In-situ study of laser-induced novel ripples formation on SiC surface by an oblique-illumination high-resolution imaging setup

After decades of research, formation and development of laser induced periodic surface structures (LIPSS) was proposed to be closely related to the roughness of sample surface and multipulse incubation effect. To exclude the influence of the stochastic distribution of surface roughness on ripple formation and further study the multipulse incubation mechanism, there is an urgent need to induce ripples in situ and monitor the evolution of ripples synchronously. In this paper, single-site pulse-by-pulse irradiation was carried out on single-crystalline SiC surface and the induced ripples morphologies were in-situ captured by an oblique-illumination high -resolution imaging setup. A type of novel ripples with spacing located between traditional low-spatial-frequency-LIPSS and high-spatial-frequency-LIPSS was reported. In-situ imaging shows that concentric annular ripples were first induced by the 1st pulse, which were possibly caused by the Fresnel diffraction of a slightly divergent light passing through the aperture-shaped entrance pupil of microscopic objective lens. Simulations based on finite difference time domain method (FDTD) showed that next pulse with linear polarization acting on the annular ripples induced an asymmetric light distribution, leading to the generation of ripples perpendicular to laser polarization. Further simulations showed that the local field enhancement in the grooves of ripples led to the deepening of grooves and the resulting decrease of local ripple spacing with pulse number. Comparison of the morphologies of ablation spots induced by single-site pulse-by-pulse irradiation and successive pulse irradiation demonstrated that ripples developed faster in the former, further indicating the role of the pre-formed structures in promoting growth of ripples. Our oblique-illumination imaging technology provides direct observation of ripples evolution and the novel ripples here are useful for extending the ripples application in surface tailoring.