arXiv subjects

K. L. Ishikawa

Publications and source records attributed to K. L. Ishikawa.

4 recordsLinked to original sources

arXiv2026

Three-dimensional real-space electron dynamics in graphene driven by strong laser fields

We theoretically investigate the three-dimensional (3D) electron dynamics of graphene in real space under strong laser fields using time-dependent density functional theory (TDDFT). We successfully reproduce the reversal of current direction originating from the cancellation of two oppositely directed residual currents, as previously predicted by Morimoto et al. [Y. Morimoto et al., New J. Phys. 24, 033051 (2022)]. By distinguishing contributions from individual orbitals, our results validate the two-level system approximation and also emphasize that the first-principles approach agrees better with experimental results for light-driven residual current, especially in extremely strong fields. Furthermore, our 3D model reveals that the real-space atomic-scale current induced by strong laser fields is concentrated slightly above and below the graphene basal plane, rather than strictly within it. The two oppositely directed currents exhibit a pronounced height separation in the out-of-plane direction, indicating that the ring current is not confined to the graphene plane but forms a rotating 3D circulation loop which is absent in the reduced-dimensional model.

cond-mat.mes-hall↗

arXiv2012

Photoelectron Angular Distributions for Two-photon Ionization of Helium by Ultrashort Extreme Ultraviolet Free Electron Laser Pulses

Phase-shift differences and amplitude ratios of the outgoing $s$ and $d$ continuum wave packets generated by two-photon ionization of helium atoms are determined from the photoelectron angular distributions obtained using velocity map imaging. Helium atoms are ionized with ultrashort extreme-ultraviolet free-electron laser pulses with a photon energy of 20.3, 21.3, 23.0, and 24.3 eV, produced by the SPring-8 Compact SASE Source test accelerator. The measured values of the phase-shift differences are distinct from scattering phase-shift differences when the photon energy is tuned to an excited level or Rydberg manifold. The difference stems from the competition between resonant and non-resonant paths in two-photon ionization by ultrashort pulses. Since the competition can be controlled in principle by the pulse shape, the present results illustrate a new way to tailor the continuum wave packet.

physics.atom-ph↗

arXiv2009

Fine-scale oscillations in the wavelength- and intensity-dependence of high-harmonic generation: connection with channel closings

We investigate the connection of recently identified fine-scale oscillations in the dependence of the yield of the high-harmonic generation (HHG) on wavelength lambda of a few-cycle laser pulse [K. Schiessl, K.L. Ishikawa, E. Persson, and J. Burgdorfer, Phys. Rev. Lett. 99, 253903 (2007)] to the well-known channel closing (CC) effect. Using the Lewenstein model of HHG, we identify the origin of the oscillations as quantum interference of many rescattering trajectories. By studying the simultaneous variations with intensity and wavelength, different models for the interference of channel closing peaks can be tested. Contrary to theoretical predictions for short-range potentials, the peaks are not located neither at nor just below the CC condition, but a significant shift is observed. The long Coulomb tail of the atomic potential is identified as the origin of the shift.

physics.atom-ph↗

arXiv2007

Quantum path interference in the wavelength dependence of high-harmonic generation

We investigate the dependence of the intensity of radiation due to high-harmonic generation (HHG) as a function of the wavelength of the fundamental driver field. Superimposed on a smooth power-law dependence observed previously we find surprisingly strong and rapid fluctuations on a fine wavelength scale. We identify the origin of these fluctuations in terms of quantum path interferences with up to five returning orbits significantly contributing.

physics.atom-ph↗