Intrinsic chirality in two-dimensional dimer photonic crystals enabled by cross-polarization-suppressed bound states in the continuum

Breaking in-plane and out-of-plane symmetries in photonic crystals is key to achieving intrinsic chirality, enabling chiroptical responses and chiral emission. Bound states in the continuum enhance quality factors and light-matter interactions, but achieving both high quality factor and true intrinsic chirality is challenging, as three-dimensional chiral structures are complex to fabricate and conventional two-dimensional designs often rely on polarization conversion. Here, we demonstrate an intrinsically chiral two-dimensional planar photonic crystal slab based on a quasi-bound state in the continuum that breaks only in-plane symmetry using nanohole dimers. The proposed slab selectively couples to circular polarization regardless of incident direction without out-of-plane perturbations. Co-polarized circular dichroism and the degree of circular polarization of chiral emission reach −0.96 and 0.73, respectively, with a high quality factor of 4892. Our design is compatible with standard top-down lithography and offers potential for chiral lasing and sensing.