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

TL;DR

This paper demonstrates that by introducing an excitation-phase degree of freedom, it is possible to merge an exceptional point and a quasi-bound state in the continuum in nanophotonic cavities, leading to enhanced mode quality factors.

Contribution

The study reveals a new mechanism to merge EP and QBIC in nanophotonic cavities using excitation-phase control, supported by coupled mode theory and numerical simulations.

Findings

Mode Q factor can be enhanced by over one order of magnitude in EP-QBIC regime.

Q factor can increase over 15 times in plasmonic structures due to QBIC formation.

Conditions for forming EP-QBIC and conventional QBIC states are systematically identified.

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.