Effects of Pulse Accumulation on Femtosecond-Laser-Induced Periodic Surface Structures in Si and SiC

To investigate the effects of pulse accumulation in femtosecond laser processing, we developed an in-situ observation system by incorporating an optical window into a scanning electron microscope (SEM).This enabled in-situ, pulse-by-pulse monitoring of evolution of laser-induced periodic surface structures (LIPSS) on single-crystal silicon (Si) and silicon carbide (SiC) under identical irradiation conditions.On Si, LIPSS initially appeared with a period comparable to the laser wavelength and progressively increased in both period and groove width with successive pulses, indicating a positive feedback mechanism.In contrast, SiC exhibited stable LIPSS with a constant period of 160-200 nm throughout the irradiation.Additionally, focused ion beam (FIB) pretreatment on Si surfaces enhanced local electric fields, resulting in groove widening and preferential LIPSS formation near the modified regions.These findings support the hypothesis that initial surface morphology plays a critical role in modulating ablation dynamics through local field enhancement.