Surface topography of ultrashort laser-irradiated CaF2

A single-crystal CaF2 (111) was irradiated with single and multiple laser (Ti:sapphire, 800nm, 25fs) shots at fluences ranging from 0.25 to 1.5Jcm-2. In this fluence regime, a single laser pulse usually leads to typical bump-like features ranging from 200nm to 1.5m in diameter and 10-50nm in height. These bumps are related to compressive stresses due to a pressure build-up induced by fast laser heating and their subsequent relaxation. When CaF2 is irradiated with successive (in our case 20) shots at a laser fluence of 1.5Jcm-2, nanocavities at the top of the microbumps are observed. The formation of these nanocavities is regarded as an explosion and is attributed to the explosive expansion generated by shock waves due to laser-induced plasma after the nonlinear absorption of the laser energy by the material. Such kinds of surface structures at the nanometre scale could be attractive for nanolithography.