Electromagnetic resonance modes on a two-dimensional tandem grating and its application for broadband absorption in the visible spectrum

Abstract

In this work, we numerically investigate the electromagnetic resonances on two-dimensional tandem grating structures. The base of a tandem grating consists of an opaque Au substrate, a SiO2 spacer, and a Au grating (concave type); that is, a well-known fishnet structure forming Au/SiO2/Au stack. A convex-type Au grating (i.e., topmost grating) is then attached on top of the base fishnet structure with or without additional SiO2 spacer, resulting in two types of tandem grating structures. In order to calculate the spectral reflectance and local magnetic field distribution, the finite-difference time-domain method is employed. When the topmost Au grating is directly added onto the base fishnet structure, the surface plasmon and magnetic polariton in the base structure are branched out due to the geometric asymmetry with respect to the SiO2 spacer. If additional SiO2 spacer is added between the topmost Au grating and the base fishnet structure, new magnetic resonance modes appear due to coupling between two vertically aligned Au/SiO2/Au stacks. With the understanding of multiple electromagnetic resonance modes on the proposed tandem grating structures, we successfully design a broadband absorber made of Au and SiO2 in the visible spectrum.