Profile control of femtosecond laser-fabricated moth-eye structures on Si substrate

Abstract The present study focuses on controlling the antireflective (AR) characteristics of one-dimensional moth eye structures on a high-resistivity silicon substrate by adjusting the profile (i.e., refractive index distribution) of its micro-tapers using femtosecond laser processing. Moth eye structures that comprise periodically arranged tapers with varying sizes and profiles (triangle, parabola, stair, and triangle–stair) were fabricated using femtosecond laser ablation via a fine adjustment of the processing pattern. The AR characteristics of these samples were experimentally evaluated by a standard terahertz (THz) time-domain spectroscopy and modeled by employing the method of Finite-difference time-domain (FDTD). The measurement data agreed well with the simulation results, and the power reflectance of these structures was induced from approximately 30% (bare silicon substrate) to a very low value (less than 5%) for a broad THz band. Furthermore, the profiled structures showed a varying power reflectance distribution with respect to the frequency in the THz region, especially for the low reflection one (