Trapped Mode Excitation in Dielectric Metasurfaces with an Inhomogeneous Superstrate

Abstract

In the electromagnetic wave theory, the term “trapped mode” (also known as a dark mode and bound state in the continuum) is used to describe modes of a system that are weakly coupled to free space. In electromagnetic metasurfaces, to excite a trapped mode by a field of incident radiation, a certain perturbation is introduced into their unit cells to break spatial symmetry. Here we discuss an alternative mechanism of excitation of trapped modes in metasurfaces by introducing inhomogeneity into the upper layer (superstrate) covering the structure. The finite-size metasurface under study is made of dielectric disk-shaped resonators regularly arranged on a thick substrate. The dielectric properties of an inhomogeneous superstrate are described by the randomized Weierstrass function, which is widely used when modeling polymer mixtures. In our study, we establish a relationship between the excitation conditions of the trapped mode and the degree of introduced disorder. The issues of the quality factor of the trapped mode and the features of the electromagnetic near-field localization in the metasurface are discussed as well. The results obtained are important for implementing metasurface-based spasers and nanolasers to reveal the relation between the disorder degree and system coherence.