Ultrafast dynamics of femtosecond laser-induced high spatial frequency periodic structures on silicon surfaces

Femtosecond laser-induced periodic surface structures (LIPSS) have been extensively studied over the past few decades. In particular, the period and groove width of high-spatial-frequency LIPSS (HSFL) is much smaller than the diffraction limit, making it a useful method for efficient nanomanufacturing. However, compared with the low-spatial-frequency LIPSS (LSFL), the structure size of the HSFL is smaller, and it is more easily submerged. Therefore, the formation mechanism of HSFL is complex and has always been a research hotspot in this field. In this study, regular LSFL with a period of 760 nm was fabricated in advance on a silicon surface with two-beam interference using an 800 nm, 50 fs femtosecond laser. The ultrafast dynamics of HSFL formation on the silicon surface of prefabricated LSFL under single femtosecond laser pulse irradiation were observed and analyzed for the first time using collinear pump-probe imaging method. In general, the evolution of the surface structure undergoes five sequential stages: the LSFL begins to split, becomes uniform HSFL, degenerates into an irregular LSFL, undergoes secondary splitting into a weakly uniform HSFL, and evolves into an irregular LSFL or is submerged. The results indicate that the local enhancement of the submerged nanocavity, or the nanoplasma, in the prefabricated LSFL ridge led to the splitting of the LSFL, and the thermodynamic effect drove the homogenization of the splitting LSFL, which evolved into HSFL.