Modeling of ultrafast phase change processes in a thin metal film irradiated by femtosecond laser pulse trains

Ultrashort laser pulses can be generated in the form of a pulse train. In this paper, the ultrafast phase change processes of a 1-μm free-standing gold film irradiated by femtosecond laser pulse trains are simulated numerically. A two-temperature model coupled with interface tracking method is developed to describe the ultrafast melting, vaporization and resolidification processes. To deal with the large span in time scale, variable time steps are adopted. A laser pulse train consists of several pulse bursts with a repetition rate of 0.5∼1 MHz. Each pulse burst contains 3∼10 pulses with an interval of 50 ps∼10 ns. The simulation results show that with such a configuration, to achieve the same melting depth, the maximum temperature in the film decreases significantly in comparison to that of a single pulse. Although the total energy depositing on the film will be lifted, more energy will be transferred into the deeper part, instead of accumulating in the sub-surface layer. This leads to lower temperature and temperature gradient, which is favorable in laser sintering and laser machining.