arXiv subjects

Muneaki Hase

Publications and source records attributed to Muneaki Hase.

At least 19 recordsLinked to original sources

arXiv2026

Spectroscopic signatures of mode-selective electron-phonon coupling in transient reflectivity change on charge-density-wave TiSe$_2$

We report on time- and spectrally resolved pump-probe spectroscopy measurement on the charge-density wave (CDW) state of TiSe$_2$ at helium temperatures. The non-oscillatory component of the spectrally resolved signals exhibit a sign reversal near 1.5 eV, which was interpreted as a redshift of the optical transition energy originated from the interband transition between the Se $p$-based valence band to the Ti $d$-derived conduction band. Furthermore, the oscillatory components exhibit distinct spectral dependences between the optical phonons and the CDW-derived modes, suggesting the mode-selective coupling.

cond-mat.mtrl-sci↗

arXiv2026

Phonon scattering mechanisms in WTe$_2$ observed by ultrafast coherent phonon spectroscopy

Revealing the mechanisms of phonon scattering is crucial for understanding material properties such as transport characteristics and optical responses. It can be discussed by measuring the temperature dependence of the phonon energy and lifetime. To gain insight into these mechanisms in Weyl semimetal T$_d$-WTe$_2$, we investigated coherent phonons using time-resolved pump-probe spectroscopy in a wide temperature range from 4.6 to 300 K. The temperature dependence of the frequency and decay rate of the two high-frequency optical modes was described by the conventional anharmonic phonon-phonon scattering model. In contrast, the low-frequency mode at 2.4 THz exhibited anomalous behavior, which can be interpreted as the contribution of phonon-electron scattering.

cond-mat.mtrl-sci↗

arXiv2026

An ultrafast diamond nonlinear photonic sensor

The integration of light and materials technology is key to the creation of innovative sensing technologies. Sensing of electric and magnetic fields, and temperature with high spatio-temporal resolution is a critical task for the development of the next-generation of nanometer-scale quantum devices. Color centers in diamonds are attractive for potential applications owing to their characteristic quantum states, although they require metallic contacts for the introduction of external microwaves. Here, we build an ultrafast diamond nonlinear photonic sensor to assess the surface electric field; an electro-optic sensor based on nitrogen-vacancy centers in a diamond nanotip breaks the spatial-limit of conventional pump-probe techniques. The 10-fs near-infrared optical pulse modulates the surface electric field of a 2D transition metal dichalcogenide and we monitor the dynamics of the local electric field at nanometer-femtosecond spatio-temporal resolutions. Our nanoscopic technique will provide new horizons to the sensing of advanced nano materials.

physics.optics↗

arXiv2025

Role of interlayer shear phonons on lattice symmetry switching in the transition metal dichalcogenide WTe$_{2}$

Coherent phonon control using ultrashort pulse trains is the key to realizing structural phase transitions in solids by non-thermal pathways. By combining double-pulse excitation and time-resolved second harmonic generation techniques under high-density electronic excitation in a 2D layered material, WTe$_{2}$, we demonstrate that the lattice symmetry switching from the Weyl semimetallic T$_{d}$ to the semimetallic 1T$^{\prime}$ phases is independent of the amplitude of the coherent interlayer shear phonons after the arrival of the second pump pulse. This finding provides new insights into the mechanisms for symmetry switching that electronic excitation-driven shear sliding plays a dominant role.

cond-mat.mtrl-sci↗

arXiv2025

Plasmon-enhanced ultrafast time-resolved spectroscopy of NV-containing diamond

We investigated ultrafast nonlinear optical effects in nitrogen-vacancy (NV)-containing diamond which is in contact with a gold-coated blazed diffraction grating using a pump-probe reflectivity technique. The reflectivity change caused by optical Kerr effect and two-photon absorption was enhanced several times because of the electric field enhancement induced by the propagating surface plasmon (PSP). Furthermore, by performing measurements with varying the incident angle of the pump beam and numerical simulations of the electric field using the Finite Difference Time Domain method, signal enhancement due to the PSP was confirmed both experimentally and theoretically. This study paves the way for applications based on enhanced nonlinear optical effects in diamond.

physics.optics↗

arXiv2024

Cooperative dynamic polaronic picture of diamond color centers

Polarons can control carrier mobility and can also be used in the design of quantum devices. Although much effort has been directed into investigating the nature of polarons, observation of defect-related polarons is challenging due to electron-defect scattering. Here we explore the polaronic behavior of nitrogen-vacancy (NV) centers in a diamond crystal using an ultrafast pump-probe technique. A 10-fs optical pulse acts as a source of high electric field exceeding the dielectric breakdown threshold, in turn exerting a force on the NV charge distribution and polar optical phonons. The electronic and phononic responses are enhanced by an order of magnitude for a low density of NV centers, which we attribute to a combination of cooperative polaronic effects and scattering by defects. First-principles calculations support the presence of dipolar Fröhlich interaction via non-zero Born effective charges. Our findings provide insights into the physics of color centers in diamonds.

cond-mat.mtrl-sci↗

arXiv2024

Spin dependent bandgap renormalization and state filling effect in Bi$_2$Se$_3$ observed by ultrafast Kerr rotation

We investigate the ultrafast spin dynamics of the prototypical topological insulator $\mathrm{Bi_{2}Se_{3}}$ using time-resolved Kerr-rotation (polarization-change) measurements across near-infrared wavelengths. The Kerr-rotation angle $Δθ_{K}$ of $\mathrm{Bi_{2}Se_{3}}$ was found to significantly depend on photon energy around a resonance transition ($\sim 1.0\ \mathrm{eV}$) of bulk states, as well as the ellipticity of the pump light, in the presence of spin excitation. The observed photon-energy dependence of $Δθ_{K}$ can be well simulated by assuming spin-dependent refractive-index changes in the presence of band-gap renormalization and state-filling effect upon photoexcitation. Our study delivers comprehensive insights into the opto-spintronic properties of bulk $\mathrm{Bi_{2}Se_{3}}$ and the fundamental physical processes underlying polarization changes. These findings are expected to be crucial in developing ultrafast magneto-optical memory devices, which can perform read-and-write operations in the Terahertz regime.

cond-mat.mtrl-sci↗

arXiv2023

Ultrafast melting of charge-density wave fluctuations at room temperature in ${1T-TiSe_2}$ monitored under non-equilibrium conditions

We investigate the ultrafast lattice dynamics in ${1T-TiSe_2}$ using femtosecond reflection pump-probe and pump-pump-probe techniques at room temperature. The time-domain signals and Fourier-transformed spectra show the $A_{1g}$ phonon mode at 5.9 THz. Moreover, we observe an additional mode at $\approx$ 3 THz, corresponding to the charge-density wave (CDW) amplitude mode, which is generally visible below T$_c \approx 200\ $K. We argue that the emergence of the CDW amplitude mode at room temperature can be a consequence of fluctuations of order parameters, based on the additional experiment using the pump-pump-probe technique, which exhibited suppression of the AM signal within the ultrafast time scale of $\sim$ 0.5 ps.

cond-mat.mtrl-sci↗

arXiv2023

Terahertz emission from transient currents and coherent phonons in layered MoSe$_2$ and WSe$_2$

Terahertz (THz) time-domain emission spectroscopy was performed on layered 2H-MoSe2 and 2H-WSe2. The THz emission shows an initial cycle attributed to surge currents and is followed by oscillations attributed to coherent interlayer phonon modes. To obtain the frequencies of the interlayer vibrations, analysis of the THz emission waveforms were performed, separating the two contributions to the total waveform. Results of the fitting show several vibrational modes in the range of 5.87 to 32.75 cm-1 for the samples, attributed to infrared-active interlayer shear and breathing modes. This study demonstrates that THz emission spectroscopy provides a means of observing these low frequency vibrational modes in layered materials.

cond-mat.mtrl-sci↗

arXiv2023

Detection of Mechanical Deformation Induced by Ultrafast Laser Irradiation upon a Metallic Cantilever

In this work, we systematically investigated the ultrafast optical properties of aluminum (Al) thin films on silicon cantilevers using a microscopic femtosecond optical pump-probe technique to explore the effect of light irradiation upon cantilevers while considering radiation pressure and photothermal effects. The ultrafast laser pulses used for the study were less than 30 fs in pulse duration and 830 nm in wavelength, and the photon energy (1.49 eV) of the light pulses is close to the interband transition threshold (ITT) of Al. Therefore, the change in ITT due to the strain of the cantilevers induced by the light irradiation is detected through the change in the transient reflectivity, which is dominated by the thermalized (Th) electron signal. We uncovered the position dependency of the transient reflectivity change and the Th electronic signal amplitude of the 100 nm-thick Al films on 160 $μ$m, 200 $μ$m and 240 $μ$m-length cantilevers, and these results are in excellent agreement with two temperature model-based curve fits. Furthermore, to understand the effect of light irradiation, we derived equations for the position-dependent radiation pressure effect and the photothermal effect, and demonstrated that thermal expansion-induced changes in ITT dominate the position dependence of the signal intensity. Our findings offer avenues for exploring strain effects on ultrafast properties and applications for ultrafast scanning probe microscopy.

physics.optics↗

arXiv2023

Highly-efficient third-harmonic generation from ultrapure diamond crystals

We report on a direct generation of efficient and wavelength-tunable third-harmonic generation (THG) from ultrapure electronic-grade (EG) diamond crystals. Under an ultrafast infrared excitation at 1280 nm, the considerably high optical conversion efficiency of ~ 0.7% at a THG wavelength of 427 nm is obtained, and the THG signal can be tuned over ultra-broadband range from 420 to 730 nm. We argue that the THG efficiency is originating from minimum absorption loss and phase-matching conditions in EG diamond. Enhanced THG from EG diamond crystal represents a new paradigm for establishing efficient diamond-based frequency converters, quantum sensing, and quantum communications platforms.

physics.optics↗

arXiv2022

Ultrafast dynamics of coherent phonons and phonon-polaritons in lithium niobate crystals

This study investigates the ultrafast dynamics of coherent phonons and phonon-polaritons in lithium niobate (LiNbO$_{3}$) crystals using reflective pump-probe spectroscopy with 25-fs time resolution. In addition to several coherent optical phonon modes, the electro-optic sampling measurements explored the coexistence of phonon-polariton E-modes near 3.8 and 14.9 THz, which agrees considerably with the theoretical phonon-polariton dispersion curves. We also discovered that a time lag ($Δ$t $\approx$ 0.2 ps) between the coherent optical phonon and phonon-polariton originates from the coupling time for propagating phonon-polariton. Our findings provide a possible application of LiNbO$_{3}$ for ultrafast electro-optic phonon modulators with bandwidths greater than 10 THz.

cond-mat.mtrl-sci↗

arXiv2022

Ultrafast opto-magnetic effects induced by nitrogen-vacancy centers in diamond crystals

The current generation of quantum sensing technologies using color centers in diamond crystals is primarily based on the principle that the resonant microwave frequency of the luminescence between quantum levels of the nitrogen-vacancy (NV) center varies with temperature, electric and magnetic fields. This principle enables us to measure, for instance, magnetic and electric fields, as well as local temperature with nanometer resolution in conjunction with a scanning probe microscope (SPM). However, the time resolution of conventional quantum sensing technologies has been limited to microseconds due to the limited luminescence lifetime. Here, we investigate ultrafast opto-magnetic effects in diamond crystals containing nitrogen-vacancy NV centers to improve the time resolution of quantum sensing to sub-picosecond time scales. The spin ensemble from diamond NV centers induces an inverse Cotton-Mouton effect (ICME) in the form of a sub-picosecond optical response in a femtosecond pump-probe measurement. The helicity and quadratic power dependence of the ICME can be interpreted as a second-order opto-magnetic effect in which ensembles of NV electron spins act as a source for the ICME. The results provide fundamental guidelines for enabling high-resolution spatial-time quantum sensing technologies when combined with SPM techniques.

quant-ph↗

arXiv2022

Temperature-dependent second-harmonic generation from color centers in diamond

Under infrared ultrashort pulse laser stimulation, we investigated temperature-dependent second-harmonic generation (SHG) from nitrogen-vacancy (NV)-introduced bulk diamond. The SHG intensity decreases in the temperature range of 20-300 $^{\circ}C$, due to phase mismatching caused by refractive index modification. We discovered that optical phonon scattering outperforms acoustic phonon one in NV diamond by fitting the temperature dependence of SHG intensity using a model based on the bandgap change via the deformation potential interaction. This study presents an efficient and viable way for creating diamond-based nonlinear optical temperature sensing.

physics.optics↗

arXiv2022

Second-Harmonic Generation in Bulk Diamond Based on Inversion Symmetry Breaking by Color Centers

Breaking inversion symmetry in solids plays a central role in nonlinear optics because it can change material properties, such as producing even-order nonlinear optical (NLO) effects. Although a centrosymmetric diamond has been developed as a photonic platform, including waveguides and light sources, the NLO effects in bulk diamonds are limited to the third-order. Thus, exploiting more powerful second-order NLO effects, such as second harmonic generation (SHG), are still challenging. Here we explore symmetry-breaking-induced second-order NLO effects in bulk diamonds using the color center, nitrogen-vacancy center. Exciting with ultrashort laser pulses, SHG and third-harmonic generation (THG) are simultaneously observed at the same time, exhibiting characteristic intensity patterns, depending on both the excitation fluence and the angle of light polarization. We uncovered that SHG serves as the source for THG by the cascading process. Our findings will offer NLO effect-based quantum sensing by diamond color centers, such as imaging of the electromagnetic field by electro-optic effects on the nanofemto scale.

physics.optics↗

arXiv2022

Photo-induced Tellurium segregation in $\mathrm{MoTe_2}$

We report on the coherent phonon spectroscopy of polymorphic $\mathrm{MoTe_2}$ single crystals using a femtosecond-pulsed laser to investigate the relationship between structural phase transitions and photo-thermal effects induced by high-density laser excitation. Even when a femtosecond pulsed laser was used, which generally induces fewer heat accumulation effects than the case for a CW laser, tellurium segregation was observed in coherent phonon spectra with increasing excitation density, while no structural phase transition among polymorphs of $\mathrm{MoTe_2}$ was observed. The use of higher photon-energy excitation, however, was found to suppress tellurium segregation.

cond-mat.mtrl-sci↗

arXiv2021

Effect of heteroepitaxial growth on LT-GaAs: ultrafast optical properties

Epitaxial low temperature grown GaAs (LT-GaAs) on silicon (LT-GaAs/Si) has the potential for terahertz (THz) photoconductive antenna applications. However, crystalline, optical and electrical properties of heteroepitaxial grown LT-GaAs/Si can be very different from those grown on semi-insulating GaAs substrates (reference). In this study, we investigate optical properties of an epitaxial grown LT-GaAs/Si sample, compared to a reference grown under the same substrate temperature, and with the same layer thickness. Anti-phase domains and some crystal misorientation are present in the LT-GaAs/Si. From coherent phonon spectroscopy, the intrinsic carrier densities are estimated to be ~$10^{15}$ cm$^{-3}$ for either sample. Strong plasmon damping is also observed. Carrier dynamics, measured by time-resolved THz spectroscopy at high excitation fluence, reveals markedly different responses between samples. Below saturation, both samples exhibit the desired fast response. Under optical fluences $\geq$ 54 $μ$ J/cm$^2$, the reference LT-GaAs layer shows saturation of electron trapping states leading to non-exponential behavior, but the LT-GaAs/Si maintains a double exponential decay. The difference is attributed to the formation of As-As and Ga-Ga bonds during the heteroepitaxial growth of LT-GaAs/Si, effectively leading to a much lower density of As-related electron traps.

cond-mat.mtrl-sci↗

arXiv2021

Ultrafast scattering dynamics of coherent phonons in Bi$_{1-x}$Sb$_{x}$ in the Weyl semimetal phase

We investigate ultrafast phonon dynamics in the Bi$_{1-x}$Sb$_{x}$ alloy system for various compositions $x$ using a reflective femtosecond pump-probe technique. The coherent optical phonons corresponding to the A$_{1g}$ local vibrational modes of Bi-Bi, Bi-Sb, and Sb-Sb are generated and observed in the time domain with a few picoseconds dephasing time. The frequencies of the coherent optical phonons were found to change as the Sb composition $x$ was varied, and more importantly, the relaxation time of those phonon modes was dramatically reduced for $x$ values in the range 0.5--0.8. We argue that the phonon relaxation dynamics are not simply governed by alloy scattering, but are significantly modified by anharmonic phonon-phonon scattering with implied minor contributions from electron-phonon scattering in a Weyl-semimetal phase.

cond-mat.mtrl-sci↗