Laser fluence dependence of ripple formation on fused silica by femtosecond laser irradiation

Laser-induced periodic surface structures (LIPSS, ripples) have attracted great attentions in the fields of surfaces and interfaces, yet their origin still remains puzzling, especially for dielectrics. In this study, a comparative study of the formation of two distinct classes of ripples on fused silica is provided by carefully adjusting the key parameters of laser fluence and pulse number. Two different ablation regimes are identified for LIPSS formation. The threshold fluence of F = 4.07 J/cm² is defined for the transition from high spatial frequency LIPSS to low spatial frequency LIPSS, which is independent of pulse number. Pulse-to-pulse results show the unique formation processes for each class of LIPSS in different fluence regimes. In the fluence range below the transition threshold (2.54 J/cm² < F < 4.07 J/cm²), the formation of high spatial frequency LIPSS starts from randomly distributed nanogrooves perpendicularly orientated to E, indicating the excitation of surface-plasmon polaritons as a primary formation mechanism. In the higher fluence regime (F > 4.07 J/cm²), the development of a thin rim surrounding an ablation crater into straight ridges spreading across the crater is observed for the formation of low spatial frequency LIPSS, suggesting a thermal and fluid process involved in the LIPSS formation. The results show that laser fluence plays a key role in controlling the origin of ripples, while pulse number mainly contributes to the subsequent growth of microstructures.