Topological Directional Coupler

Abstract Interferometers and beam splitters are fundamental building blocks for photonic neuromorphic and quantum computing machinery. In waveguide‐based photonic integrated circuits, beam‐splitting is achieved with directional couplers that rely on transition regions where the waveguides are adiabatically bent to suppress back‐reflection. In this study, a novel, compact approach for introducing guided mode coupling is presented. Along the multimodal domain wall between topological photonic crystals, the photonic spin is conserved to suppress back‐reflection, and the topological protection of the valley degree of freedom is relaxed to implement tunable beam splitting. Rapid advancements in chip‐scale topological photonics suggest that the proposed simultaneous utilization of multiple topological degrees of freedom could benefit the development of novel photonic computing platforms.