Observation of intrinsic chiral bound states in the continuum.

Photons with spin angular momentum possess intrinsic chirality, which underpins many phenomena including nonlinear optics 1 , quantum optics 2 , topological photonics 3 and chiroptics 4 . Intrinsic chirality is weak in natural materials, and recent theoretical proposals 5-7 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 8 . Therefore, most of the experimental attempts 9-11 showing strong circular dichroism rely on false/extrinsic chirality by using either oblique incidence 9,10 or structural anisotropy 11 . 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.