The morphological and optical characteristics of femtosecond laser-induced large-area micro/nanostructures on GaAs, Si, and brass

We systematically study the morphological and optical characteristics of the large-area micro/nanostructures produced by femtosecond laser irradiation on GaAs, Si, and brass. The experimental results demonstrate that along with the increase of laser fluence, significant changes in the surface morphology can be observed, and the most prominent phenomenon is the enlarging of the feature size of formed structures. Interestingly, by the fourier analysis of the treated areas, a peculiar phenomenon can be revealed: as laser fluence increases, the spatial frequencies of the structures change following a specific law — the allowed main frequencies are discrete, and appear to be a sequence of 2f, f, f/2, f/4, and f/8 (f is the fundamental frequency corresponding to the near-subwavelength ripples). In our opinion, the new frequency components of f/2, f/4, and f/8 originate in the 2-order, 4-order, and 8-order grating coupling. The law can offer us new insights for the evolving mechanisms of a variety of laser-induced micro/nanostructures in different scales. Furthermore, the optical characteristics of the treated surface are strongly dependent on the morphological characteristics that are mainly determined by laser fluence, such as the feature size of the micro/nanostructures, the topology of the surface morphology, the surface roughness, and the irregular degree of the formed structures. In general, as laser fluence increases in a moderate range, the specular reflectance of the structured surface would be significantly reduced. However, if laser fluence is excessive, the anti-specular-reflection effect would be much weakened. In ideal laser fluence, the micro/nanostructures produced by the near-infrared laser can achieve an ultra-low specular reflectance in the visible and near-infrared spectral region, which exhibits an attracting application prospect in the field of utilizing solar energy.