplitude exceeds 200 kV/cm, the generation of excitons by the THz pump becomes prominent. The mechanism is described by the above-gap excitation of electrons and holes in SWNTs due to the impact excitation process induced by the intense THz electric field. © Springer Science+Business Media, LLC 2012.

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Terahertz profilometer by time-domain polarimetry

Naoya Yasumatsu, Shinichi Watanabe

2012 CONFERENCE ON LASERS AND ELECTRO-OPTICS (CLEO) (IEEE) 2012

Accepted, ISSN 2160-9020

高強度テラヘルツパルスによるカーボンナノチューブの超高速非線形光学現象

渡邉紳一, 島野亮

月刊オプトロニクス 2011.08

Research paper (scientific journal), Joint Work

Intense terahertz pulse induced exciton generation in carbon nanotubes

S. Watanabe, N. Minami, and R. Shimano

Optics Express (OPTICAL SOC AMER) 19 ( 2 ) 1528 - 1538 2011.01

Research paper (scientific journal), Joint Work, Accepted, ISSN 1094-4087

We have investigated the highly nonlinear terahertz (THz) light-matter interaction in single-walled carbon nanotubes (SWNTs). The high-peak THz electric- field (similar to 0.7 MV/cm) and the low effective mass of carriers result in their ponderomotive energy exceeding the bandgap energy of semiconducting SWNTs. Under such an intense THz pulse irradiation, the interband excitation that results in the generation of excitons occurs, although the THz photon energy (similar to 4 meV) is much smaller than the gap energy of SWNTs (similar to 1 eV). The ultrafast dynamics of this exciton generation process is investigated by THz pump and optical probe spectroscopy. The exciton generation mechanism is described by impact excitation process induced by the strong THz E-field. Such intense THz pulse excitation provides a powerful tool to study nonlinear terahertz optics in non-perturbative regime as well as nonlinear transport phenomena in solids with ultrafast temporal resolution. (c) 2011 Optical Society of America

Room temperature terahertz electro-optic modulation by excitons in carbon nanotubes

T. Ogawa, S. Watanabe, N. Minami, and R. Shimano

Applied Physics Letters (AMER INST PHYSICS) 97 ( 4 ) 041111 2010.07

Research paper (scientific journal), Joint Work, Accepted, ISSN 0003-6951

We have investigated the electroabsorption of aligned micelle-wrapped single-walled carbon nanotubes induced by intense terahertz (THz) pulses by using THz-pump and optical-probe spectroscopy at room temperature. The electroabsorption signal is enhanced at exciton resonances near the optical communication wavelength. The signal showed a quadratic dependence on the THz electric field and instantaneously followed the THz oscillation (similar to 1 ps). The observed ultrafast electro-optic modulation with a THz bandwidth suggests the potential functionality of carbon nanotubes in high-speed electro-optic devices. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3470105]

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Ultrafast photo-induced insulator-to-metal transition in the spin density wave system of (TMTSF)2PF6

S. Watanabe, R. Kondo, S. Kagoshima, and R. Shimano

Physica B 405 ( 11 ) S360-362 2010.06

Research paper (scientific journal), Joint Work, Accepted

Ultrafast photo-induced insulator-to-metal transition in the spin density wave system of (TMTSF)(2)PF6

Shinichi Watanabe, Ryusuke Kondo, Seiichi Kagoshima, Ryo Shimano

PHYSICA B-CONDENSED MATTER (ELSEVIER SCIENCE BV) 405 ( 11 ) S360 - S362 2010.06

Accepted, ISSN 0921-4526

We demonstrate a photo-induced insulator-to-metal (I-M) transition in an organic linear chain compound (TMTSF)(2)PF6 [bis-(tetramethyl-tetraselenafulvalene)hexafluorophosphate] in the spin-density-wave (SOW) ground state by optical pump and terahertz probe experiments We observe the collapse of SOW gap in the optical conductivity spectrum at 3 ps after the photo-excitation indicating the ultrafast I-M transition in the SOW system Temporal evolution of the terahertz reflectivity change shows that the photo-induced metallic state sustains longer than 400 Ps (C) 2009 Elsevier B V All rights reserved

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Observation of ultrafast photoinduced closing and recovery of the spin-density-wave gap in (TMTSF)2PF6

S. Watanabe, R. Kondo, S. Kagoshima, and R. Shimano

Physical Review B 80 ( 22 ) 220408(R) 2009.12

Research paper (scientific journal), Joint Work, Accepted

Observation of ultrafast photoinduced closing and recovery of the spin-density-wave gap in (TMTSF)(2)PF6

Shinichi Watanabe, Ryusuke Kondo, Seiichi Kagoshima, Ryo Shimano

PHYSICAL REVIEW B (AMER PHYSICAL SOC) 80 ( 22 ) 220408(R) 2009.12

Accepted, ISSN 1098-0121

Photoexcited dynamics of a spin-density-wave (SDW) state in an organic linear-chain compound bis-(tetramethyl-tetraselenafulvalene)hexafluorophosphate, (TMTSF)(2)PF6, is investigated by optical pump and terahertz probe experiments. After the ultrashort laser-pulse excitation, a sudden closing followed by a recovery of the SDW gap is clearly observed in the far-infrared spectrum. The recovery time is found to diverge toward the phase-transition temperature, T-SDW, while the gap remains open at T-SDW. The observed critical slowing down behavior is interpreted in terms of three-dimensional fluctuation which enlarges near the phase-transition temperature.

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Ultrafast terahertz probe of spin-density wave dynamics in organic conductor (TMTSF)(2)PF6

S. Watanabe, R. Kondo, S. Kagoshima, R. Shimano

2009 CONFERENCE ON LASERS AND ELECTRO-OPTICS AND QUANTUM ELECTRONICS AND LASER SCIENCE CONFERENCE (CLEO/QELS 2009), VOLS 1-5 (IEEE) 2993 - + 2009

Accepted

Spin-density-wave gap in (TMTSF)2PF6 probed by reflection-type terahertz time-domain spectroscopy

S. Watanabe, R. Kondo, S. Kagoshima, and R. Shimano

Physica Status Solidi (b) 245 ( 12 ) 2688-2691 2008.10

Research paper (scientific journal), Joint Work, Accepted

高空間分解能を持つ超小型テラヘルツ分光装置の開発

渡邉紳一, 島野亮

分光研究 57 ( 2 ) 81 2008

Research paper (scientific journal), Joint Work

Compact terahertz time domain spectroscopy system with diffraction-limited spatial resolution

S. Watanabe, R. Shimano

Review of Scientific Instruments (AMER INST PHYSICS) 78 ( 10 ) 103906 2007.10

Research paper (scientific journal), Joint Work, Accepted, ISSN 0034-6748

A compact and rigid terahertz time domain spectroscopy system is presented. The size of the device is 20 mm diameter times 61 mm length with four parabolic-shaped concave mirrors dug in it to effectively focus a terahertz beam on a sample. The device has no chromatic aberration over the whole bandwidth of the beam (0.3-2 THz), and an effective numerical aperture of about 0.45 is achieved, which has a capability to image the structure whose size is almost the same as the wavelength. Frequency resolved images clearly show this performance. We also show that quantitative retrieval of the complex refractive index of the structure as small as twice of the wavelength is possible. (C) 2007 American Institute of Physics.

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Narrow (~4 meV) inhomogeneous broadening and its correlation with confinement potential of pyramidal quantum dot arrays

K. Leifer, E. Pelucchi, S. Watanabe, F. Michelini, B. Dwir, E. Kapon.

Applied Physics Letters 91 ( 8 ) 081106 2007.08

Research paper (scientific journal), Joint Work, Accepted

Narrow (approximate to 4 meV) inhomogeneous broadening and its correlation with confinement potential of pyramidal quantum dot arrays

K. Leifer, E. Pelucchi, S. Watanabe, F. Michelini, B. Dwir, E. Kapon

APPLIED PHYSICS LETTERS (AMER INST PHYSICS) 91 ( 8 ) 81106 2007.08

Accepted, ISSN 0003-6951

The inhomogeneous broadening in the luminescence spectra of ordered arrays of pyramidal GaAs/AlGaAs semiconductor quantum dots (QDs) was studied as a function of the dot size. Dot arrays with inhomogeneous broadening as small as 4.1 meV and a corresponding ground state to first excited state transition separation of 28 meV were obtained. By evaluating the QD energy levels using a multiband k center dot p model, the authors estimated that the observed inhomogeneous broadening corresponds to dot height fluctuations of about 1-2 ML across the array.(c) 2007 American Institute of Physics.

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Mechanisms of quantum dot energy engineering by metalorganic vapor phase epitaxy on patterned nonplanar substrates

E. Pelucchi, S. Watanabe, K. Leifer, Q. Zhu, B. Dwir,P. De Los Rios, E. Kapon.

Nano Letters (AMER CHEMICAL SOC) 7 ( 5 ) 1282-1285 - 1285 2007.03

Research paper (scientific journal), Joint Work, Accepted, ISSN 1530-6984

A novel technique for tuning the strength of quantum confinement in site-controlled semiconductor quantum dots (QDs) is introduced and investigated theoretically and experimentally. The method makes use of controlled local growth rates during metalorganic vapor phase epitaxy on patterned arrays of inverted pyramids. A model accounting for precursor migration and adatom incorporation predicts the tuning in QD thickness as a function of the pattern parameters. The results are in good agreement with experimental findings. This technique offers means for designing QD photonic structures with potential applications in QD-based cavity quantum electrodynamics and quantum information processing.

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Very compact THz-TDS imaging system with diffraction limited spatial resolution

Shinichi Watanabe, Ryo Shimano

2007 JOINT 32ND INTERNATIONAL CONFERENCE ON INFRARED AND MILLIMETER WAVES AND 15TH INTERNATIONAL CONFERENCE ON TERAHERTZ ELECTRONICS, VOLS 1 AND 2 (IEEE) 56 - 57 2007

Accepted

Patterning of confined-state energies in site-controlled semiconductor quantum dots

S. Watanabe, E. Pelucchi, K. Leifer, A. Malko, B. Dwir, E. Kapon.

Applied Physics Letters (AMER INST PHYSICS) 86 ( 24 ) 243105 2005.06

Research paper (scientific journal), Joint Work, Accepted, ISSN 0003-6951

We demonstrate control of the confined-state energies of semiconductor quantum dots (QDs) grown on prepatterned substrates. The InGaAs/AlGaAs QDs self-order at the apex of self-limiting, inverted pyramids whose locations are fixed by lithography. The confinement energy in the dots is systematically varied across the substrate by changing the pattern of the pyramid array in their vicinity. The resulting energy- and site-controlled QDs show systematic and reproducible shifts of their emission wavelengths as well as antibunched photon emissions from confined single excitons. Such QDs should be useful for applications in quantum information processing and quantum communication devices, e.g., multiple-wavelength single-photon emitters. (c) 2005 American Institute of Physics.

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Site- and energy-controlled pyramidal quantum dot heterostructures

E. Kapon , E. Pelucchi, S. Watanabe, A. Malko, M. H. Baier, K. Leifer, B. Dwir, F. Michelini, and M. A. Dupertuis

Physica E (ELSEVIER SCIENCE BV) 25 ( 2-3 ) 288-297 - 297 2004.08

Research paper (scientific journal), Joint Work, Accepted, ISSN 1386-9477

The formation mechanisms, structure and optoelectronic properties of Ga(In)As/(Al)GaAs quantum dot (QD) heterostructures grown by organometallic chemical vapor deposition on patterned (111)B GaAs substrates are reviewed. With this approach, it is possible to prepare high-quality semiconductor QDs whose position on a substrate is precisely controlled via a pre-growth photolithography step. The dots are formed at the center of an inverted, tetrahedral pyramid and are surrounded by distinct, low-dimensional barriers (quantum wires and quantum wells). Reproducible neutral and charged exciton states are observed in the QDs and are probed using microphotoluminescence and photon correlation spectroscopy. Single- and correlated-photon emissions are systematically detected from specific single and biexciton states. The emission spectra of light emitting diodes incorporating single pyramidal dots indicate the possibility of preferential injection of charge carriers via self-ordered wires connected to the dots. Finally, photonic crystal structures incorporating pyramidal QDs with controlled energy states are demonstrated. (C) 2004 Elsevier B.V. All rights reserved.

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Site-controlled quantum dots grown in inverted pyramids for photonic crystal applications

E. Pelucchi, S. Watanabe, K. Leifer, B. Dwir, E. Kapon.

Physica E (ELSEVIER SCIENCE BV) 23 ( 3-4 ) 476-481 - 481 2004.07

Research paper (scientific journal), Joint Work, Accepted, ISSN 1386-9477

We report on the growth and optical properties of various configurations of sub-micron pitch dense arrays of pyramidal quantum dots (QDs) grown by organometallic chemical vapour deposition on patterned substrates. We show that the effective growth rate of these QDs is influenced by the ratio between the free {1 1 1}B area and {1 1 1}A exposed facets surrounding them. This provides a powerful technique for engineering the energy level structure of ordered QD arrays by means of geometrical patterning of the growth template. Such technique should be particularly useful for applications in photonic crystals incorporating QDs with tailored absorption and/or emission properties. (C) 2004 Elsevier B.V. All rights reserved.

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Growth and optical characterization of dense arrays of site-controlled quantum dots grown in inverted pyramids

S. Watanabe, E. Pelucchi, B. Dwir, M. Baier, K. Leifer, E. Kapon.

Physica E (ELSEVIER SCIENCE BV) 21 ( 2-4 ) 193-198 - 198 2004.03

Research paper (scientific journal), Joint Work, Accepted, ISSN 1386-9477

We report on the growth and optical properties of dense arrays of single GaAs/AlGaAs quantum dot (QD) heterostructures with pitches as small as 300 nm. The samples were grown by organometallic chemical vapor deposition in dense inverted pyramids on {1 1 1}B GaAs substrate pre-patterned using electron beam lithography and wet chemical etching. The growth conditions such as deoxidation and growth temperatures, growth rates, and V/III ratio, had to be chosen quite differently from those employed with micron-size pyramids. Low-temperature micro-photoluminescence and cathodoluminescence spectra of the samples show distinct luminescence from the QDs with a linewidth of less than 1 meV and uniform emission energy for an ensemble of 900 QDs. The possibility of incorporating such QD arrays inside optical microcavity structures is also discussed. (C) 2003 Elsevier B.V. All rights reserved.

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Dense uniform arrays of site-controlled quantum dots grown in inverted pyramids

S. Watanabe, E. Pelucchi, B. Dwir, M. Baier, K. Leifer, E. Kapon.

Applied Physics Letters (AMER INST PHYSICS) 84 ( 15 ) 2907-2909 - 2909 2004.02

Research paper (scientific journal), Joint Work, Accepted, ISSN 0003-6951

We report on the growth and optical properties of homogeneous, dense arrays of site-controlled, single GaAs/AlGaAs quantum dot (QD) heterostructures with periodicities as small as 300 nm. The samples were grown by organometallic chemical vapor deposition on (111)B GaAs substrates containing dense inverted pyramid recess patterns prepared by electron beam lithography and wet chemical etching. Low-temperature microphotoluminescence spectra of the samples show distinct luminescence from the QDs with 1-3 meV linewidth. Low-temperature cathodoluminescence spectrally resolved images reveal uniform emission energy within an ensemble of 900 QDs. (C) 2004 American Institute of Physics.

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High uniformity of site-controlled pyramidal quantum dots grown on prepatterned substrates

M. Baier, S. Watanabe, E. Pelucchi, E. Kapon

Applied Physics Letters (AMER INST PHYSICS) 84 ( 11 ) 1943-1945 - 1945 2004.01

Research paper (scientific journal), Joint Work, Accepted, ISSN 0003-6951

We studied the uniformity of site-controlled, pyramidal InGaAs/AlGaAs semiconductor quantum dots (QDs) grown by organometallic chemical vapor deposition on prepatterned substrates. The inhomogeneous broadening of the QD ground state emission has been determined to be 7.6 meV by statistical single QD photoluminescence spectroscopy on a set of 120 individual QD structures. Taking into account the ground-to-excited state separation of 55 meV, such a small value has not yet been observed in QD systems where other growth mechanisms are employed. Moreover, a high reproducibility of the sharp QD emission features in the single exciton regime has been observed. (C) 2004 American Institute of Physics.

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Vertically polarized lasing and photoluminescence in a ridge quantum-wire laser

S. Watanabe, S. Koshiba, M. Yoshita, M. Baba, H. Sakaki, and H. Akiyama

Physical Review B 68 ( 19 ) 193304 2003.12

Research paper (scientific journal), Joint Work, Accepted

Imaging of emission patterns in a T-shaped quantum wire laser

Y. Takahashi, S. Watanabe, M. Yoshita, H. Itoh, Y. Hayamizu, H. Akiyama, L. N. Pfeiffer, and K. W. West,

Applied Physics Letters (AMER INST PHYSICS) 83 ( 20 ) 4089-4091 - 4091 2003.09

Research paper (scientific journal), Joint Work, Accepted, ISSN 0003-6951

Spatially and spectrally resolved microscopic images of spontaneous and stimulated emissions are imaged at the mirror facets of a GaAs T-shaped quantum wire laser with high uniformity. Laser emission from the one-dimensional ground state reveals a circular image located at the core of a T-shaped optical waveguide but significantly smaller in area than the low power spontaneous emission from the same waveguide. These images unambiguously allow assignment of all spontaneous and laser emissions to the wire ground state and respective intersecting wells in the structure. (C) 2003 American Institute of Physics.

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High-quality InxGa1-xAs/Al0.30Ga0.70As quantum dots grown in inverted pyramids

E. Pelucchi, M. Baier, Y. Ducommun, S. Watanabe, E. Kapon

Physica Status Solidi (b) (WILEY-V C H VERLAG GMBH) 238 ( 2 ) 233-236 - 236 2003.07

Research paper (scientific journal), Joint Work, Accepted, ISSN 0370-1972

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Polarization dependence of the optical interband transition defined by the spatial variation of the valence p-orbital Bloch functions in quantum wires

S. Watanabe, M. Yoshita, S. Koshiba, and H. Akiyama

Japanese Journal of Applied Physics (INST PURE APPLIED PHYSICS) 41 ( 10 ) 5924-5936 - 5936 2002.10

Research paper (scientific journal), Joint Work, Accepted, ISSN 0021-4922

We develope a method to express wave functions of hole states in semiconductor quantum wire (QWR) structures based on spatial variation of the valence p-orbital Bloch functions, to show how envelope wave functions relate to polarization-dependent interband transition. A wave function of a hole state is obtained solving the Schrodinger equation based on the 4 x 4 Luttinger Hamiltonian, and then recomposed by means of six bases of three p-orbital Bloch functions with two spin components. As a result, the hole wave function is expressed by six envelope wave functions for the six bases, Then. interband optical transition matrix elements with x-, y-, and z-polarizations are separately given by overlap integrals between envelope wave functions of holes for p(x), p(y), and p(z) orbitals and those of electrons. We also calculate the wave functions for a modeled ridge QWR structure with mirror symmetry as well as for an asymmetric structure, and discuss the polarization dependence of the optical transition.

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Lasing from a single-quantum wire

Y. Hayamizu, M. Yoshita, S. Watanabe, H. Akiyama, L. N. Pfeiffer, and K. W. West

Applied Physics Letters 81 ( 26 ) 4937-4939 2002.10

Research paper (scientific journal), Joint Work, Accepted

Transformation of GaAs (001)-(111)B facet structure by surface diffusion during molecular beam epitaxy

S. Koshiba, Y. Nakamura, T. Noda, S. Watanabe, H. Akiyama, and H. Sakaki

Journal of Crystal Growth 227-228 62-66 2001.07

Research paper (scientific journal), Joint Work, Accepted

Transformation of GaAs (001)-(111)B facet structure by surface diffusion during molecular beam epitaxy on patterned substrates

S Koshiba, Y Nakamura, T Noda, S Watanabe, H Akiyama, H Sakaki

JOURNAL OF CRYSTAL GROWTH (ELSEVIER SCIENCE BV) 227 62 - 66 2001.07

Accepted, ISSN 0022-0248

The mechanisms of molecular beam epitaxy growth on the mesa patterned substrates which results in the formation of sharp ridge structure were investigated. A simple model, based oil the scanning electron microscope measurements, nicely explains the time dependent growth rates as well as the change of the facet shape during the formation of ridge structure originating from the strong Ga atoms migration. The estimated number of atoms crossing the boundary changes according to the alternating facet shape and this indicates the instability in the facet growth. As a result, the mechanism of mesa growth was clarified, which becomes self-regulatory while the facet shape and/or the boundary condition vary as the growth progresses. (C) 2001 Elsevier Science B.V. All rights reserved.

Microscopy of electronic states contributing to lasing in ridge quantum-wire laser structure

S. Watanabe, S. Koshiba, M. Yoshita, H. Sakaki, B. Motoyoshi, and H. Akiyama

Applied Physics Letters (AMER INST PHYSICS) 75 ( 15 ) 2190-2192 - 2192 1999.10

Research paper (scientific journal), Joint Work, Accepted, ISSN 0003-6951

Distribution of electronic states inside ridge quantum-wire (QWR) laser cavity was investigated using spatially and spectrally resolved top-view photoluminescence (PL) imaging method. PL inhomogeneity in QWR has shown that the electronic states were perturbed by the fluctuation of vertical thickness in the scale of similar to nm, while the optical waveguide was not. The PL images of QWR were traced up to the lasing condition to examine the lasing origin. (C) 1999 American Institute of Physics. [S0003-6951(99)00541-0].

Selective molecular beam epitaxy (MBE) growth of GaAs AlAs ridge structures containing 10 nm scale wires and side quantum wells (QWs) and their stimulated emission characteristics

S. Koshiba, S. Watanabe, Y. Nakamura, M. Yamauchi, M. Yoshita, M. Baba, H. Akiyama, and H. Sakaki

Journal of Crystal Growth (ELSEVIER SCIENCE BV) 201-202 810-813 - 813 1999.05

Research paper (scientific journal), Joint Work, Accepted, ISSN 0022-0248

We report on the formation and optical characteristics of ridge waveguide structures containing quantum wires (QWRs). To form a waveguide? we adopted digital alloys, i.e. short-period superlattices (SLs), rapid changes of As fluxes using valved cells, growth temperatures (T-s), and introduction of growth interruptions. Stimulated emission from these ridge QWR laser structures was observed at temperatures 4.7-290 K by optical pumping. (C) 1999 Elsevier Science B.V. All rights reserved.

Stimulated emission in ridge quantum wire laser structures measured with optical pumping and microscopic imaging methods

S. Watanabe, S. Koshiba, M. Yoshita, H. Sakaki, B. Motoyoshi, and H. Akiyama

Applied Physics Letters (AMER INST PHYSICS) 73 ( 4 ) 511-513 - 513 1998.05

Research paper (scientific journal), Joint Work, Accepted, ISSN 0003-6951

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