Non-Hermitian chiral coalescence spawning from a quasi-bound state in the continuum

Non-Hermitian chiral coalescence associated with polarization in optical scattering systems has been routinely realized and applied. However, the scattering exceptional points (EPs) associated with polarization obtained by modes with broadband response and high dissipation constrain its further application in narrowband optics. Here, as a scheme, distinct from the implementation pathway of traditional EPs, we introduce a quasi-bound state in the continuum based on a quadrupole mode as the response environment for EP generation, achieving an EP with high-quality-factor characteristics on a metasurface. Furthermore, we demonstrate the robustness of EPs as intrinsic features of non-Hermitian systems, independent of specific parameter choices. Finally, we present an example of direct chirality detection and display, showcasing the maximum spin-selective property induced by EPs. Our approach unveils the potential of scattering chiral EPs for applications in narrowband optics.