Metasurface demonstration of exceptional points partitioned by a line of bound states in the continuum

Bound states in the continuum (BICs) and coalesced states at exceptional points (EPs) represent two anomalous resonant states in non-Hermitian systems, attracting extensive interest due to their theoretical implications and practical applications in fields such as sensing, lasing, and nonlinear light-matter interactions. Understanding the relationship between BICs and EPs is therefore of central importance. Recent studies have demonstrated their coexistence in various optical systems, including gratings, photonic crystal slabs, and individual dielectric inclusions. In this paper, we design a simple coupled-split-ring metasurface to demonstrate the connections between BICs and EPs, providing a direct and intuitive mapping of the theoretical predictions from the coupled-mode theory. Specifically, a pair of EPs with opposite chirality is partitioned by a BIC line in the parameter space, indicating that during the process of adiabatic evolution between EPs, one of the modes transitions through a BIC state. Our findings unveil a more comprehensive picture of mode coupling in metasurfaces, paving the way for innovative non-Hermitian system designs and potentially revolutionizing the integration of BICs and EPs within a unified platform for advanced technological applications.