Excitation - melting - ablation: Theoretical investigations of key processes during ultrashort pulsed laser machining

摘要

Ultrashort laser pulse interaction with material involves a number of specialities as compared to longer irradiations. Applying femtosecond laser pulses, the fundamental physical processes such as excitation, melting and ablation are temporally separated, allowing a separate investigation of each of them. The irradiated material passes through highly non-equilibrium states of different kinds on different timescales after irradiation. Thus, the theoretical description of the investigated processes may differ strongly from the classical descriptions valid for equilibrium or steady-state conditions. On a femtosecond timescale we investigate the non-equilibrium of the laser-excited electron gas. With the help of a detailed microscopic approach we study the applicability of simplified macroscopic descriptions of laser absorption and free-electron excitation. We study different melting processes occuring on different timescales in the picosecond regime. The nature of the melting process depends on the laser and material parameters, respectively. Material removal, i.e. ablation, occurs on a pico-to nanosecond time scale, depending on excitation strength. We show theoretical and experimental investigations of the expansion dynamics of the excited material.