Deepening of nanograting structures on Si by a two-step laser spatial-selective amorphization strategy combined with chemical etching

Abstract

Femtosecond laser induced periodic surface structures (LIPSSs) have been implemented on various materials. However, its further development in manipulation of optical properties is hindered by the shallow depth. Herein, the paper proposes a novel method of two-step spatial-selective amorphization for producing the deep regular nanogratings on silicon by femtosecond laser and chemical etching with HF acid. In the first step, the spatially periodic amorphization of silicon surface is fabricated by the surface plasmon excitation of the incident femtosecond laser, the chemical etching of which produces the shallow gratings with a period of 765 nm, or so-called prefabricated-LIPSSs (pre-LIPSSs). During the second step, the spatial-selective amorphization of sample takes place by exploiting the local field enhancement of laser irradiation within the grooves. After chemical etching, the deep local field enhancement assisted LIPSSs (LFE-LIPSSs) are formed, with a three-fold larger depth of 155 nm but an invariable period. Comparing with the pre-LIPSSs, the LFE-LIPSSs show 45% increase in anti-reflection performance at around 700 nm and surface-enhanced Raman scattering (SERS) anisotropy ratio is increased about four times with 4-MBA molecules as probe molecules. This work provides a promising approach to fabricate deeper LIPSSs for more effectively controlling the optical properties of materials by femtosecond laser.