Femtosecond Laser Pulse Train Effects on Optical Characteristics and Nonequilibrium Energy Transport in Metal Thin Films Considering Quantum Effects

The objective of this study is to investigate numerically the electron-phonon interactions and the nonequilibrium energy transfer in metal thin films irradiated by ultrashort pulse train lasers. During laser irradiation, in particular, the temporal and spatial variations of optical properties are discussed and the influence of pulse number per train and pulse separation time is also examined. The present study uses the well-established two temperature model in describing laser-solid matter interactions and it also adopts 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 pulse number per train increases, the nonequilibrium state between electrons and phonons disappears gradually because of the energy relaxation and the low electron thermal conductivity. From the results, the electron-electron and electron-phonon collision frequencies are changed significantly with the pulse number per train and the separation time per pulse, and they affect considerably reflectivity and absorption rate, leading to the change of ablation mechanism of thin metal films for the pulse train laser heating.