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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