Effects of supra-wavelength periodic structures on the formation of 1D/2D periodic nanostructures by femtosecond lasers

Abstract

Femtosecond laser-induced periodic surface structure (fs-LIPSS) is a high-throughput parallel laser patterning method to fabricate various surface structures of sub-micrometer scale; the morphology and regularity of the structured surface determine its function. In this paper, single and double femtosecond laser beams are used to fabricate nanostructures on a silicon substrate. In the experiments, it is found that the appearance of supra-wavelength structures with periods longer than the laser wavelength not only deteriorate the regularity of the sub-wavelength periodic ripples but also hinder the fabrication of the deep sub-wavelength ripples when a single femtosecond laser is used. In addition, when double time-delayed femtosecond laser beams are used, 2D nanostructures, such as hexagonally arranged nano-hole array, cannot be fabricated once the supra-wavelength structures appear. The formation mechanism of the supra-wavelength periodic structures, which cannot be explained by the interference between the incident laser and surface plasmon polaritons, may be attributed to the hydrothermal wave on the molten surface layer. These findings provide insights to understand the mechanisms of 1D and 2D nanostructures produced by single and dual femtosecond laser beams, which is critical for fabricating functional surfaces.