Maximally Chiral Emission via Chiral Quasibound States in the Continuum

Abstract Although numerous natural materials exhibit chiral optical phenomena, they are typically very weak. Chiral nanophotonic structures can significantly enhance the chiroptical responses and provide unprecedented design flexibility. However, achieving extreme chirality that approaches the ultimate theoretical limit remains challenging. Here, chiral quasibound states in the continuum are realized in the visible range, and maximally chiral emission from a perovskite metasurface is demonstrated. A perovskite film is spin‐coated on a patterned glass substrate. Grayscale lithography is employed to control the etching depths in the substrate and induce out‐of‐plane symmetry breaking. An extremely high level of chiral emission is experimentally achieved in the normal direction at room temperature. Chiral emission is maximally enhanced for one helicity via critical coupling, while strongly suppressed for the other helicity. The physical mechanism is explained using the reciprocity principle. Approaching the ultimate limit of chiral responses may lead to far‐reaching consequences in various important applications as well as fundamental studies.