Time-Reversal Invariant Topological Photonic Alloy

Conventional approaches generate quantum spin Hall (QSH) and quantum valley Hall (QVH) effects by breaking spatial inversion symmetry across the entire photonic crystal. Modulating the bulk transmission gaps within such periodic frameworks has traditionally relied on the global modification of structural parameters throughout the lattice, which often presents substantial challenges for device reconfigurability and postfabrication adaptability. While recent advances have begun to explore localized tuning mechanisms to overcome these limitations, in this work, we demonstrate a fundamentally distinct mechanism for generating topology: the random, local breaking of crystalline spatial symmetries in a reciprocal photonic alloy. Using a photonic alloy platform based on random substitutional disorder in a parallel-plate metal waveguide, we show that disorder-driven local breaking of z-direction mirror and in-plane symmetries can open topological gaps to trigger QSH and QVH phase transitions, respectively. Importantly, this symmetry-engineered disorder manifests exclusively as diagonal perturbations, inherently suppressing backscattering in interspin/valley channels and thereby supporting robust helical edge states and valley kink states. Rigorous topological characterization via an improved, mode-mixing-free reflection-phase winding method reveals a striking size-dependent scaling behavior in both configurations, where the threshold doping concentration required to trigger the topological phase transition asymptotically vanishes in the thermodynamic limit. By enabling flexible, on-demand bulk gap engineering simply through random doping tuning─rather than relying on magnetic constituents or global structural reconfigurations─this photonic alloy platform not only deepens understanding of disorder-induced topological physics but also offers a scalable, highly efficient design strategy for integrated topological optical communications.