Unclassical ripple patterns in single-crystal silicon produced by femtosecond laser irradiation

Laser-induced periodic surface structures (LIPSS) in single-crystal silicon upon irradiation with multiple linearly polarized femtosecond (fs) laser pulses (120 fs, 800 nm, 1 kHz) were investigated under different laser fluence and pulse number. Unclassical ripples (U-ripples), which were nearly parallel to the polarization of the laser beam, were observed to form gradually on the top of classical ripples with the effective pulse number. Their periods were significantly longer than the laser wavelength, and increased with increasing both the laser fluence and pulse number in the current study. The relationship between the types of ripple patterns and their parametric dependence was established. The mechanism of U-ripple formation was attributed to the capillary wave, arising from the inhomogeneous temperature gradient combined with the electric field of the pulses in the molten surface layer.

► Transition from classical ripples to unclassical ripples. ► Laser fluence has a significant effect on the unclassical ripple period. ► Relationship between structures and their parametric dependence is established. ► Capillary wave is responsible for the formation of unclassical ripples.