Plasma evolution during metal ablation with ultrashort laser pulses.

We report on time-resolved measurements of the plasma evolution during metal ablation with ultrashort laser pulses in the range from 200 fs to 3.3 ps. The plasma transmission exhibits two distinctive minima. Almost total attenuation is observed a few nanoseconds after the ablation pulse, while a second decrease of the transmission to approximately 50% is observed after about 150 ns. Images taken with a gated ICCD-camera confirm these data and allow determining the expansion velocity of the plasma plume. The attenuation in the first nanoseconds can be attributed to electrons and sublimated mass emitted from the target surface, while attenuation after several 10 ns is due to particles and droplets after a thermal boiling process. The possibility of a normal or an explosive boiling process, also called phase explosion, is discussed. Despite of the physical insight into the ablation process, these data provide valuable information for scaling the speed of ultrashort pulse laser materials processing in a fluence regime of several J/cm2 since they allow estimating the maximum usable pulse repetition rate.