Time resolved detection of particle removal from dielectrics on femtosecond laser ablation

The dynamics of the ablation process in different oxides (amorphous and crystalline quartz, sapphire and MgO) after intensive laser excitation was studied by a combination of the femtosecond pump-probe technique with optical surface scattering. A 120-fs laser pulse (λ = 800 nm) was focused onto the sample with a fluence at least twice the single-shot surface damage threshold. A much weaker probe laser pulse optically aligned collinear to the pump beam illuminated the excited area at variable delay times up to 100 ps. We observe a material dependent increase in the scattering signal after a certain delay time which we interpret as the onset of the ablation process. This occurs much earlier for amorphous and crystalline silica than for sapphire or MgO, as a consequence of differences in the electron-phonon coupling strength. The morphology of the irradiated spot on sapphire, amorphous and crystalline quartz is indicative of melting and vaporisation in contrast to that on MgO which clearly shows fracturing.