Details of a Researcher - Watanabe, Shinichi

Papers - Watanabe, Shinichi

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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

<p>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.</p>

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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