Tunable Electromagnetic Flow Control in Valley Photonic Crystal Waveguides

The exploration of the binary valley degree of freedom in topological photonic systems has inspired many intriguing optical phenomena such as the photonic Hall effect, robust delay lines, and perfect out-coupling refraction. In this work, we experimentally demonstrate the tunability of electromagnetic flow in a valley photonic crystal waveguide. By continuously controlling the phase differences of a microwave monopolar antenna array, the flow of electromagnetic waves can split into different directions according to the chirality of the phase vortex, and the splitting ratio varies from 0.9 to 0.1. Topological valley transport of edge states is also observed at the photonic domain wall. Tunable edge state dispersion, i.e., from gapless valley-dependent modes to gapped flat bands, is found at the photonic boundary between a valley photonic crystal waveguide and a perfect electric conductor, leading to the tunable frequency bandwidth of high transmission. Our work paves the way to the controllable and dynamic modulation of electromagnetic flow in topological photonic systems.