Relationship between total reflection and Fabry-Perot bound states in the continuum

TL;DR

This paper explores the relationship between total reflection phenomena and the formation of Fabry-Perot bound states in the continuum within periodic dielectric structures, revealing conditions under which total reflection correlates with BICs.

Contribution

It clarifies the connection between total reflection and FP-BICs, showing that symmetry and wavenumber influence the existence of BICs near total reflection points.

Findings

FP-BICs are found near total reflection when wavenumber is zero or layers are symmetric.

Asymmetric layers with nonzero wavenumber do not support FP-BICs near total reflection.

Total reflection does not always imply the presence of FP-BICs, depending on symmetry and wavenumber.

Abstract

Bound states in the continuum (BICs) have interesting properties and important applications in photonics. A particular class of BICs are found in Fabry-Perot (FP) cavities formed by two parallel periodic dielectric layers separated by a distance $h$. A periodic dielectric layer can totally reflect a plane incident wave with a particular frequency and a particular wavenumber. Existing FP-BICs are found when $h$ is close to the values deduced from a phase-matching condition related to the reflection coefficient, but they are obtained in FP-cavities where the periodic layers have a reflection symmetry in the periodic direction. In this paper, we further clarify the connection between total reflections and FP-BICs. Our numerical results indicate that if the wavenumber is zero or the periodic layers have a reflection symmetry in the periodic direction, FP-BICs can indeed be found near the parameters of total reflections. However, if the wavenumber is nonzero and the periodic layer is asymmetric (in the periodic direction), we are unable to find a FP-BIC (with a frequency and a wavenumber near those of a total reflection) by tuning $h$ or other structural parameters. Consequently, a total reflection does not always lead to a FP-BIC even when the parameters of the FP-cavity are tuned.