Solitary Spectrally-Discrete Bound States in the Continuum in an Open System

TL;DR

This paper demonstrates that anisotropic waveguides with two radiation channels can support isolated, spectrally-discrete bound states in the continuum (BICs) that are lossless and exist at specific frequencies and directions.

Contribution

It introduces a new class of BICs in open systems with anisotropic materials where continuous mode families are absent, supporting discrete, lossless states.

Findings

Discrete BICs exist at specific frequencies and directions.

BICs are isolated, lossless needle-like states.

These BICs emerge from radiating leaky modes.

Abstract

Bound states in the continuum (BICs) exist in a variety of physical systems where they appear as lossless propagating states surrounded by radiating modes. However, in the case of open systems, they coexist with continuous families of guided states, which may be modes or other BICs, located in different regions of the frequency-momentum parameter space. Here we report that waveguiding structures comprising anisotropic materials with two radiation channels where continuous families of modes are not possible whatsoever can support discrete BICs that exist for a single frequency and a single propagation direction. The results are isolated spectrally-discrete BIC states existing as lossless needles emerging from a sea of radiating leaky modes.