Influence of ambient pressure on surface structures generated by ultrashort laser pulse irradiation

We report an experimental investigation on the surface structures induced by linearly polarized ? 900 fs laser pulses, at ? = 1055 nm, on silicon at different values of the ambient pressure, from ~ 10-4 mbar to one atmosphere. Our experimental findings address interesting influences of the surrounding pressure on: (1) the spatial period of ripples; (2) the formation of micro-grooves; (3) the shape of the structured area. Moreover, the effects of various states of polarization in vacuum as well as of circularly polarized pulses in air vs vacuum are also addressed. We identify as one possible key element of such experimental observations: the fact that as the pressure raises the ablated nanoparticles produced during the femtosecond ablation process of the target get deposited more and more on the sample surface covering the irradiated spot area and influencing the structuring process.