Continuous Ring of Unidirectional Guided Resonances Induced by Isotropic Interband Coupling

Unidirectional guided resonances (UGRs)-topological polarization singularities in momentum space with intrinsic unidirectional radiation-have recently garnered widespread attention. However, previous studies have been largely limited to realizing only a few discrete UGRs. We demonstrate an unprecedented continuous ring of UGRs in a hexagonal bilayer cylinder array, whose formation is governed by isotropic interband coupling, ensuring robust and azimuthally continuous unidirectional emission. Furthermore, by leveraging an in-plane inversion symmetry-protected bound state in the continuum at the Γ point, the continuous ring of UGRs acquires its topological properties from the enclosed bound state in the continuum, manifesting as a phase vortex with a nontrivial topological charge. This unique combination of continuous unidirectionality and a global topological charge provides a robust platform for developing novel devices, such as unidirectional vortex lasers.