Merging exceptional point and quasi-bound state in the continuum in nanophotonic cavities

Abstract: In conventional eigenvalue analyses of non-Hermitian systems, the coupling of two modes does not lead to the coexistence of an exceptional point (EP) and a quasi-bound state in the continuum (QBIC) at the same spectral position. Here, we theoretically demonstrate that introducing an excitation-phase degree of freedom enables the merging of an EP and a QBIC (or even a BIC) in nanophotonic cavities. Using coupled mode theory, we reveal the underlying mechanism of this phenomenon and further validate it via numerical simulations on practical stacked structures. In the EP-QBIC regime, the mode quality (Q) factor can be enhanced by over one order of magnitude. Moreover, we systematically investigate the formation conditions for EP-QBIC states and conventional QBICs. Additionally, introducing an excitation-phase degree of freedom in a pure plasmonic structure allows the Q factor to increase by over 15 times due to QBIC formation-even breaking through the limit imposed by material loss.