Ablation of metallic targets by high-intensity ultrashort laser pulses

Fundamental characteristics and underlying mechanisms of ultrafast ablation of aluminum and copper materials by short, from $50\phantom{\rule{0.3em}{0ex}}\mathrm{fs}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}24\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$, infrared laser pulses with relatively high-energy fluences are investigated in theory and experiment. Two different ablation regimes, nonthermal and thermal, are revealed to exist depending on the laser parameters. Theoretical analyses provide a detailed description of the underlying mechanisms, laser-induced stress and phase explosion, playing a dominant role in these ablation processes. Their induced ablation rates as a function of pulse duration are also simulated at several laser fluences. The interplay of the two competing mechanisms is found to be responsible for mutual transitions between thermal and nonthermal ablations. With the help of a modified two-temperature mode through considering e-e-collision dominated electron diffusions, a thermal-nonthermal ablation transition law is obtained successfully. The critical pulse length, acting as a division of the two ablation regimes, is demonstrated to reduce with increasing laser fluence. Both hole-drilling and line-scribing experiments of Al targets by using ultrashort laser pulses are performed for the confirmation of our theory. Morphological changes of the ablated areas are characterized through scanning electron microscopy. Transitions between the two different ablation regimes are achieved by proper selection of the laser pulse time duration for a given laser fluence. Experimental results are consistent with theoretical predictions.