Surface ablation оf aluminum and silicon by ultrashort laser pulses of variable width

Single-shot thresholds of surface ablation of aluminum and silicon via spallative ablation by infrared (IR) and visible ultrashort laser pulses of variable width $\tau_{\mathrm{las}}$ ($0.2$–$12$ ps) have been measured by optical microscopy. For increasing laser pulse width $\tau_{\mathrm{las}}<3$ ps, a drastic (threefold) drop of the ablation threshold of aluminum has been observed for visible pulses compared to an almost negligible threshold variation for IR pulses. In contrast, the ablation threshold in silicon increases threefold with increasing $\tau_{\mathrm{las}}$ for IR pulses, while the corresponding thresholds for visible pulses remained almost constant. In aluminum, such a width-dependent decrease in ablation thresholds has been related to strongly diminished temperature gradients for pulse widths exceeding the characteristic electron-phonon thermalization time. In silicon, the observed increase in ablation thresholds has been ascribed to two-photon IR excitation, while in the visible range linear absorption of the material results in almost constant thresholds.