arXiv subjects

Shinichi Watanabe

Publications and source records attributed to Shinichi Watanabe.

At least 19 recordsLinked to original sources

arXiv2026

Motion-Tracking Terahertz Time-Domain Spectroscopy Enabled by Time-Programmable Frequency Combs for Moving-Object Sensing

Terahertz time-domain spectroscopy (THz-TDS) provides direct access to both spectral information and time-of-flight features, making it attractive for dynamic sensing. However, conventional high-speed THz-TDS methods typically rely on a fixed delay trajectory, limiting their ability to selectively acquire only the relevant temporal window while tracking target motion. Here, we demonstrate motion-tracking THz-TDS based on time-programmable frequency combs (TPFCs), in which the THz measurement window is actively locked to a moving time-domain waveform. By continuously modulating the phase-lock set point of one TPFC, we perform real-time apodized acquisition of a specific temporal window centered on the main THz peak at rates up to 308 Hz. Simultaneously, the residual peak position within the window is detected in real time and fed back to the other comb, enabling the measurement window to follow target displacements exceeding the original acquisition window. The feedback command together with the residual peak position further enables reconstruction of the relative target displacement. As a proof-of-concept demonstration, we use a moving gold mirror to simulate respiration- and heartbeat-related motion superimposed on a much larger displacement representing body motion, and successfully resolve the two vital-sign-related components from the measured displacement. This work extends arbitrary-delay control with TPFCs to THz-TDS and establishes a motion-compensated spectroscopy platform for simultaneous displacement tracking and THz waveform acquisition.

physics.optics↗

arXiv2026

Ultralong pump-probe movies of magnon and phonon dynamics from ultrafast generation to microsecond relaxation

The long lifetimes of magnons and phonons make them attractive for information-processing devices, highlighting the importance of visualizing their spatiotemporal dynamics from generation through relaxation. Ultrafast pump-probe spectroscopy is a powerful tool for investigating their early-stage dynamics after impulsive excitation; however, their long-lived nature makes it challenging to comprehensively track their evolution across all relevant time scales while maintaining sufficient temporal resolution. Here, we demonstrate spatiotemporal tracking of magnon and phonon dynamics over more than seven orders of magnitude in time, from 500 femtoseconds to 20 microseconds, using $4 \times 10^7$ sampled time points enabled by the highly precise time base of optical frequency combs. The resulting spatiotemporal movie, consisting of $4.5 \times 10^{5}$ frames, captures their generation, coherent motion, propagation, and relaxation, providing a powerful platform for exploring their full dynamical evolution.

physics.optics↗

arXiv2026

Quantitative cavity-enhanced photothermal dynamics in TMDC-integrated ultrahigh-Q microcavities

We investigate photothermal effects in monolayer transition metal dichalcogenides (TMDCs) integrated with an ultrahigh-Q silica microcavity. Launching a continuous-wave laser into a cavity resonance enables controlled intracavity heating, allowing direct observation of excitonic photoluminescence (PL) modulation. A distinct redshift of the PL peak energy is observed as the pump wavelength is tuned across resonance. This behavior is quantitatively reproduced by a temperature-dependent bandgap model that combines the Varshni relation with the thermo-optic response of the microcavity, from which the local temperature rise can be estimated. We further find that PL collected through a fiber waveguide exhibits spectral and temporal characteristics markedly different from free-space emission, indicating selective coupling of the microcavity to specific excitonic channels. These results provide a quantitative framework for understanding photothermal effects in TMDC-microcavity hybrid systems and offer a versatile approach for all-optical control and probing of thermal states in integrated nanophotonic devices.

physics.optics↗

arXiv2026

Ultrafast acoustic modulation of second-harmonic generation in monolayer transition metal dichalcogenides

High-speed modulation and deterministic control of optical nonlinear processes in nanomaterials are essential for realizing future nanoscale optoelectronic devices. Applying strain is a ubiquitous and versatile approach to deform atomically thin materials, allowing direct modification of their electronic and optical properties. Yet, strain engineering of nonlinear processes has so far relied predominantly on static approaches, which inherently limit modulation speed, reproducibility, and device scalability. Here, we demonstrate ultrafast acoustic modulation of second-harmonic (SH) generation in monolayer transition metal dichalcogenides using surface acoustic waves (SAWs). By employing a fully phase-synchronized SH measurement combined with stroboscopic surface displacement detection, we directly visualize dynamic SH modulation at a frequency of 226 MHz. Moreover, theoretical modeling and determination of photoelastic coefficients enable quantitative extraction of the SAW-induced dynamic strain. Our results establish a direct link between acoustic fields and optical nonlinearities, providing a robust platform for dynamic strain engineering in two-dimensional nanophotonic devices.

physics.app-ph↗

arXiv2026

Spatial mapping of quantum-dot dynamics across multiple timescales at low temperature using remote asynchronous optical sampling

Quantum dots (QDs) offer significant potential for applications in quantum information and optoelectronic devices; however, conventional time-resolved spectroscopy cannot generally simultaneously extract both long-lived relaxation dynamics and short-lived quantum beats from ensemble measurements. This limitation arises from the inherent trade-off between temporal resolution and total acquisition time. Here, we demonstrate that asynchronous optical sampling based on a fiber-delivered frequency comb enables simultaneous observation of QD dynamics across multiple timescales. By integrating a galvanometric scanner, we achieve spatial mapping over a $1 \times 1$-\si{\milli\meter}$^2$ area at 441 discrete points in 30.1~min, a measurement that would otherwise require more than 12~days. At each location, both quantum beats and relaxation lifetimes are resolved, giving physical insights into QD ensembles that were previously inaccessible and paving the way for rapid feedback in device fabrication.

physics.optics↗

arXiv2024

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

Dissipative Kerr soliton microcombs have inspired various intriguing applications such as spectroscopy, ranging, telecommunication, and high purity microwave generation. Mechanically actuated soliton microcombs provide enhanced controllability and flexibility for Kerr solitons, thus enabling technological progress to be made on such practical applications. Here, we present architectures for coherent dual-comb techniques and ultralow-noise microwave generation by exploiting the mechanical actuation of ultrahigh-Q crystalline microresonators. By unifying a pump laser, we demonstrate highly coherent dual-soliton combs using distinct resonators with slightly different repetition rates. We also report significant phase noise reduction achieved by directly generating Kerr solitons from a sub-Hz linewidth ultrastable laser. This study paves the way for further advancements in a wide variety of applications based on Kerr soliton microcombs.

physics.optics↗

arXiv2024

Fluctuations in Spin Dynamics Excited by Pulsed Light

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.

cond-mat.mes-hall↗

arXiv2023

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

Surface acoustic waves (SAWs) coupled to magnons have attracted much attention because they allow for the long-range transport of magnetic information which 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 on 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.

cond-mat.mtrl-sci↗

arXiv2023

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

We report on an Er-doped fiber (EDF)-laser-based dual-comb system that allows us to perform triggerless asynchronous optical sampling (ASOPS) pump-probe measurements of ultrafast demagnetization and spin precession in magnetic materials. Because the oscillation frequencies of the two frequency-comb light sources are highly stabilized, the pulse-to-pulse timing jitter is sufficiently suppressed and data accumulation without any trigger signals is possible. To effectively induce spin precession in ferromagnetic thin films, the spectral bandwidth of the output of one of the EDF frequency comb sources is broadened by a highly nonlinear fiber and then amplified at a wavelength of about 1030 nm by a Yb-doped fiber amplifier. The output of the other frequency comb source is converted to about 775 nm by second harmonic generation. We used this system to observe the ultrafast demagnetization and spin precession dynamics on the picosecond and nanosecond time scales in a permalloy thin film. This time-domain spectroscopy system is promising for the rapid characterization of spin-wave generation and propagation dynamics in magnetic materials.

physics.app-ph↗

arXiv2022

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

By using two mutually phase-locked optical frequency combs with slightly different repetition rates, we demonstrate asynchronous optical-sampling terahertz time-domain spectroscopy (ASOPS THz-TDS) without using any trigger signals or optical delay lines. Due to a tight stabilization of the repetition frequencies, it was possible to accumulate the data over 48 minutes in a triggerless manner without signal degradation. The fractional frequency stability of the measured terahertz signal is evaluated to be $\sim$8.0$\times$10$^{- 17}$ after 730 s. The frequency accuracy of the obtained terahertz spectrum is ensured by phase-locking the two frequency combs to a frequency standard. To clarify the performance of our system, we characterized the absorption line of water vapor around 0.557 THz. The good agreement of the measured center frequency and linewidth with the values predicted from the HITRAN database verifies the suitability of our ASOPS THz-TDS system for precise measurements.

physics.optics↗

arXiv2022

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

We report on the measurement of terahertz electric-field vector waveforms by using a system that contains no mechanical moving parts. It is known that two phase-locked femtosecond lasers with different repetition rates can be used to perform time-domain spectroscopy without using a mechanical delay stage. Furthermore, an electro-optic modulator can be used to perform polarization measurements without rotating any polarizers or waveplates. We experimentally demonstrate the combination of these two methods and explain the analysis of data obtained by such a system. Such a system provides a robust platform that can promote the usage of polarization-sensitive terahertz time-domain spectroscopy in basic science and practical applications. For the experimental demonstration, we alter the polarization of a terahertz wave by a polarizer.

physics.optics↗

arXiv2022

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

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 and an ultra-wide dynamic range. The thickness and the refractive index at 193.414 THz of a silicon wafer determined by this method are 0.52047(3) mm and 3.4756(3), respectively, without any prior knowledge of the refractive index.

physics.optics↗

arXiv2021

Loading ultracold atoms onto nonlinear Bloch states and soliton states in bichromatic lattices

We simulate and analyze an experimental method of loading interacting ultracold atoms onto nontrivial quantum states such as nonlinear Bloch wave and soliton solutions in a 1-dimensional bichromatic lattice. Of standard bands, inverted bands, and bands with Dirac-like points permitted by a bichromatic lattice, we consider the case of an inverted band and examine the loading process in terms of nonlinear Bloch waves formed by an aggregate of ultracold atoms described by the mean-field model. Specifically, we solved the Gross-Pitaevskii equation numerically and found an appropriate standing wave-pulse sequence for the inverted band, which sequence proved to be a suitable protocol for producing soliton solutions. In addition, we examined the effect of an external potential and dynamical instabilities for the post-loading process. We also provide an appropriate data set for future experimental realization of our findings.

cond-mat.quant-gas↗

arXiv2018

Fast and selective inter-band transfer of ultracold atoms in bichromatic lattices permitting Dirac points

An experimental group at Beijing[Yueyang Zhai, ${\it et. al.}$, Phys. Rev. A ${\bf 87}$, 063638 (2013)] introduced the method of standing-wave pulse sequence for efficiently preparing ultracold bosonic atoms into a specific excited band in a 1-dimensional optical lattice. Here, we report a theoretical extension of their work to the problem of 1-dimensional bichromatic superlattice. We find that varying the lattice parameters leads to the so-called Dirac point where a pair of excited bands crosses. This paper thus discusses ${\it simultaneously}$ the efficient excitation of the wave packet to the proximity of the Dirac point and its subsequent dynamics in the force field of a parabolic trap. With the aid of a toy model, we theoretically unravel the mechanism of the efficient preparation, and then numerically explore optimal pulse-sequence parameters for a realistic situation. We find an optimized sequence of a bichromatic optical lattice that excites more than 99% of the atoms to the 1st and 2nd excited bands within 100 $μ$s without the harmonic trap. Our main finding is that the system permitting the Dirac point possesses a region of parameters where the excited energy bands become nearly parabolic, conducive to robust coherence and isochronicity. We also provide an appropriate data set for future experimentation, including effects of the atom-atom interaction by way of the mean-field nonlinear term.

cond-mat.quant-gas↗

arXiv2017

A significantly stable mode of the ultracold atomic wave packet in amplitude modulated parabolic optical lattices

We show that a conspicuous wave packet of ultracold noninteracting Bosonic atoms emerges in a 1-dimensional parabolic optical lattice as in the setup of the Aarhus experiment [P. L. Pedersen ${\it et}$ ${\it al.}$, Phys. Rev. A ${\bf 88}$, 023620 (2013)], given the lattice height is harmonically modulated with a particular amplitude at a resonant frequency. We show that this wave packet, coined "${\it 4bandPWP}$" here, executes stable time-wise periodic motion for infinitely long time. We apply the Floquet theory to analyze the parameter dependence of ${\it 4bandPWP}$ in detail. Our analysis shows that it consists mainly of two principal Floquet eigenstates of the periodically driven Hamiltonian. The informative Husimi representation yields temporal slices of the phase space of ${\it 4bandPWP}$, visually identifying moments where the inter-band transitions take place. The provided data should aid the experiment in locating ${\it 4bandPWP}$.

quant-ph↗

arXiv2016

Dynamics of fermions in an amplitude modulated lattice

We study dynamics of fermions loaded in an optical lattice with a superimposed parabolic trap potential. In the recent Hamburg experiments [J.Heinze et.al., Phys. Rev. Lett. 110, 085302 (2013)] on quantum simulation of photoconductivity, a modulation pulse on the optical lattice transferred part of the population of the lowest band to an excited band, leaving a hole in the particle distribution of the lowest band. Subsequent intricate dynamics of both excited particles and holes can be explained by a semiclassical approach based on the evolution of Wigner function. Here we provide a more detailed analysis of the dynamics taking into account the dimensionality of the system and finite temperature effects, aiming at reproducing experimental results on longer timescales. A semiclassical wave packet is constructed more accurately than in the previous theory. As a result, semiclassical dynamics indeed reproduces experimental data and full quantum numerical calculations with much better accuracy. In particular, fascinating phenomenon of collapse and revival of holes is investigated in a more detail. We presume the experimental setup can be used for deeper exploration of nonlinear waves in fermionic gases.

cond-mat.quant-gas↗

arXiv2014

Single-particle Analysis of Non-interacting Ultracold Bosons in Amplitude Modulated Parabolic Optical Lattice

Ultracold atoms in the combined potential of a parabolic trap and an optical lattice is considered a promising tool for coherent manipulation of matter wave packets. The recent Aarhus experiment[P. L. Pedersen et al., Phys. Rev. A. 88, 023620 (2013)] produced wave packets by applying the optical lattice's amplitude modulation to a Bose-Einstein condensate (BEC) of $^{87}$Rb. The present paper renders a theoretical account with single-particle analysis of this experimental production of the wave packets and their subsequent time-evolution. We focus on the one-dimensional non-interacting bosonic system as a fundamental starting point for accurate quantum analysis and for further investigation of similar experiments. We show that a simple Rabi-oscillation model gives a good description of the wave packet production in terms of the inter-band transition while the first-order perturbation theory proves inadequate, that is the recent experiment already reached the realm of high-order couplings. As a natural extension, we demonstrate enhancement of the wave packet production by the two-step Rabi-oscillation method using either single frequency or dual frequencies. We assess the high-order Bragg reflection and Landau-Zener transition at a band gap with the aid of rigorous quantum time-propagation as well as the semi-classical theory exploited earlier by the Hamburg experiment [J. Heinze et al., PRL 107, 135303(2011)]. Complicated reflections and splittings of the wave packet during free evolution may be largely attributed to the intertwining of these two effects.

cond-mat.quant-gas↗

arXiv2013

Stochastic and equilibrium pictures of the ultracold FFR molecular conversion rate

The ultracold molecular conversion rate occurring in an adiabatic ramp through a Fano-Feshbach resonance is studied and compared in two statistical models. One model, the so-called stochastic phase space sampling (SPSS)[E.Hodby et al., PRL.94 120402(2005)] evaluates the overlap of two atomic distributions in phase space by sampling atomic pairs according to a phase-space criterion. The other model, the chemical equilibrium theory(ChET)[S.Watabe and T.Nikuni, PRA.77 013616(2008)] considers atomic and molecular distributions in the limit of the chemical and thermal equilibrium. The present study applies SPSS and ChET to a prototypical system of K+K K2 in all the symmetry combinations, namely Fermi-Fermi, Bose-Bose, and Bose-Fermi cases. To examine implications of the phase-space criterion for SPSS, the behavior of molecular conversion is analyzed using four distinct geometrical constraints. Our comparison of the results of SPSS with those of ChET shows that while they appear similar in most situations, the two models give rise to rather dissimilar behaviors when the presence of a Bose-Einstein condensate (BEC) strongly affects the molecule formation.

physics.atom-ph↗