Controlling ablation mechanisms in sapphire by tuning the temporal shape of femtosecond laser pulses

We have analyzed the influence of the temporal pulse shape on femtosecond (fs) laser-induced surface ablation processes in sapphire. To this end, single transform-limited (TL), stretched, and third-order-dispersion (TOD) shaped fs pulses have been used, while the dynamics of the interaction were analyzed by fs-resolved microscopy and correlated with plasma emission intensity and crater morphology. The modification of the pulse shape enables changing the ablation mechanism from a strong, thermally mediated ablation process to a gentle ablation process mediated by Coulomb explosion (CE), with respective ablation depths of 100–200 nm and 5–10 nm. Analysis of the transient optical response allows direct comparison of the transient plasma carrier densities involved, observing comparable peak values for both processes. For strong ablation induced by TL pulses, a direct relation between plasma density and local ablation depth is found, but this does not hold for the CE-mediated process observed for TOD-shaped pulses. For TOD-shaped pulses at very high fluence, a different ablation mechanism involving explosive boiling is identified. This mechanism leads to the formation of deep craters with reduced lateral extension and steep walls. This amount of control over the ablation mechanisms by a simple selection of the pulse shape should be of interest for new surface structuring approaches.