Comparison of Theoretical Models of Electron-Phonon Coupling in Thin Gold Films Irradiated by Femtosecond Pulse Lasers

Abstract

This study reports on a comparison of theoretical models for electron-phonon coupling that is substantially associated with non-equilibrium energy transport in thin gold films irradiated by femtosecond pulse lasers. Three published electron-phonon coupling models were analyzed with the use of a well-established two-temperature model to describe non-equilibrium energy transport between electrons and phonons. Based on the numerical results, at lower fluence, all models showed nearly similar tendencies, whereas at higher fluence, constant electron-phonon coupling forced unrealistically long electron-phonon equilibration times and spatially long diffusive regions as it failed to intrinsically consider the effect of a high number density of excited d-band electrons. Even at higher fluence, however, both Lin’s and Chen’s models yielded physically reasonable results, showing converging electron-phonon equilibration times and steep gradients in the spatial lattice temperature profiles at higher laser fluence. In particular, Lin’s model predicts nonlinear characteristics of heat capacity and lattice temperature with respect to laser fluence better than Chen’s model. Moreover, the electron-phonon relaxation time increased with laser fluence, whereas at laser fluence greater than 0.05 J/cm², the thermal equilibrium time was nearly independent of the laser fluence. Thus, it was concluded that Lin’s model better predicted the electron-phonon coupling phenomena in thin metal films irradiated by ultra-short pulse lasers.