Papers - Watanabe, Shinichi

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

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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

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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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We develop a method to precisely determine the polarization state of each comb tooth by a combination of dual-comb spectroscopy and rotating compensator polarimetry. This method is useful for measuring birefringent and dichroic materials.

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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

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Interpretation of the vector waveform detected by using the electro-optic (EO) sampling method in the terahertz time-domain spectroscopy is presented, especially when there is a velocity difference between the elliptically-polarized terahertz pulse and the optical probe pulse in the EO crystal. The formulation can be used to interpret the time-domain vector waveform detected by the EO sampling method using the symmetry of the EO crystal without resorting to the polarizers.

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

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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

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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

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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

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We experimentally show that continuously changing polarity of an elliptically-polarized terahertz electric-field can be used to image a height profile of semiconductors, metals, and their hybrid samples with a depth resolution of similar to 1 mu m.

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

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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

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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

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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