# Bound States in the Continuum in Bilayer Photonic Crystal with TE-TM Cross-Coupling

## TL;DR

This paper explores the design and analysis of bilayer photonic crystals that support high-Q bound states in the continuum (BICs) influenced by TE-TM polarization coupling and symmetry considerations, offering new insights into multilayer photonic systems.

## Contribution

It introduces a novel bilayer photonic crystal design demonstrating BICs with TE-TM coupling and broken mirror symmetry, expanding understanding beyond single polarization models.

## Key findings

- High-Q BIC states with TE-TM coupling are achievable in bilayer structures.
- Broken mirror-flip symmetry enhances TE-TM coupling and BIC quality factors.
- BICs exhibit unique spatial characteristics and layer selectivity.

## Abstract

Bound states in the continuum (BICs) in photonic crystals represent the unique solutions of wave equations possessing an infinite quality-factor. We design a type of bilayer photonic crystal and study the influence of symmetry and coupling between TE and TM polarizations on BICs. The BIC modes possess $C_{3v}$ symmetry in the x-y plane while the mirror-flip symmetry in the z-direction is broken, and they provide selective coupling into different layers by varying frequency. The enhanced TE-TM coupling due to broken mirror-flip symmetry in the z-direction gives rise to high-Q factor BIC states with unique spatial characteristics. We show the emergence of such BIC states even in the presence of coupling between the TE- and TM-like modes, which is different from the existing single polarization BIC models. We propose to study BICs in multilayer systems, and the results may be helpful in designing photonic settings to observe and manipulate BICs with various symmetries and polarizations for practical applications.

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Source: https://tomesphere.com/paper/1906.06102