High-$Q$ resonances in periodic photonic structures

The support of high-$Q$ resonances in all-dielectric nanostructures are necessary for the enhancement of light-matter interactions. By introducing some perturbations into periodic structures to enable an engineered coupling between the supported bound modes and the exterior environment, quasibound modes can consequently be achieved. However, the nature of those modes is still in need of clear investigation. In this work, we employ two geometrically similar binary gratings and demonstrate with both numerical and experimental results that two fundamentally different bound modes may lie behind the high-$Q$ resonances reported in the literature: the bound state in the continuum (BIC) and the guided modes in subwavelength photonic lattices. These two modes have distinct dependences of the $Q$ factor on the wave vector and when they are converted into quasibound (leaky) modes, the same behaviors are retained. We find that only those high-$Q$ resonances with the predecessor of guided modes have robust $Q$ factors over a large range of wave vectors and frequencies, which suggests the high tunability of resonances with comparable $Q$ values by the incident angle. In contrast, the BIC provides more versatility for dealing with optical resonances with both high-$Q$ factors and exotic polarizations. These findings correct the general misunderstanding of the origin of high-$Q$ resonances in periodic photonic structures and will significantly broaden the landscape of all-dielectric nanophotonics.