The Metamolecular Predictive Method of Exceptional Point and Quasi-Bound State In the Continuum

Exceptional point (EP) and quasi-bound state in the continuum (QBIC) are unique physical phenomena in non-Hermitian metasurfaces. The exceptional topological (ET) phase, representing a novel state where the system's energy approaches zero, has been extensively investigated in EP. However, the meta-atom design and prediction of EP and QBIC remain intricate and challenging, with most research relying on the Exhaustive method to determine the structures. Furthermore, studies on QBIC primarily focus on the Fano phenomenon, often neglecting the polarized zero point and ET phase associated with QBIC. This study utilizes the coupling theory and the Jones matrix to model the polarization behavior of metamolecules. A polarization transmission matrix is utilized to predict the metamolecular structures of EP and QBIC, thus avoiding the ambiguity and complexity inherent in the Exhaustive method. An analysis of the metamolecular zero-point characteristics of EP and QBIC within the framework of coupled equations reveals that the EP is associated with a cross-polarized zero point, and the QBIC is associated with a co-polarized zero point. Owing to the different characteristics of the zero points, the metamolecule of EP induces chirality within the ET phase, whereas the metamolecule of QBIC induces achirality within the ET phase, but displays chirality within the spectrum. This study proposes a metamolecular predictive method for EP and QBIC and introduces an innovative methodology for designing chiral metasurfaces.