Ablation of metals by ultrashort laser pulses

Ablation of Fe by ultrashort laser pulses with durations 0.1, 1, and 5 ps were investigated experimentally. The laser fluence varied from the ablation threshold up to 100 J cm-2. Above 1 J cm-2, the ablation rate depended on the laser pulse duration, with the shortest pulse producing the highest value. A change in the ablation rate as the laser fluence increased was also observed. These results were analysed using molecular dynamics simulations. We show that the change in the ablation rate is connected to an overheating of the material above the critical point, which results in a steep rise of the pressure developed. Furthermore, due to the electron heat diffusion, the overheated volume increases and involves material located deeper than the skin depth. An increase in the pulse duration results in a decrease in the degree of overheating