Observation of Perfectly-Chiral Exceptional Point via Bound State in the Continuum.

Chirality is one of the fundamentals of nature with strong ties to asymmetry. In wave physics, it is conventionally characterized by asymmetric scattering of circularly polarized waves but suffers from two-state polarization. To overcome the limitation, here we demonstrate the concept of extreme chirality regarding orbital angular momentum (OAM) helicity, originating from a chiral quasibound state in the continuum held by a mirror-symmetry-broken metasurface. Empowered by the intrinsic OAM-selective coupling nature of the metasurface, the system arrives at a peculiar state where the left-handed incident vortex is completely absorbed while the right-handed counterpart is totally reflected, namely, a perfectly-chiral exceptional point. The realization of asymmetric OAM modulation creates the possibility to explore chirality with unlimited states. Our work raises a new paradigm for the study of extreme OAM chirality and enriches the physics of chiral wave-matter interaction.