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 eigen-frequencies 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 excep-tional 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.