Laser ablation of sapphire with ultrashort pulses

We present the results of our investigations on the surface damage/ablation threshold and processing morphology for sapphire after single and multiple laser pulse irradiation at 800 nm in the picosecond and sub-picosecond duration range. The threshold for ablation drops sharply for multiple laser shot irradiation, due to material dependent incubation effects. We observe two distinctively etch phases: `gentle' and `strong'. Monitoring the mechanism and dynamics of the ion expulsion using combination of time-of-flight mass spectroscopy and femtosecond pump-probe technique, we identified Coulomb explosion as the dominant mechanism for ion emission in the `gentle' etch phase on a time scale of 1 ps. The momenta of the emitted ions are equal under these conditions. After sufficient incubation the damage threshold decreases and the ablation is shifted towards the `strong' phase. The velocity distributions shift to lower values, evidence for `phase explosion' is seen and the ions tend here to equal kinetic energies.