ULTRASHORT PULSED LASER ABLATION OF DIELECTRICS

摘要

Ultrashort pulsed laser ablation of dielectrics (sapphire) has been investigated using ex-situ morphological examinations with in-situ time-of-flight mass spectrometry. Two different ablation phases have been identified, a "gentle" phase with low ablation rates and high ionization efficiency, and a "strong" etch-phase with higher ablation rates, but with lower structure quality. Comparison of the energy and momentum distributions of ejected ions, neutrals, and electrons allows distinguishing between non-thermal and thermal processes that lead to material removal. Fast positive ions with charge-scaled momenta are resulting from the Coulomb explosion of the upper layers in the gentle phase. Pump-probe studies reveal the dynamics of excitation and energy transfer to the lattice. After longer incubation, evidence for phase explosion can be derived from both the morphology of the surface and the results of the in-situ experiments. Based on its contribution to the charge-induced ejection of particles, electron emission is also examined. Prompt electrons are evidenced as a direct product of excitation. Their dynamics support the idea of a repulsion of material due to electrostatic energy accumulation balanced by heat deposition. Finally, the knowledge gained from the fundamental studies is used to advocate the tremendous potential of femtosecond pulses for transparent materials micro-machining.