On the formation and features of the supra-wavelength grooves generated during femtosecond laser surface structuring of silicon

We have investigated the formation process of supra-wavelength quasi-periodic surface structures, commonly termed as grooves, produced during femtosecond laser surface structuring of a silicon target, in air. The experiments have been carried out at two wavelengths, namely 513 nm and 1026 nm, and in multi-pulse irradiation conditions. Both static, i.e., irradiating the same target location, and dynamic, namely scanning the beam along a line, configurations have been explored. We have also carried out simulations of the process by using a multiscale numerical model, based on the two-temperature model coupled to a fluid dynamics approach. The comparison of experimental and numerical results has allowed us to clearly establish the role of hydrothermal waves in the grooves formation process.