Full Asymmetric Radiation Control Through Multi-Channel Bound States in the Continuum.

Bound states in the continuum (BICs) are waves exhibiting theoretically infinite quality factors, offering a powerful mechanism for extreme light confinement in photonic structures. Although breaking vertical structural symmetry in BICs-supporting systems can induce asymmetric radiation, the radiated power typically remains partitioned between opposing half-spaces. Furthermore, achieving arbitrary control over the amplitude ratio and phase difference of these counter-propagating beams presents a significant challenge, thereby limiting sophisticated beam manipulation within a single half-space. In this work, we delve into BICs within the superwavelength regime, where photonic structures inherently support multiple diffraction orders. We systematically investigate the far-field polarization states and associated topological properties of these individual diffraction channels. Critically, by engineering a configuration that supports two co-propagating diffraction orders directed into the same half-space, we demonstrate comprehensive and continuous control over the resulting unidirectional guided resonances (UGRs). Full tunability of both the directionality (spanning from -1 to 1) and the relative phase difference (spanning from -π to π) between these two co-propagating beams is achieved. This versatile manipulation of multiple beams radiating concertedly into a specific direction opens new avenues for various advanced applications.