Abstract
Abstract When two nonorthogonal resonances are coupled to the same radiation channel, avoided crossing arises and a bound state in the continuum (BIC) appears with appropriate conditions 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 (1D) 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.
Highlights
The ability to confine light to limited regions is of fundamental importance in both basic science and practical applications
We show that unidirectional-bound state in the continuum (BIC) are accompanied by avoided crossings due to guided-mode resonances with different transverse parities in asymmetric photonic lattices
We show that unidirectional-BICs are accompanied by avoided crossings due to two guided-mode resonances with different transverse parities in asymmetric photonic lattices
Summary
The ability to confine light to limited regions is of fundamental importance in both basic science and practical applications. Electromagnetic waves can be localized in photonic structures by separating specific eigenmodes away from the continuum of radiating modes. Friedrich–Wintgen BICs, which are generally found in the vicinity of the avoided crossing of two dispersion curves, arise because of the destructive interference of two guided-mode resonances coupled to the same radiation channel [32]. The aim of the present paper is to address the fundamental properties of avoided crossings and BICs due to coupled guided-mode resonances in one-dimensional (1D) leaky-mode photonic lattices. The Friedrich–Wintgen BIC can be found near the avoided crossing in the photonic band structure without the fine tuning of structural parameters. We investigated BICs and avoided crossing due to two different waveguide modes in photonic lattice slabs with symmetric and asymmetric cladding layers through finite element method (FEM) simulations and temporal coupled-mode formalism. The Q factor of the unidirectional-BIC is finite but its radiation power in the upward or downward direction is significantly smaller than that in the opposite direction
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