Artificial uniaxial and biaxial dielectrics with use of two-dimensional subwavelength binary gratings

TLDR

The study investigates two-dimensional symmetric and asymmetric subwavelength binary gratings and designs two- and three-level gratings using effective indices and quarter-wave Tschebyscheff synthesis to suppress reflections over a broad band. A method is presented for determining the three effective indices of a 2‑D subwavelength grating, including a theoretical formalization of the index parallel to the surface normal, and these indices are used with quarter-wave Tschebyscheff synthesis to design gratings. An asymmetric 2‑D binary grating behaves like a biaxial thin film, a symmetric grating like a uniaxial thin film, and a three‑level grating on a substrate with ns = 3.0 reduces reflections below 0.1 % from 8 µm to 12 µm.

Abstract

Two-dimensional symmetric and asymmetric subwavelength binary gratings are investigated. A method for determining the three effective indices of a two-dimensional (2-D) subwavelength grating is presented, as well as a theoretical formalization for the effective index parallel with the normal to the surface. It is shown that a 2-D asymmetric binary grating on the surface of a dielectric substrate is analogous to a biaxial thin film. If the grating is symmetric, then the two effective indices perpendicular to the normal are equal, and the grating is analogous to a uniaxial thin film. Using these effective indices and the quarter-wave Tschebyscheff synthesis technique, we designed two- and three-level binary gratings to suppress reflections over a broad band. It is shown that for a substrate index of ns = 3.0 a three-level 2-D binary grating reduced reflections below 0.1% from 8 μm to 12 μm.