High-Order Janus Bound States in the Continuum Assisted by Unidirectional Guided Resonances.

Existing realizations of Janus bound states in the continuum (BICs) rely primarily on manipulating polarization singularities derived from accidental BICs. Here, we introduce a distinct strategy to engineer high-order Janus BICs by leveraging interband-coupling-induced unidirectional guided resonances (UGRs). By breaking the out-of-plane symmetry in a square-lattice photonic crystal slab, we manipulate the momentum-space evolution of UGRs, driving them to coalesce with an intrinsic symmetry-protected BIC at the Brillouin zone center. This process gives rise to a high-order Janus BIC exhibiting extreme radiative asymmetry: it yields an upward-radiated vortex beam with a topological charge of -3, while maintaining a +1 charge downward. Our findings establish a novel paradigm for tailoring momentum-space polarization singularities, unlocking new potential for generating vortex beams with direction-dependent topological charges in advanced optical information processing.