Loss-Induced Bulk-Boundary Detachment in a Photonic Chern Insulator

Chiral edge states (CESs) in Chern insulators enable one-way propagation without backscattering loss. However, they are usually limited to narrow bandwidths within independent band gaps, and the intrinsic loss is generally viewed as a detrimental factor for transport. Here, leveraging non-Hermitian physics in topological photonics, we fabricate a two-dimensional photonic Chern insulator with multiple topological band gaps embedded in absorptive backgrounds. In the original lossless scenario, multiple bulk bands exist, each separated by gaps traversed by CESs due to the strong coupling between the CESs and bulk states. With increasing bulk loss, a series of exceptional points at specific Bloch momenta appear between the CESs and projected bulk states. After the exceptional point transitions, they enter the weak coupling (decoupling) regime and behave as bulk-boundary detachment in the complex-frequency plane. Microwave experiments demonstrate this loss-broadened CES across three band gaps, achieving a 27% relative bandwidth. These results highlight the loss-induced detachment effect between different-dimensional topological edge and bulk states, shedding light on broadband applications by adding moderate losses.