Formation of periodic nanoripples on silicon and germanium induced by femtosecond laser pulses

We report on the generation of high spatial frequency LIPSS (HSFL) in silicon and germanium induced by nanojoule femtosecond laser pulses at high repetition rate in air. The periodicity of the ripples is found to be of 125 nm for silicon and of 98 nm for germanium. The formation of these HSFL in silicon and germanium seems to be induced by second harmonic generation (SHG) and the periods agree quite well with a /2n-law. Reproducible and accurate patterning of uniform HSFL on surface areas up to 100 mm2 can be performed using a versatile laser processing workstation.