Higher-order exceptional points in waveguide-coupled microcavities: perturbation induced frequency splitting and mode patterns

Exceptional points are degeneracies in the spectrum of non-Hermitian open systems where at least two eigenfrequencies and simultaneously the corresponding eigenstates of the Hamiltonian coalesce. Especially, the robust construction of higher-order exceptional points with more than two degenerate eigenfrequencies and eigenstates is challenging but yet worthwhile for applications. In this paper, we reconsider the formation of higher-order exceptional points through waveguide-coupled microring cavities and asymmetric backscattering. In this context, we demonstrate the influence of perturbations on the frequency splitting of the system. To generate higher-order exceptional points in a simple and robust way, a mirror-induced asymmetric backscattering approach is used. In addition to the exceptional-point enhanced sensing capabilities of such systems, also a cavity-selective sensitivity is achieved for particle sensing. The results are motivated by an effective Hamiltonian description and verified by full numerical simulations of the dielectric structure.