Wavefront control in valley Hall photonic topological insulators via sub-element rotation

This study presents a sub-element rotation-based symmetry-breaking mechanism for a quantum valley Hall photonic topological insulator (PTI) with a bandgap of 15.8–19.0 GHz. The topological phase is switched between the K and K′ points in a hexagonal lattice space by selectively rotating only the ring element within the patch-loaded hexapod unit cell, without requiring active electronic components or external magnetic field bias. The proposed waveguides employ two PTI domains with opposing valley Chern numbers on a printed circuit board platform and are fed directly by microstrip lines with an open-circuit stub for impedance matching. This proposed three-port topological waveguide has two operating states, both of which demonstrate low loss and high isolation within the band. As a proof of concept, two of the three-port single-pole double-throw (SPDT) topological waveguides are designed, fabricated, and measured. The two fabricated devices achieve an insertion loss of 2.4 dB and an isolation of 33.2 dB in state 1, and an insertion loss of 1.0 dB and an isolation of 26.0 dB in state 2, both at 17 GHz. The SPDT topological waveguide allows wave routing between selected ports by the rotating ring element within each unit cell. Furthermore, it only requires rotation of four unit cells. Chern number analysis, together with simulation and experimental results, demonstrates robust transmission and verifies the topologically protected edge states. The sub-element rotation mechanism proposed in this study is compatible with mechanical actuation methods and offers a promising solution for achieving reconfigurable topological wave routing.