Emission of nano-particles during ultrashort laser irradiation of silicon targets

We have experimentally observed that nano-particles emission is a peculiar feature of matter removal during ultrashort (fs) laser pulse irradiation of silicon followed by vacuum expansion. Time-resolved optical emission spectroscopy has been used to characterize the plume expansion dynamics, while atomic force microscopy analysis of silicon nano-particles deposited after 100 fs laser ablation has shown an average particle size of about 7 nm, with a pretty narrow size distribution. This is in agreement with the predictions of recent theoretical models on the mechanisms underlying ablation of solids in this ultrashort temporal regime. Finally, a phenomenological model of nano-particles cooling during their vacuum expansion has allowed us to single out radiative cooling as the most effective cooling mechanism in the late stage of plume expansion. (authors)