Formation mechanism of femtosecond laser-induced high spatial frequency ripples on semiconductors at low fluence and high repetition rate

Periodic high spatial frequency ripples structures (HSFL) have been generated in silicon (Si) and in germanium (Ge) at very low fluence below or close to the melting fluence threshold, at different wavelengths and at high repetition rate femtosecond laser pulses (80 MHz, 700-950 nm, 170 fs). HSFL initiation, formation, and arrangement combine structural modification of the surface initiated by heat accumulation of successive pulses with second harmonic generation. HSFL are wavelength dependent and the refractive index plays a central role on their periodicities. HSFL spacing follows quite well a law of = / 2 n*, where n * is the modified femtosecond laser excited refractive index as a function of the wavelength for Si and Ge.