Exceptional points in a topological photonic system

Abstract

Exceptional points as branch singularities describe peculiar degeneracies of non-Hermitian systems that do not obey energy conservation. However, this work proposes for the first time that exceptional points can also exist in Hermitian topological systems, for example, the photonic topological waveguide coupled with two degenerate counter-propagation topological whispering gallery modes. Such a photonic architecture can be designed by crystal-symmetry-protected photonic topological insulators based on air rods in conventional dielectric materials. The relevant exceptional point reveals the breaking of the parity-time symmetry, reflected by the dip number changing from two to one in the optical transmission spectra of the system. Achieving exceptional points in Hermitian photonic topological systems possibly opens a new avenue toward robust optical devices with exceptional-point-based unique properties and functionalities.