Strong hybridization of edge and bulk states in dimerized PT-symmetric coupled waveguide chain

We study mode coupling of an edge state with bulk states in a coupled waveguide chain in the presence of gain and losses. In a low non-Hermiticity regime, it is found that the edge state associated with the interface between two topologically different waveguide chains is well isolated from the bulk states, sharing the exact same features as the well-known Su-Schrieffer-Heeger Model. As the non-Hermiticity increases, the two bands of the overall waveguide chain merge together, which leads to the overlap in the band diagram between the edge state and states of lower bands. We find that before the PT-symmetry breaking, the edge state is strongly coupled to bulk states of lower bands, evident by the anti-crossing feature in the band diagram. The strong mode hybridization is verified by a non-Hermitian coupled mode theory developed from reaction conservation, and is further examined using phenomenological models. We envisage that the strong coupling between the edge state and the bulk state may be useful in expanding our understanding in topological photonics in non-Hermitian condition, as well as in applications such as mode conversion between edge and bulk states.