arXiv subjects

Tomohito Otobe

Publications and source records attributed to Tomohito Otobe.

25 records · Page 2Linked to original sources

arXiv2021

Valley polarization control in WSe2 monolayer by a single-cycle laser pulse

Abstract The valley degree of freedom in two-dimensional materials provides an opportunity to extend the functionalities of valleytronics devices. Very short valley lifetimes demand the ultrafast control of valley pseudospin. Here, we theoretically demonstrate the control of valley pseudospin in WSe2 monolayer by single-cycle linearly polarized laser pulse. We use the asymmetric electric field controlled by the carrier-envelope phase (CEP) to make the valley polarization between K and K'-point in the Brillouin zone (BZ). Time-dependent density functional theory with spin-orbit interaction reveals that no valley asymmetry and its CEP dependence is observed within the linear-optical limit. In the nonlinear-optical regime, linearly polarized pulse induces a high degree of valley polarization and this polarization is robust against the field strength. Valley polarization strongly depends and oscillates as a function of CEP. The carrier density distribution forms nodes as the laser intensity increases, our results indicate that the position of the carrier density in the BZ can be controlled by the laser intensity. From the analysis by the massive Dirac Hamiltonian model, the nodes of the carrier density can be attributed to the Landau-Zener-Stückelberg interference of wave packets of the electron wave function.

physics.optics↗

arXiv2021

Semi-classical description of electron dynamics in extended systems under intense laser fields

We propose a semi-classical approach based on the Vlasov equation to describe the time-dependent electronic dynamics in a bulk simple metal under an ultrashort intense laser pulse. We include in the effective potential not only the ionic Coulomb potential and mean-field electronic Coulomb potential from the one-body electron distribution but also the exchange-correlation potential within the local density approximation (LDA). The initial ground state is obtained by the Thomas-Fermi model. To numerically solve the Vlasov equation, we extend the pseudo-particle method, previously used for nuclei and atomic clusters, to solids, taking the periodic boundary condition into account. We apply the present implementation to a bulk aluminum (FCC) conventional unit cell irradiated with a short laser pulse. The optical conductivity, refractive index, extinction coefficient, and reflectivity as well as energy absorption calculated with the Vlasov-LDA method are in excellent agreement with the results by the time-dependent density functional theory and experimental references.

physics.plasm-ph↗

arXiv2020

Strong-Field Electron Dynamics in Solids

Solid-state materials have recently emerged as a new stage of strong-field physics and attosecond science. The mechanism of the electron dynamics driven by an ultrashort intense laser pulse is under intensive discussion. Here we theoretically discuss momentum-space strong-field electron dynamics in graphene and crystalline dielectrics and semiconductors. First, within massless Dirac fermion and tight-binding models for graphene, we rigorously derive intraband displacement and interband transition, which form the basis for understanding solid-state strong-field physics including high-harmonic generation (HHG). Then, based on the time-dependent Schrödinger equation for a one-dimensional model crystal, we introduce a simple, multiband, momentum-space three-step model that incorporates intraband displacement, interband tunneling, and recombination with a valence band hole. We also analyze how the model is modified by electron-hole interaction. Finally, actual three-dimensional materials are investigated. We present a time-dependent density-matrix method whose results for HHG are compared with experimental measurement results. Moreover, we describe the dynamical Franz-Keldysh effect in femtosecond time resolution, i.e., the time-dependent modulation of a dielectric function under an intense laser field, using a real-time time-dependent density functional theory.

cond-mat.mtrl-sci↗

arXiv2019

Macroscopic electron-hole distribution in silicon and cubic silicon carbide

Electron excitations at silicon and 3C-SiC surfaces caused by an intense femtosecond laser pulse can be calculated by solving the time-dependent density functional theory and the Maxwell's equation simultaneously. The energy absorption, carrier density, and electron-hole quasi-temperatures decrease exponentially in 100 nm from the surface. The electron and hole quasi-temperatures have finite values even at large distances from the surface because of a specific photo-absorption channel. Although the quasi-temperature in the silicone shows smooth exponential descrease, 3C-SiC shows stepwise decrease because of the change of concerning bands. The quasi-temperature depends not only on the excitation process, i.e., tunnel and multi-photon absorption, but also on the band structure significantly.

physics.app-ph↗

arXiv2018

Deformation of an inner valence molecular orbital in ethanol by an intense laser field

Valence molecular orbitals play a crucial role in chemical reactions. Here we reveal that an intense laser field deforms an inner valence orbital (10a') in the ethanol molecule. We measure the recoil-frame photoelectron angular distribution (RFPAD), which corresponds to the orientation dependence of the ionization probability of the orbital, using photoelectron-photoion coincidence momentum imaging with a circularly polarized laser pulse. Ab initio simulations show that the orbital deformation depends strongly on the laser field direction, and that the measured RFPAD cannot be reproduced without taking the orbital deformation into account. Our findings suggest that the laser-induced orbital deformation occurs prior to electron emission on a sub-optical-cycle time scale.

physics.chem-ph↗

arXiv2018

SALMON: Scalable Ab-initio Light-Matter simulator for Optics and Nanoscience

SALMON (Scalable Ab-initio Light-Matter simulator for Optics and Nanoscience, http://salmon-tddft.jp) is a software package for the simulation of electron dynamics and optical properties of molecules, nanostructures, and crystalline solids based on first-principles time-dependent density functional theory. The core part of the software is the real-time, real-space calculation of the electron dynamics induced in molecules and solids by an external electric field solving the time-dependent Kohn-Sham equation. Using a weak instantaneous perturbing field, linear response properties such as polarizabilities and photoabsorptions in isolated systems and dielectric functions in periodic systems are determined. Using an optical laser pulse, the ultrafast electronic response that may be highly nonlinear in the field strength is investigated in time domain. The propagation of the laser pulse in bulk solids and thin films can also be included in the simulation via coupling the electron dynamics in many microscopic unit cells using Maxwell's equations describing the time evolution of the electromagnetic fields. The code is efficiently parallelized so that it may describe the electron dynamics in large systems including up to a few thousand atoms. The present paper provides an overview of the capabilities of the software package showing several sample calculations.

physics.comp-ph↗

arXiv2013

First-principles simulation of the optical response of bulk and thin-film α-quartz irradiated with an ultrashort intense laser pulse

A computational method based on a first-principles multiscale simulation has been used for calculating the optical response and the ablation threshold of an optical material irradiated with an ultrashort intense laser pulse. The method employs Maxwell's equations to describe laser pulse propagation and time-dependent density functional theory to describe the generation of conduction band electrons in an optical medium. Optical properties, such as reflectance and absorption, were investigated for laser intensities in the range $10^{10} \, \mathrm{W/cm^{2}}$ to $2 \times 10^{15} \, \mathrm{W/cm^{2}}$ based on the theory of generation and spatial distribution of the conduction band electrons. The method was applied to investigate the changes in the optical reflectance of $α$-quartz bulk, half-wavelength thin-film and quarter-wavelength thin-film and to estimate their ablation thresholds. Despite the adiabatic local density approximation used in calculating the exchange--correlation potential, the reflectance and the ablation threshold obtained from our method agree well with the previous theoretical and experimental results. The method can be applied to estimate the ablation thresholds for optical materials in general. The ablation threshold data can be used to design ultra-broadband high-damage-threshold coating structures.

cond-mat.mtrl-sci↗