Femtosecond laser pulse train effect on optical characteristics and nonequilibrium heat transfer in thin metal films

Abstract

The objective of this study is to numerically investigate the electron-phonon interactions and the nonequilibrium energy transfer in metal thin films irradiated by ultrashort pulse train lasers. In particular, the temporal and spatial variations in the optical properties during laser irradiation are discussed; the influence of the number of pulses per train and the pulse separation time are also examined. The present study uses the well-established two-temperature model to describe laser-solid interactions and the quantum approach to determine various properties such as electron heat capacity, electron thermal conductivity, collision frequencies, reflectivity, and absorption rates. It is found that as the number of pulses per pulse train increases, the nonequilibrium state between electrons and phonons gradually disappears because of the energy relaxation and the low electron thermal conductivity. The results show that the electron-electron and electron-phonon collision frequencies vary significantly with the number of pulses per train and the separation time per pulse, and that they considerably affect the reflectivity and absorption rate, in turn leading to a change in the ablation mechanism of thin metal films for pulse train laser heating.