Bound states in the continuum in strong-coupling and weak-coupling regimes under the cylinder-ring transition

TL;DR

This paper investigates the transition of quasi-bound states in the continuum (BICs) from strong to weak coupling regimes in dielectric structures, revealing how their spectral features evolve during the cylinder-ring transition.

Contribution

It provides a detailed analysis of quasi-BIC behavior during the cylinder-ring transition, highlighting the crossover from strong to weak coupling regimes and discussing Fano resonance applicability.

Findings

Quasi-BICs transition from avoided crossing to intersection as coupling weakens.

Strong-coupling quasi-BICs involve interference of three waves in the far field.

Fano resonance concept is valid only under weak coupling conditions.

Abstract

Bound states in the continuum (BIC) have been at the forefront of research in optics and photonics over the past decade. It is of great interest to study the effects associated with quasi-BICs in the simplest structures, where quasi-BICs are very pronounced. An example is a dielectric cylinder, and in a number of works, quasi-BICs have been studied both in single cylinders and in structures composed of cylinders. In this work, we studied the properties of quasi-BICs during the transition from a homogeneous dielectric cylinder in an air environment to a ring with narrow walls while increasing the diameter of the inner air cylinder gradually. The results demonstrate the quasi-BIC crossover from the strong-coupling to the weak-coupling regime, which manifests itself in the transition from avoided crossing of branches to their intersection with the quasi-BIC being preserved on only one straight branch. In the regime of strong-coupling and quasi-BIC, three waves interfere in the far-field zone: two waves corresponding to the resonant modes of the structure and the wave scattered by the structure as a whole. The validity of the Fano resonance concept is discussed, since it describes the interference of only two waves under weak coupling conditions.