Pulse-width-dependent surface ablation of copper and silver by ultrashort laser pulses

The single-shot spallation thresholds for copper and silver surfaces demonstrate a considerable IR-laser (1030 nm) pulse-width dependent increase over a range of 0.2-12 ps for the former material and a very weak increase for the latter one, while the corresponding thresholds for visible (515 nm) laser pulses remain almost constant. The IR-laser increase of the ablation thresholds is related to two-photon interband (d-s) absorption in copper, contrasting with the linear absorption of visible laser pulses in this material. In silver, common weakly sublinear dependences on the laser pulsewidth were observed, ruling out possible multi-photon-either three(four)-photon in IR, or two-photon in the visible range-interband transitions in this material. Moreover, electron-lattice thermalization times of 1-2 ps were evaluated for these materials in the spallative ablation regime, contrasting strongly with the previously theoretically predicted multi-picosecond thermalization times.