Phonon-induced long-lasting nonequilibrium in the electron system of a laser-excited solid

TL;DR

This paper investigates how electron-phonon interactions cause a prolonged nonequilibrium state in laser-excited copper electrons, revealing effects on thermalization and electron-phonon coupling during ultrafast laser excitation.

Contribution

It provides a detailed Boltzmann collision integral analysis of electron-phonon interplay, highlighting a long-lasting nonequilibrium in noble metals after ultrashort laser pulses.

Findings

Strong nonequilibrium during and after laser pulse

Fast thermalization of most electrons

Long-lasting nonequilibrium due to electron-phonon scattering

Abstract

Electron-electron thermalization and electron-phonon relaxation processes in laser-excited solids are often assumed to occur on different timescales. This is true for the majority of the conduction band electrons in a metal. However, electron-phonon interactions can influence the thermalization process of the excited electrons. We study the interplay of the underlying scattering mechanisms for the case of a noble metal with help of a set of complete Boltzmann collision integrals. We trace the transient electron distribution in copper and its deviations from a Fermi-Dirac distribution due to the excitation with an ultrashort laser pulse. We investigate the different stages of electronic nonequilibrium after an excitation with an ultrashort laser-pulse of 800 nm wavelength and 10 fs pulse duration. Our calculations show a strong nonequilibrium during and directly after the end of the laser pulse. Subsequently, we find a fast thermalization of most electrons. Surprisingly, we observe a long-lasting nonequilibrium, which can be attributed to the electron-phonon scattering. This nonequilibrium establishes at energies around peaks in the density of states of the electrons and persists on the timescale of electron-phonon energy relaxation. It influences in turn the electron phonon coupling strength.