Bound states in the continuum in a waveguide system of zero-index materials with a defect embedded

Abstract

The bound states in the continuum (BICs) have attracted great research interest recently due to the high quality factor (Q factor) resonance (quasi-BIC) when the BIC condition slightly deviates. As an important electromagnetic system, the waveguide is very favored for many high-Q applications due to its small-size and easy integration. In this paper, we report the realization of the BICs in a waveguide system of zero-index materials (ZIMs) by embedding a defect inside. By analytically investigating the reflectivity and eigenfrequency of the waveguide system, we show how a resonant state turns into a BIC with the decrease of the material parameter of the ZIM. The theoretical predictions are confirmed with numerical simulations. As an application, such a waveguide system is exploited to realize refractive index sensing, which shows high sensitivity and figure of merit. Our study may pave a new way for realizing high-Q applications in the waveguide system, such as sensors, filters, and nonlinear devices.