Ablation of crystalline oxides by infrared femtosecond laser pulses

We use focused laser pulses with duration of 180fs and wavelength of 800nm to study the interactions of high power near-infrared light with the surfaces of single-crystal transparent oxides (sapphire, LaAlO3, SrTiO3, yttria-stabilized ZrO2, and MgO); the morphologies of the ablation craters are studied by atomic force microscopy and scanning electron microscopy. With the exception of LaAlO3, the high temperature annealing of these oxide crystals produces atomically flat starting surfaces that enable studies of the morphology of ablation craters with subnanometer precision. The threshold fluence for ablation is determined directly from atomic-force microscopy images and increases approximately linearly with the band gap of the oxide. For all oxides except sapphire, the depth of the ablation crater increases approximately as the square root of the difference between the peak laser fluence and the threshold fluence for ablation. Sapphire shows unique behavior: (i) at laser fluences within 1J∕cm2 of the threshold for ablation, the depth of the ablation crater increases gradually instead of abruptly with laser fluence, and (ii) the rms roughness of the ablation crater shows a pronounced minimum of <0.2nm at a laser fluence of 1J∕cm2 above the threshold.