Imaginary-gap protection of topological edge states in lossy photonic crystals

Topological edge states (TESs) in valley photonic crystals (VPCs) hold immense potential for robust, backscattering-immune wave routing. Conventionally, the realization of these states relies strictly on opening a real bandgap, within which intrinsic material dissipation is universally viewed as detrimental to topological protection. Here, we demonstrate a loss-driven approach to topological transport by engineering a non-Hermitian VPC with spatially selective dissipation. We reveal that, rather than degrading transport, the introduction of targeted macroscopic loss drives a pronounced splitting of the imaginary eigenvalues, thereby opening a broad imaginary bandgap. This spectral transition explicitly detaches the TESs from the hybridized bulk continuum in the complex-frequency plane. To elucidate this loss-induced bulk-boundary decoupling, we introduce a subspace decomposition framework that projects the system into an asymptotically Hermitian boundary subspace. Within this framework, we analytically confirm the effective restoration of spatial orthogonality and tight mode confinement. Consequently, these decoupled TESs sustain highly efficient transmission and exhibit extraordinary robustness against structural imperfections and inhomogeneous dissipation. Ultimately, our findings recast non-Hermitian dissipation from a fundamental limitation into a powerful, actively engineered degree of freedom for advanced topological photonics.