Dynamics of the formation of laser-induced periodic surface structures on dielectrics and semiconductors upon femtosecond laser pulse irradiation sequences

The formation of laser-induced periodic surface structures (LIPSS) upon irradiation of fused silica and silicon with multiple (N-DPS) irradiation sequences consisting of linearly polarized femtosecond laser pulse pairs (pulse duration similar to 150 fs, central wavelength similar to 800 nm) is studied experimentally. Nearly equal-energy double-pulse sequences are generated allowing the temporal pulse delay Delta t between the cross-polarized individual fs-laser pulses to be varied from -40 ps to + 40 ps with a resolution of similar to 0.2 ps. The surface morphologies of the irradiated surface areas are characterized by means of scanning electron and scanning force microscopy. Particularly for dielectrics in the sub-ps delay range striking differences in the orientation and spatial characteristics of the LIPSS can be observed. For fused silica, a significant decrease of the LIPSS spatial periods from similar to 790 nm towards similar to 550 nm is demonstrated for delay changes of less than similar to 2 ps. In contrast, for silicon under similar irradiation conditions, the LIPSS periods remain constant (similar to 760 nm) for delays up to 40 ps. The results prove the impact of laser-induced electrons in the conduction band of the solid and associated transient changes of the optical properties on fs-LIPSS formation.