Femtosecond laser-induced periodic surface structures on fused silica: The impact of the initial substrate temperature

The formation and properties of laser-induced periodic surface structures (LIPSS) were investigated upon fs-laser irradiation of fused silica at different initial substrate temperatures, T_S. For substrate heating between room temperature, T_RT, and T_S = 1200 °C, a continuous wave CO₂ laser was used as the radiation source. The surface structures generated in the air environment at normal incidence with five successive fs-laser pulses (pulse duration, τ = 300 fs, laser wavelength, λ = 1025 nm, repetition frequency, f_rep = 1 kHz) were characterized by using optical microscopy, scanning electron microscopy, and 2D-Fourier transform analysis. The threshold fluence of fused silica was systematically investigated as a function of T_S. It was shown that the threshold fluence for the formation of low-spatial frequency LIPSS (LSFL) decreases with increasing T_S. The results reveal that the initial spatial period observed at T_RT is notably increased by increasing T_S, finally leading to the formation of supra-wavelength LIPSS. The findings are discussed in the framework of the electromagnetic interference theory, supplemented with an analysis based on thermo-convective instability occurring in the laser-induced molten layer. Our findings provide qualitative insights into the formation mechanisms of LIPSS, which allow improvements of the control of nanostructure formation to be made for corresponding applications of dielectric materials in the future.