Understanding inter-site mutual mode interaction in coupled physical systems is essential to comprehend large compound systems, as this local interaction determines the successive multiple inter-site energy transfer efficiencies. In the present study, we demonstrate that only the non-Hermitian coupling can correctly account for the light transfer between two coupled optical cavities. We also reveal that the non-Hermitian coupling effect becomes crucial as the system dimension decreases. Our results provide important insight for handling general-coupled devices in the subwavelength regime.

A large number of different types of second-order non-Hermitian degeneracies called exceptional points (EPs) were found in various physical systems depending on the mechanism of coupling between eigenstates. We show that these EPs can be hybridized to form higher-order EPs, which preserve the original properties of the initial EPs before hybridization. For a demonstration, we hybridize chiral and supermode second-order EPs, where the former and the latter are the results of intra-disk and inter-disk mode coupling in an optical system comprised of two Mie-scale microdisks and one Rayleigh-scale scatterer. The high sensitivity of the resulting third-order EP against external perturbations in our feasible system is emphasized.

Open quantum and wave systems can exhibit non-Hermitian degeneracies called exceptional points, where both the eigenvalues and the corresponding eigenstates coalesce. Previously, such exceptional points have been investigated in dielectric microcavities in terms of optical modes which are well confined inside the cavity. However, beside these so-called “internal modes” with a relatively high quality factor, there exists another kind of mode called “external modes,” which have a large decay rate and almost zero intensity inside the cavity. In the present paper, we demonstrate the physical significance of the external modes via the occurrence of exceptional points of internal–external mode pairs for transverse electric polarization. Our numerical studies show that these exceptional points can be achieved by either a boundary deformation of the microdisk or by introducing absorption into a circular cavity.