Femtosecond laser ablation of transparent dielectrics: measurement and modelisation of crater profiles

Femtosecond lasers are promising tools for micro-machining requiring high surface quality and reliability. One of the many remarkable features in the case of transparent dielectrics is the shallowness of ablated craters, a still unexplained phenomena. We have measured the shape of ablated craters in corundum (α-Al2O3) for various pulse duration (from 60 fs to 14 ps). The evolution of ablated craters depth and shape with laser intensity depends on the pulse duration. Below 1 ps there is a clear threshold above which the depth increases dramatically from 0 to 100 nm, and then slowly increases with laser intensity. For longer pulses, the craters are much more shallow—about 20 nm, and we observe above a second threshold the appearance of a deeper hole in the centre of the first. The fundamental laser–matter interaction mechanisms at work during ablation are discussed. A model aiming to link these microscopic processes and the macroscopic measurements of ablation craters is presented and its results are compared to the experimental data.