Influence of the Wavelength on Femtosecond Laser Ablation Thresholds and Incubation Coefficients of Silicon and Germanium

When using an ultrafast laser to irradiate the surface of a material, one can induce different changes in it depending on a set of laser parameters, such as the laser wavelength, laser fluence, and the total number of pulses.In our materials of interest, we are mainly concerned by two different laser-mater interaction regimes: the laser ablation regime and the sub-ablation modification regime, which are separated by the ablation threshold fluence.This fluence, depends on the laser wavelength, and the number of pulses (incubation effect).In this work, we investigated these dependencies, for mono-crystalline silicon (Si) and germanium (Ge).We measured the ablation thresholds for irradiation with 5, 10 and 50 pulses at 343, 515 and 1030 nm, for a laser pulse duration of 350 femtosecond (fs).The ablation thresholds of Si and Ge for these laser conditions are traced experimentally.The single-shot ablation thresholds for the 3 wavelengths are deduced, as well as the incubation coefficient, both of which are based on the experimental data from our study.We then discuss a number of factors which may contribute to the ablation thresholds and incubation coefficient when operating at different wavelengths.