Influence of femtosecond laser pulse repetition rate on thermochemical laser-induced periodic surface structures formation by focused astigmatic Gaussian beam

In this paper, we present the study of the thermochemical laser-induced periodic surface structures (TLIPSS) formation on a thin chromium film under femtosecond laser (λ  =  1026 nm, τ  =  232 fs) irradiation focused on the astigmatic spot with an axis ratio of  ≈1/10 and a width along the long side of 155 µm. The effect of the number of laser pulses and the repetition rate (2–200 kHz) on the formation of the structures is investigated. Variation of the number of pulses allowed us to observe the dynamics of the formation of structures. It has been found that the ordering of TLIPSS structures increases with increasing repetition rate of laser pulses, which is explained by the heat accumulation effect on the oxidation process and, consequently, on the formation of TLIPSS structures in the case of a high repetition rate of pulses, which is confirmed by the results of numerical modelling. This effect leads to a different oxidation dynamics and the formation of various types of oxides: predominance of CrO₂ in the case of a high-rate impact and an increase in the contribution of Cr₂O₃ in the case of a low repetition rate of pulses. (letter)