Bound states in the continuum accompanied by avoided crossings in leaky-mode photonic lattices

TL;DR

This paper investigates the formation of bound states in the continuum (BICs) and avoided crossings in leaky-mode photonic lattices, revealing how symmetry and mode coupling influence their properties and potential applications.

Contribution

It provides both numerical and analytical insights into BICs and avoided crossings in one-dimensional photonic lattices, highlighting effects of symmetry and mode coupling on their characteristics.

Findings

Friedrich-Wintgen BICs with infinite lifetime occur in symmetric lattices.

Quasi-BICs with finite lifetime appear in asymmetric lattices due to incomplete radiation suppression.

Unidirectional-BICs exhibit significantly reduced radiation in one direction, influenced by mode parity.

Abstract

When two nonorthogonal resonances are coupled to the same radiation channel, avoided crossing arises and a bound state in the continuum (BIC) appears in parametric space. This paper presents numerical and analytical results on the properties of avoided crossing and BIC due to the coupled guided-mode resonances in one-dimensional leaky-mode photonic lattices with slab geometry. In symmetric photonic lattices with up-down mirror symmetry, Friedrich-Wintgen BICs with infinite lifetime are accompanied by avoided crossings due to the coupling between two guided modes with the same transverse parity. In asymmetric photonic lattices with broken up-down mirror symmetry, quasi-BICs with finite lifetime appear with avoided crossings because radiating waves from different modes cannot be completely eliminated. We also show that unidirectional-BICs are accompanied by avoided crossings due to guided-mode resonances with different transverse parities in asymmetric photonic lattices. The Q factor of a unidirectional-BIC is finite, but its radiation power in the upward or downward direction is significantly smaller than that in the opposite direction. Our results may be helpful in engineering BICs and avoided crossings in diverse photonic systems that support leaky modes.