Topological Nature of Radiation Asymmetry in Bilayer Metagratings

Manipulating radiation asymmetry of photonic structures is of particular interest in many photonic applications such as directional optical antenna, high efficiency on-chip lasers, and coherent light control. Here, we proposed a term of pseudopolarization to reveal the topological nature of radiation asymmetry in bilayer metagratings. Robust pseudopolarization vortex with an integer topological charge exists in $P$-symmetry metagrating, allowing for tunable directionality ranging from $\ensuremath{-}1$ to 1 in synthetic parameter space. When $P$-symmetry breaking, such vortex becomes pairs of $C$ points due to the conservation law of charge, leading to the phase difference of radiation asymmetry from $\ensuremath{\pi}/2$ to $3\ensuremath{\pi}/2$. Furthermore, topologically enabled coherent perfect absorption is robust with customized phase difference at will between two counterpropagating external light sources. This Letter can not only enrich the understanding of two particular topological photonic behaviors, i.e., bound state in the continuum and unidirectional guided resonance, but also provide a topological view on radiation asymmetry, opening an unexplored avenue for asymmetric light manipulation in on-chip laser, light-light switch, and quantum emitters.