Observation of intrinsic chiral bound states in the continuum.

Photons with spin angular momentum possess intrinsic chirality, which underpins many phenomena including nonlinear optics, quantum optics, topological photonics and chiroptics. Intrinsic chirality is weak in natural materials, and recent theoretical proposals aimed to enlarge circular dichroism by resonant metasurfaces supporting bound states in the continuum that enhance substantially chiral light-matter interactions. Those insightful works resort to three-dimensional sophisticated geometries, which are too challenging to be realized for optical frequencies. Therefore, most of the experimental attempts showing strong circular dichroism rely on false/extrinsic chirality by using either oblique incidence or structural anisotropy. Here we report on the experimental realization of true/intrinsic chiral response with resonant metasurfaces in which the engineered slant geometry breaks both in-plane and out-of-plane symmetries. Our result marks, to our knowledge, the first observation of intrinsic chiral bound states in the continuum with near-unity circular dichroism of 0.93 and a high quality factor exceeding 2,663 for visible frequencies. Our chiral metasurfaces may lead to a plethora of applications in chiral light sources and detectors, chiral sensing, valleytronics and asymmetric photocatalysis.