Details of a Researcher - Watanabe, Shinichi

Papers - Watanabe, Shinichi

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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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