Effects of Femtosecond Laser Ablation Regimes on the Formation and Evolution of Periodic Structures

Fabrication of metal and semiconductor surface periodic structures (LIPSS) by femtosecond laser is a hot topic in ultrafast laser fabrication recently. Developments of formation and evolution of the LIPSS period have received extensive attention because of its great application value and complex physical background. It is well known that increase of single pulse energy density and cumulative pulse number will eventually lead LIPSS period to be fuzzy or disappearance. In this paper, a new phenomenon has been found that the LIPSS appears damaged or disappears in the medium laser fluence range from $\boldsymbol{23.4}\ \mathbf{mJ}/\mathbf{cm}^{\boldsymbol{2}}$ to $\boldsymbol{52.6}\ \mathbf{mJ}/\mathbf{cm}^{\boldsymbol{2}}$, in addition, the opposite law of LIPSS evolution was observed in this experiment. According to the experimental results, this phenomenon is attributed to the difference of femtosecond laser ablation mechanisms in different laser fluence intervals, and the effects of these two mechanisms on the LIPSS periodic evolution are discussed. This study extends our deep understanding of the mechanism of femtosecond laser-induced LIPSS formation, and provides a certain reference value for future application transformation and reasonable control of experimental parameters to optimize the preparation of LIPSS.