Merging the characteristics of an exceptional point and a quasi-bound state in the continuum in nanophotonic cavities

Abstract

In conventional eigenvalue analyses of non Hermitian two mode systems, mode coupling cannot produce the simultaneous occurrence of an exceptional point (EP) and a quasi bound state in the continuum (QBIC) at the same spectral position. This work shows that this limitation originates from the eigenvalue framework itself. By introducing an excitation phase degree of freedom, the interference between radiation channels can be reshaped without modifying the intrinsic eigenmodes of the system, thereby overcoming this constraint. Based on coupled mode theory, we demonstrate that the excitation phase enables the merging of an EP and a QBIC (and even a BIC) in nanophotonic cavities, and we validate this mechanism through full wave simulations of practical stacked structures. In the EP-QBIC regime, the mode quality (Q) factor is enhanced by more than one order of magnitude. We further systematically analyze the formation conditions of EP-QBIC states and conventional QBICs. Moreover, in a purely plasmonic structure, introducing an excitation phase leads to a more than 15 fold increase in the Q factor due to QBIC formation-surpassing the theoretical limit imposed by intrinsic material loss.