Ultrafast laser surface irradiation of silicon: Effects of repetition rate in vacuum and air

The process of surface structuring of a silicon target with femtosecond laser pulses (pulse width ?180 fs; wavelength ??1030 nm) at repetition rates varying from 0.01 to 200 kHz is investigated in vacuum and in air, in static irradiation conditions. The threshold fluence for the formation of a shallow crater, as well as the surface features generated inside the crater, are analyzed at different repetition rate for a fixed sequence of N laser pulses. Our experimental results evidence a clear difference in the morphology of the surface structures generated in vacuum and air, addressing a change in the laser-target energy coupling at the higher pulse repetition rates. The observed behavior is rationalized in terms of the effects induced by the presence of nanoparticle debris, as well as of a plume shielding occurring at repetition rates larger than 10 kHz in air. Both such effects are hindered for vacuum irradiation conditions. Our experimental findings highlight the impact of plume shielding, and nanoparticles coverage on laser material processing in air at high repetition rates. These observations, thus, provide new useful insights in the field of fast laser processing essentially required in many applications of nanoscience and nanotechnologies.