Direct measurement of non-equilibrium phonon occupations in femtosecond laser heated Au films

TL;DR

This study employs ultrafast electron diffraction to directly observe the non-equilibrium dynamics of phonon populations in femtosecond laser-heated gold films, revealing momentum-dependent thermalization timescales that challenge traditional models.

Contribution

It provides the first direct measurement of non-equilibrium phonon occupations and their momentum-dependent thermalization timescales in laser-excited gold films.

Findings

Phonon populations near X and K points grow within 2.3 and 2.9 ps.

Mean-square atomic displacements increase with a 4.7 ps time constant.

Thermalization persists beyond 10 ps, contradicting two-temperature model predictions.

Abstract

We use ultrafast electron diffraction to detect the temporal evolution of phonon populations in femtosecond laser-excited ultrathin single-crystalline gold films. From the time-dependence of the Debye-Waller factor we extract a 4.7 ps time-constant for the increase in mean-square atomic displacements. We show from the increase of the diffuse scattering intensity that the population of phonon modes near the X and K points in the Au fcc Brillouin zone grows with timescales of 2.3 and 2.9 ps, respectively, faster than the Debye-Waller average. We find that thermalization continues within the initially non-equilibrium phonon distribution after 10 ps. The observed momentum dependent timescale of phonon populations is in contrast to what is usually predicted in a two-temperature model.