Low spatial frequency laser-induced periodic surface structures in fused silica inscribed by widely tunable femtosecond laser pulses

The formation and evolution of laser-induced periodic surface structures in fused silica under irradiation of widely tunable (in the1−3 μm range) linearly polarized femtosecond (200 fs) pulses was studied experimentally. The structures were inscribed inhigh fluence regime (exceeding the surface ablation threshold for a single pulse) and characterized by using scanning electronmicroscopy and two dimensional Fourier transform. The results revealed rapid (after irradiation with a few successive pulses)formation of periodic laser-induced periodic surface structures aligned parallel to laser polarization, whose period increases withincreasing the inscription wavelength, obeying the λ/n law. With further increase of number of pulses, the generated structuresgradually reorganize into laser polarization-independent low spatial frequency annular structures associated with formation ofthe damage crater, which fully established after irradiation with a few tens of successive laser pulses. This particular evolutionscenario was observed over the entire wavelength tuning range of incident pulses.