Melting, Vaporization and Resolidification in a Thin Gold Film Irradiated by Multiple Femtosecond Laser Pulses

Melting, vaporization and resolidification in a gold thin film subject to multiple femtosecond laser pulses are numerically studied in the framework of the two-temperature model. The solid-liquid phase change is modeled using kinetics controlled model that allows the interfacial temperature deviates from the melting point. The kinetics controlled model also allows superheating in the solid phase during melting and undercooling in the liquid phase during resolidification. Superheating of the liquid phase caused by nonequilibrium evaporation of the liquid phase is modeled by adopting the wave hypothesis, instead of Clausius-Clapeyron equation. Melting depth, ablation depth, and maximum temperature in both liquid and solid are investigated and the result is compared with that from Clausius-Clapeyron equation based vaporization model. The vaporization wave model predicts a much higher vaporization speed which leads to a deeper ablation depth. The relationship between laser processing parameters, including pulse separation time and pulse number, and phase change effect are also studied. It is found that longer separation time and larger pulse number will cause lower maximum temperature within the gold film, as well as lower depths of melting and ablation.