Ablation enhancement of metal in ultrashort double-pulse experiments

We report an ablation enhancement (both deepening and widening in ablation profiles) on nickel by ultrashort double-pulse laser irradiation, an exception of the typical reduction trend observed in ablation rates for metals irradiated with double-pulse excitations. Maximized at a pulse-separation (τ_s) of approximately 4 ps, the enhancement was observed before τ_s was increased to approximately the electron-lattice energy relaxation time. An achievement of ablation, by temporal shaping a weak pulse with energy well below the single-pulse ablation threshold into two considerably weaker pulses, was demonstrated. It is proposed that the enhanced electron-phonon coupling and drop of electronic heat conductivity, both of which result from the reductions in electron temperature (by pulse shaping), co-contribute to the widening in ablation profiles, while the former one is responsible for the deepening. This letter proposed an ablation enhancement approach to ultrashort laser processing of nickel.