Electron-phonon non-equilibrium in nanoscale gold films

Ultrashort-pulsed laser irradiation on metals creates a thermal non-equilibrium between electrons and the phonons. Previous computational studies used the two-temperature model and its variants to model this non-equilibrium. 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 paper, the non-equilibrium between energy carriers is modeled via numerical solution of the Boltzmann Transport Model (BTM) for electrons and phonons which is applicable over a wide range of length and time scales. The BTM is solved using the Discontinuous Galerkin Finite Element Method for spatial discretization and the three-step Runge Kutta temporal discretization. Temperature dependant electron-phonon coupling factor and electron heat capacity are used due to the strong electron-phonon non-equilibrium considered in this study. The results from the proposed model is compared with existing experimental studies on laser heating of macroscale materials. The model is then used to study laser heating of gold films, by varying parameters such as the film thickness, laser fluence and pulse duration. It is found that the temporal evolution of electron and phonon temperatures in nanometer size gold films are very different from the macroscale films. For a given laser fluence and pulse duration, the peak electron temperature increases with a decrease in the thickness of the gold film. Both film size as well as laser fluence significantly affect the melting time. For a fluence of 5000 J/m², and a pulse duration of 75 fs, gold films of thickness smaller than 200 nm melt before reaching electron-phonon equilibrium. However, for the film thickness of 2000 nm, even with the highest laser fluence examined, the electrons and phonons reach equilibrium and the gold film does not melt.