arXiv subjects

Kenichi L. Ishikawa

Publications and source records attributed to Kenichi L. Ishikawa.

At least 19 recordsLinked to original sources

arXiv2026

Sector-Resolved Winding Selection Rules for Structured-Light-Driven dc Currents

Structured light can generate electronic dc currents with azimuthal winding, yet the rules governing their winding order $m$ remain unclear. Here we identify sector-resolved winding selection rules using graphene as a clean two-dimensional platform. By decomposing the current response into local and gradient sectors at the current-operator level, we show that the winding order is determined not by the optical orbital angular momentum $\ell$ alone, but by the angular structure of the corresponding current operators together with projection onto the azimuthal direction. For scalar Laguerre-Gaussian beams, linear polarization yields $m=|\ell\pm1|$ in the local sector and $m=|\ell|,|\ell\pm2|$ in the gradient sector, whereas circular polarization with helicity $σ=\pm1$ selects $m=|\ell-σ|$ in the local sector and $m=|\ell+2σ|$ in the gradient sector. Numerical time-evolution calculations verify these rules and further show that helicity can select an $m=0$ branch, producing an azimuthally uniform circulating current whose radial profile determines the axial magnetic field $B_z(z)$. Our results provide a sector-resolved organizing principle for classifying and controlling structured-light-driven dc currents with tailored winding structures.

cond-mat.mes-hall↗

arXiv2026

Field-driven attosecond deflection of electron beams at the position of planar foils

While the coupling of free-electron beams and light has enabled the control of electron's quantum states and the generation of ultrashort electron pulses, it can distort the measured signal when probing a laser-driven sample with an electron beam. Here, we theoretically study optical-field-driven beam deflection that inevitably occurs at the location of a laser-excited sample. Treating electrons in a beam as classical point particles and focusing on thin planar samples, we conduct a systematic study of the instantaneous deflection amplitudes with respect to light polarization, material type, thickness, and interaction geometry. For s-polarized excitation fields, the deflection amplitude is strictly proportional to the excitation-field amplitude in a foil, making beam deflection unavoidable. Conversely, for p-polarized fields, a nontrivial relationship emerges between the deflection amplitude and the field amplitude due to the interplay of electric and magnetic fields coupled with the electron-light velocity mismatch. Crucially, we demonstrate that the deflection can be minimized even under high field strengths by selecting an optimal configuration for a given foil material and thickness. These findings provide key guidelines for designing future time-resolved imaging experiments using ultrashort electron beams.

physics.optics↗

arXiv2026

Ultrafast Energy Absorption in Silicon Controlled by Two-Color Double Pulses

We theoretically show that energy absorption in crystalline silicon can be controlled by two-color femtosecond double-pulse irradiation, in which two temporally separated pulses with different wavelengths interact sequentially with the system. Using time-dependent density functional theory, we systematically examine the wavelength and intensity dependence of the absorbed energy over peak intensities of $2\times10^{11}$-$10^{13}$ W/cm$^2$ and wavelengths of 515, 1030, and 2060 nm. We find that the mechanism governing energy absorption and the optimal wavelength combination strongly depend on the intensity regime. In the low-intensity regime, multiphoton interband absorption is dominant, and energy absorption is enhanced for pulse pairs composed of shorter wavelengths. In contrast, in the high-intensity regime, the contributions of tunneling ionization and intraband acceleration become significant, leading to enhanced absorption for longer-wavelength combinations. In the intermediate-intensity regime, a pronounced enhancement is observed when a short-wavelength pulse precedes a long-wavelength pulse. Our analysis reveals that the nonequilibrium electronic state prepared by the first pulse modifies the excitation process induced by the second pulse, thereby enhancing the absorbed energy through an increased energy gain per excited electron. In this regime, the energy absorption is governed not only by the number of excited carriers but also by the energy gain per excited electron, which can be strongly modified by the pulse sequence. These results indicate that ultrafast energy transfer in semiconductors is tunable by appropriately designing the wavelength and intensity combination of the two pulses, and provide microscopic insight into two-color strong-field excitation.

cond-mat.mtrl-sci↗

arXiv2026

Time-dependent Hole States in Multiconfigurational Time-Dependent Hartree-Fock Approaches: A Time-Domain Generalization of Extended Koopmans' Theorem

We introduce a framework for resolving electron-hole dynamics within wavefunction-based multiconfigurational time-dependent Hartree-Fock (MCTDHF) theory. Central to this framework is a time-domain generalization of the extended Koopmans' theorem, which rigorously defines time-dependent hole states through single-electron removal. From this foundation, we prove the existence of exact equations of motion for time-dependent Dyson orbitals, enabling instantaneous construction of photofragments' reduced density matrices. The formalism further yields a systematic procedure to extract hole-resolved observables, such as channel-resolved photoelectron momentum distributions, directly from time-dependent \textit{ab initio} wavefunctions. As a demonstration, we employ an attosecond $ω-2ω$ laser strategy to control hole dynamics, thereby resolving a long-standing challenge in MCTDHF simulations. This advance opens a pathway for exploring correlated multielectron dynamics in atoms and molecules under ultrafast laser fields.

physics.atom-ph↗

arXiv2025

Reduced density matrix description of high-harmonic generation in multi-electron atoms: exploring sub-cycle correlation effects

High-harmonic generation (HHG) is one of the fundamental processes at the heart of attosecond physics. Traditionally viewed as an effective single-particle effect, recent advances have focused on contributions beyond this single-particle picture to the harmonic spectrum, as well as on probing electron correlations through HHG in atoms, molecules, and solids. In this paper, we introduce a reduced density matrix description to explore and to quantify correlation effects on a sub-cycle time scale and apply this approach to prototypical multi-electron atoms. By comparing noble gas atoms (He, Ne) with alkaline earth atoms (Be, Mg) exposed to driving fields with similar Keldysh parameters, we show that the sub-cycle variation of correlation parameters differs markedly for noble gases and alkaline earth atoms. We provide an intuitive explanation of these surprising effects based on the dynamics of natural orbitals and discuss the effect of this ultrafast correlation dynamics on the HHG spectrum.

physics.atom-ph↗

arXiv2025

Strong Field Optical Hall Effect in 2D Weyl Semimetal

The study of interplay between the geometric nature of Bloch electrons and transverse responses under strong field offers new opportunities for optoelectronic applications. Here, we present a comprehensive study of the strong-field response of Weyl Dirac nodes in bilayer T'-WTe2 using time-dependent first-principles formalism. The electron dynamics is explored focusing on the mid-infrared frequency, ranging from the perturbative to nonperturbative regime. In the nonperturbative regime, the high-harmonic generation (HHG) spectra under a strong field clearly exhibit a plateau and energy cutoffs for both longitudinal and anomalous Hall (transverse) currents, with the latter being due to the large interband Berry curvature of the Weyl-Dirac semimetal. For the longitudinal harmonics, the intraband contributions increase with intensity, resulting in a complex interplay between interband polarization and intraband motions. Remarkably, if we take a comprehensive all-band perspective enabled by time-dependent density functional calculations, the anomalous Hall responses are purely attributed to the interband processes, even in the nonperturbative regime, thus Hall HHG can be crucial to understand the carrier dynamics. Our findings suggest that HHG associated with the ultrafast strong-field driven electron dynamics holds immense potential for exploring the nonlinear high Hall responses in Weyl semimetal.

cond-mat.mes-hall↗

arXiv2025

Electron Excitation Probability in Dielectrics under Two-color Intense Laser Fields

Two-color laser fields offer significantly enhanced control over electron excitation dynamics under ultrashort intense laser pulses compared to monochromatic fields. However, their strong nonlinearity necessitates computationally expensive first-principles calculations to accurately predict ionization dynamics. To overcome this challenge, we derive an analytical expression for the ionization rate in dielectrics subjected to intense two-color laser fields, refining the theoretical framework introduced in JPSJ {\bf 88}, 024706 (2019). By benchmarking our formula against first-principles calculations based on time-dependent density functional theory (TDDFT) for $α$-quartz, we demonstrate that our model captures the essential physics of ionization dynamics with remarkable qualitative accuracy, despite employing certain approximations. This analytical approach not only provides deeper physical insight but also offers a computationally efficient alternative for predicting strong-field interactions in dielectrics.

physics.optics↗

arXiv2025

Coherent Control of Ion-Photoelectron Dynamics through Rabi Oscillations: An ab initio study

We present first-principles numerical simulations of photoionization in neon induced by bichromatic extreme ultraviolet pulses with frequencies $ω$ and $2ω$, specially chosen to make $ω$ equal to the energy difference between the $2s$ and $2p$ subshells. This allows for the production of photoelectrons from the $2s$ shell by $2ω$ pulse and from the $2p$ shell by $ω$ pulse with the same energy. Using the multi-configurational time-dependent Hartree-Fock method, we explore how Rabi coupling between subshells generates coherence between the corresponding photoelectron wave packets. Our \textit{ab initio} calculations confirm the analytical results derived from the essential-states approach in [K. L. Ishikawa, K. C. Prince, and K. Ueda, J. Phys. Chem. A 127, 10638 (2023)], validating the theoretical predictions. Although we focus on the Ne $2p$ and $2s$ subshells, our approach is applicable to a broad range of systems exhibiting photoionization from multiple subshells. The laser parameters employed in our simulations are available in modern Free Electron Lasers (FELs), and we anticipate that this work could stimulate experimental investigations using FELs to study ion-photoelectron coherence and entanglement.

physics.atom-ph↗

arXiv2025

Time-dependent Hole States in Multiconfigurational Time-Dependent Hartree-Fock Approaches: Applications in Photoionization of Water Molecule

By simulating the real-time multielectron wavefunction with the multi-configurational time-dependent Hartree-Fock (MCTDHF) approach, we conduct an \textit{ab initio} study of the single-photon ionization process of a body-fixed water molecule ($\mathrm{H_2O}$) driven by attosecond pulses. To this end, we present a full-dimensional implementation of the MCTDHF method based on one-center expansions, allowing for the simulation of arbitrarily polarized lasers and multi-center polyatomic potentials. With a rigorous definition of the time-dependent hole state (TDHS) using the time-domain generalization of extended Koopmans' theorem (TD-EKT), we derive the reduced ion density matrix within the MCTDHF framework, which inherently encodes the total and channel-resolved photoionization cross sections of $\mathrm{H_2O}$. The cross sections obtained are benchmarked against existing experimental and theoretical results, validating the TDHS formalism. Furthermore, by adjusting the phase delay and intensity ratio of a pair of orthogonally polarized attosecond pulses, we explore the ultrafast control of attosecond coherence between electronic states of $\mathrm{H_2O^+}$.

physics.atom-ph↗

arXiv2025

Ultrafast Optical Control of Multi-Valley States in 2D SnS

We theoretically study the ultrafast optical control of multiple valley states in two-dimensional (2D) tin sulfide (SnS) monolayers, a member of the layered group-IV monochalcogenides, which is a promising class of materials for overcoming current challenges in valleytronics. By combining time-dependent density functional theory with Maxwells equations, we simulate how both linearly and circularly polarized ultrashort laser pulses affect the electronic excitation dynamics and valley polarization in SnS. Our results reveal that the corrugated phosphorene-like crystal structure of SnS monolayers leads to the emergence of both linear and circular dichroism, allowing flexible manipulation of multi-valley excitation by simply adjusting the light polarization. Moreover, the interplay between broken inversion symmetry and spin-orbit coupling gives rise to distinct Berry curvature effects and spin-valley coupling, thereby enabling circular dichroism. Furthermore, we propose that tuning the carrier-envelope phase of few-cycle femtosecond laser pulses can achieve sub-cycle, ultrafast switching among multiple valleys states. These findings not only deepen our understanding of valley dynamics in 2D materials but may also open new avenues for the development of valleytronic devices.

physics.optics↗

arXiv2025

Maxwell-Vlasov-Uehling-Uhlenbeck (VUU) Simulation for Coupled Laser-Electron Dynamics in a Metal Irradiated by Ultrashort Intense Laser Pulses

The description of electron-electron scattering presents challenges in the microscopic modeling of the interaction of ultrashort intense laser pulses with solids. We extend the semiclassical approach based on the Vlasov equation [Phys. Rev. B 104, 075157(2021)] to account for dynamic electron-electron scattering by introducing the Vlasov-Uehling-Uhlenbeck (VUU) equation. We further couple the VUU equation with Maxwell's equations to describe the laser pulse propagation. We apply the present approach to simulate laser-electron interactions in bulk and thin-film aluminum, focusing on energy absorption and transport. Our calculation results reveal that electron-electron scattering affects energy absorption more significantly under p-polarization than under s-polarization, highlighting the role of the non-uniform surface potential. Our simulations also show that the energy transport extends beyond the optical penetration depth, which is consistent with observations in previous laser ablation experiments. The developed Maxwell-VUU approach is expected to advance the understanding of intense laser-material interactions not only as a cost-effective alternative to the time-dependent density functional theory (TDDFT), but also by incorporating fermionic two-body collisions whose description is limited in TDDFT.

physics.plasm-ph↗

arXiv2023

Control of ion-photoelectron entanglement and coherence via Rabi oscillations

We report a theoretical investigation of photoionization by a pair of coherent, ultrashort fundamental and second-harmonic extreme-ultraviolet pulses, where the photon energies are selected to yield the same photoelectron energy for ionization of two different sub-shells. This choice implies that the fundamental energy is equal to the difference in energy of the ionic states, and they are therefore coupled by the fundamental photon. By deriving analytical expressions using the essential-states approach, we show that this Rabi coupling creates coherence between the two photoelectron wave packets, which would otherwise be incoherent. We analyze how the coupling is affected by the parameters such as relative phase, pulse width, delay between the two pulses, the Rabi coupling strength, and photoelectron energy. Our discussion mostly considers Ne 2p and 2s photoionization, but it is generally valid for many other quantum systems where photoionization from two different shells is observed.

physics.atom-ph↗

arXiv2023

Efficient simulation of multielectron dynamics in molecules under intense laser pulses: Implementation of the multiconfiguration time-dependent Hartree-Fock method based on the adaptive finite element method

We present an implementation of the multiconfiguration time-dependent Hartree-Fock method based on the adaptive finite element method for molecules under intense laser pulses. For efficient simulations, orbital functions are propagated by a stable propagator using the short iterative Arnoldi scheme and our implementation is parallelized for distributed memory computing. This is demonstrated by simulating high-harmonic generation from a water molecule and achieves a simulation of multielectron dynamics with overwhelmingly less computational time, compared to our previous work.

quant-ph↗

arXiv2022

Time-dependent multiconfiguration self-consistent-field and time-dependent optimized coupled-cluster methods for intense laser-driven multielectron dynamics

We review time-dependent multiconfiguration self-consistent-field (TD-MCSCF) method and time-dependent optimized coupled-cluster (TD-OCC) method for first-principles simulations of high-field phenomena such as tunneling ionization and high-order harmonic generation in atoms and molecules irradiated by a strong laser field. These methods provide a flexible and systematically improvable description of the multielectron dynamics by expressing the all-electron wavefunction by configuration interaction expansion or coupled-cluster expansion, using time-dependent one-electron orbital functions. The time-dependent variational principle plays a key role to derive these methods satisfying gauge invariance and Ehrenfest theorem. The real-time/real-space implementation with an absorbing boundary condition enables the simulation of high-field processes involving multiple excitation and ionization. We present a detailed, comprehensive discussion of such features of TD-MCSCF and TD-OCC methods.

physics.chem-ph↗

arXiv2022

Enhanced energy deposition and carrier generation in silicon induced by two-color intense femtosecond laser pulses

We theoretically investigate the optical energy absorption of crystalline silicon subject to dual-color femtosecond laser pulses, using the time-dependent density functional theory (TDDFT). We employ the modified Becke-Johnson (mBJ) exchange-correlation potential which reproduces the experimental direct bandgap energy $E_g$. We consider situations where the one color is in the ultraviolet (UV) range above $E_g$ and the other in the infrared (IR) range below it. The energy deposition is examined as a function of mixing ratio $η$ of the two colors with the total pulse energy conserved. Energy transfer from the laser pulse to the electronic system in silicon is dramatically enhanced by simultaneous dual-color irradiation and maximized at $η\sim 0.5$. Increased is the number of generated carriers, not the absorbed energy per carrier. The effect is more efficient for lower IR photon energy, or equivalently, larger vector-potential amplitude. As the underlying mechanism is identified the interplay between intraband electron motion in the {\it valence} band (before excitation) driven by the IR component and resonant valence-to-conduction interband excitation (carrier injection) induced by the UV component. The former increases excitable electrons which pass through the $k$ points of resonant transitions. The effect of different multiphoton absorption paths or intraband motion of carriers generated in the conduction band play a minor role.

physics.plasm-ph↗

arXiv2022

Time-dependent optimized coupled-cluster method with doubles and perturbative triples [TD-OCCD(T)] for first principles simulation of multielectron dynamics

We report the formulation of a new, cost-effective approximation method in the time-dependent optimized coupled-cluster (TD-OCC) framework [T. Sato et al., J. Chem. Phys. 148, 051101 (2018)] for first-principles simulations of multielectron dynamics in an intense laser field. The method, designated as TD-OCCD(T), is a time-dependent, orbital-optimized extension of the "gold-standard" CCSD(T) method in the ground-state electronic structure theory. The equations of motion for the orbital functions and the coupled-cluster amplitudes are derived based on the real-valued time-dependent variational principle using the fourth-order Lagrangian. The TD-OCCD(T) is size extensive and gauge invariant, and scales as O(N^7) with respect to the number of active orbitals N. The pilot application of the TD-OCCD(T) method to the strong-field ionization and high-order harmonic generation from a Kr atom is reported in comparison with the results of the previously developed methods, such as the time-dependent complete-active-space self-consistent field (TD-CASSCF), TD-OCC with double and triple excitations (TD-OCCDT), TD-OCC with double excitations (TD-OCCD), and the time-dependent Hartree-Fock (TDHF) methods.

physics.chem-ph↗

arXiv2022

Atomic real-space perspective of light-field-driven currents in graphene

When graphene is exposed to a strong few-cycle optical field, a directional electric current can be induced depending on the carrier-envelope phase of the field. This phenomenon has successfully been explained by the charge dynamics in reciprocal space, namely an asymmetry in the conduction band population left after the laser excitation. However, the corresponding real-space perspective has not been explored so far although it could yield knowledge about the atomic origin of the macroscopic currents. In this work, by adapting the nearest-neighbor tight-binding model including overlap integrals and the semiconductor Bloch equation, we reveal the spatial distributions of the light-field-driven currents on the atomic scale and show how they are related to the light-induced changes of charge densities. The atomic-scale currents flow dominantly through the network of the π bonds and are the strongest at the bonds parallel to the field polarization, where an increase of the charge density is observed. The real-space maps of the currents and changes in charge densities are elucidated using simple symmetries connecting real and reciprocal space. We also discuss the strong-field-driven Rabi oscillations appearing in the atomic-scale charge densities. This work highlights the importance of real-space measurements and stimulates future time-resolved atomic-scale experimental studies with high-energy electrons or X-rays, for examples.

physics.optics↗

arXiv2021

Exact solutions, spectrum properties, and hierarchical structures of the multiple temperature model

Recent developments of ultrafast laser pulse techniques enable us to study the subpicosecond scale dynamics out of thermal equilibrium. Multiple temperature models are frequently used to describe such dynamics where the total system is divided into subsystems each of which is in local thermal equilibrium. Typical examples include the electron-lattice two temperature model and electron-spin-phonon three temperature model. We present the exact analytical solutions of linear multiple temperature model, based on the Fourier series expansion, and discuss their properties for the case of the two and three temperature models. The solutions are linear combinations of "eigenmodes" characterized by the wave vector $\mathbf{q}$ and the well-defined mode lifetime. The eigenmode picture enables us to explore the hierarchical structure of models with respect to space, time and the coupling parameter. We also find diffusion modes unique to the three temperature model which unveils the rich physics in spite of the simplicity of the model. We prove that the eigensystem in this model is non-positive definite, which assures that the mode lifetime is always well-defined. This property clearly characterizes the model.

cond-mat.mtrl-sci↗