Abstract
Periodic microstructures — ripples oriented perpendicular to the direction of radiation polarisation are produced by irradiating crystalline silicon with the (100) orientation of the surface by femtosecond laser pulses. The behaviour of the third-harmonic signal is studied in these microstructures. It is shown that the orientation dependences of the third-harmonic signal for optically induced ripples are determined by the morphology of the latter on the irradiated silicon surface, which allows the in situ diagnostics of these microstructures.