Emission of nanoparticles during ultrashort laser irradiation of silicon targets.

We have experimentally observed that nanoparticles 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 nanoparticles deposited after 100 fs laser ablation has shown an average particle size of ≈ 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 nanoparticles cooling during their vacuum expansion has allowedus to single out radiative cooling as the most effective cooling mechanism in the late stage of plume expansion.