On the physics of material processing with femtosecond lasers

Ultrashort pulsed laser ablation of dielectrics has been investigated using ex-situ morphological examinations in combination with in-situ time-of-flight mass spectrometry of the ablated species. Analysis of the energy spectrum of the ablation products provides a wealth of information on the processes occurring during femtosecond laser ablation of materials. The presentation will focus on the case of sapphire (Al2O3) and discuss the fundamental processes in ultrashort pulsed laser sputtering. Two different ablation phases have been identified, a “gentle” phase with low ablation rates and a “strong” etch phase with higher ablation rates, but with limitation in the structure quality. A comparison of the energy and momentum distributions of ejected ions, neutrals, and electrons allows one to distinguish between non-thermal and thermal processes that lead to the macroscopic material removal. Fast positive ions with charge-scaled momenta are resulting from Coulomb explosion of the upper layers at low fluence and low number of irradiating laser pulses (“gentle” etch phase). Pump-probe studies with fs laser pulses reveal the dynamics of excitation and electron-mediated energy transfer to the lattice. At higher laser fluences or 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.