A Comparative Study of Two-Temperature and Boltzmann Transport Models for Electron-Phonon Nonequilibrium

Ultrashort-pulsed laser irradiation on metals creates a thermal nonequilibrium between electrons and the phonons. Previous computational studies used the two-temperature model and its variants to model this nonequilibrium. However, when the laser pulse duration is smaller than the relaxation time of the energy carriers or when the carriers mean free path is larger than the material dimension, these macroscopic models fail to capture the physics accurately. In this article, the nonequilibrium between energy carriers caused by a laser pulse interaction with a metal film is modeled via a numerical solution of the Boltzmann transport model (BTM) for electrons and phonons. A comparative assessment of the two-temperature model and its variants is carried out relative to the BTM. The higher order Runge-Kutta discontinuous Galerkin (RKDG) method is used for numerical discretization of the models. In this study, the gold film thickness is varied between 2-2000nm, and the laser pulse duration and fluence are varied between 5 fs to 10ps and 10-2000 J/m2, respectively. It is found that BTM shows the best agreement with the experimental data compared to the two-temperature models for the electron and phonon temperature profiles and the melting threshold fluence.