![]() 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. We also show that unidirectional-BICs are accompanied by avoided crossings due to guided-mode resonances with different transverse parities in asymmetric photonic lattices. 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. 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. 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. ![]() 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.
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