Contribution of defect on early stage of LIPSS formation.

We investigated an early stage of laser-induced periodic surface structure (LIPSS) formation to elucidate the contribution of defects on the formation. 4H-SiC crystals were irradiated by multiple pulses of femtosecond laser with different laser spot sizes. We observed the decrease in formation thresholds of high-spatial-frequency LIPSS (HSFL) and low-spatial-frequency LIPSS (LSFL) with the increased irradiated laser spot size. For smaller laser spot size, HSFL was only formed at the periphery of LSFL formation area, whereas for larger spot size, HSFL was randomly distributed within the laser spot. Our results are coincident with the hypothesis that the existence of defects in crystal contributes to the early stage on the formation of LIPSS, in which the electron excitation via one or two photon absorption in a defect site cause local nanoablation at a laser fluence under the intrinsic ablation threshold, followed by the formation of a nanovoid, which act as a scatterer, and interference of scattered wave and laser pulses lead to HSFL formation.