# Bound states in the continuum in symmetric and asymmetric photonic crystal slabs

## TL;DR

This paper introduces a semi-analytical model for bound states in the continuum in photonic crystal slabs, predicting their existence, positions, and behavior with slab thickness and symmetry breaking.

## Contribution

A novel semi-analytical multimode Fabry-Perot model that accurately predicts BICs in symmetric and asymmetric photonic crystal slabs.

## Key findings

- BICs cannot exist below a certain cut-off frequency regardless of slab thickness.
- Symmetry-protected BICs can persist even when horizontal mirror symmetry is broken, at specific slab thicknesses.
- The model effectively describes BIC dynamics in various slab configurations.

## Abstract

We develop a semi-analytical model to describe bound states in the continuum (BICs) in photonic crystal slabs. We model leaky modes supported by photonic crystal slabs as a transverse Fabry-Perot resonance composed of a few propagative Bloch waves bouncing back and forth vertically inside the slab. This multimode Fabry-Perot model accurately predicts the existence of BICs and their positions in the parameter space. We show that, regardless of the slab thickness, BICs cannot exist below a cut-off frequency, which is related to the existence of the second-order Bloch wave in the photonic crystal. Thanks to the semi-analyticity of the model, we investigate the dynamics of BICs with the slab thickness in symmetric and asymmetric photonic crystal slabs. We evidence that the symmetry-protected BICs that exist in symmetric structures at the {\Gamma}-point of the dispersion diagram can still exist when the horizontal mirror symmetry is broken, but only for particular values of the slab thickness.