Electronic nonequilibrium effect in ultrafast-laser-irradiated solids

TL;DR

This study models electronic nonequilibrium effects during ultrafast laser irradiation of solids, revealing their impact on electron-phonon coupling and phase transition thresholds, emphasizing the importance of including all relevant effects in simulations.

Contribution

It introduces a simulation scheme that simultaneously models electronic nonequilibrium, nonthermal, and nonadiabatic effects, highlighting the significance of electron-electron thermalization in ultrafast laser-material interactions.

Findings

Non-equilibrium electronic states slow down electron-phonon coupling.

Electronic excitation alters damage thresholds for phase transitions.

Models excluding electron-electron thermalization may yield qualitatively different results.

Abstract

This paper describes the effects of electronic nonequilibrium in a simulation of ultrafast laser irradiation of materials. The simulation scheme based on tight-binding molecular dynamics, in which the electronic populations are traced with a combined Monte Carlo and Boltzmann equation, enables the modeling of nonequilibrium, nonthermal, and nonadiabatic (electron-phonon coupling) effects simultaneously. The electron-electron thermalization is described within the relaxation-time approximation, which automatically restores various known limits such as instantaneous thermalization (the thermalization time ${\tau}_{e-e} \rightarrow 0$) and Born-Oppenheimer approximation (${\tau}_{e-e} \rightarrow \infty$). The results of the simulation suggest that the non-equilibrium state of the electronic system slows down electron-phonon coupling with respect to the electronic equilibrium case in all studied materials: metals, semiconductors, and insulators. In semiconductors and insulators, it also alters the damage threshold of ultrafast nonthermal phase transitions induced by modification of the interatomic potential due to electronic excitation. It is demonstrated that the models that exclude electron-electron thermalization (using the assumption of ${\tau}_{e-e} \rightarrow \infty$, such as BO or Ehrenfest approximations) may produce qualitatively different results, and a reliable model should include all three effects: electronic nonequilibrium, nonadiabatic electron-ion coupling, and nonthermal evolution of interatomic potential.