Topological THz on-chip valley–spin converter

Topological photonics offers a powerful platform for next-generation nanophotonic chips, capitalizing on their remarkable resilience to disorder and defects. Among the two-dimensional (2D) photonic topological insulators, valley-Hall (VH) and pseudo-spin-Hall (PSH) topological insulators have emerged as the most practical designs, as they do not require breaking time-reversal symmetry. These photonic topological insulators support robust edge states, demonstrating promising potential for a wide range of applications, from on-chip communication to optical computing and sensing. However, the conversion between distinct topological phases (VH and PSH) in terahertz (THz) band has not been achieved. Here we experimentally demonstrate a THz on-chip spin–valley converter through adiabatic evolution in 2D parameter space without closing the bulk bandgap. By leveraging the adiabatic phase transition, we confirm the high-efficiency conversion between two valley states in a valley–spin–valley converter. In addition, we verify the robustness of THz PSH topological energy transport through sharply twisted corners. Our findings not only advance the understanding of topological phases in photonics but also hold promise for the development of innovative photonic devices with enhanced performance and functionality.