Switchable topological property of magneto-optical photonic crystal waveguide by adjusting magnetic field strength

We theoretically and experimentally demonstrate a special phenomenon in magneto-optical photonic crystals (MOPCs), wherein the gap Chern number of a high-frequency bandgap varies with the strength of an external magnetic field. Specifically, when the magnetic field strength exceeds a critical threshold, the gap Chern number of the third bandgap undergoes a transition from a non-zero value to zero, indicating a topological phase transition in the system. This finding provides an alternative approach to alter the topological properties of a photonic bandgap without modifying the geometric structure. Based on this effect, we construct a waveguide structure composed of a square MOPC with metallic boundaries. By simply adjusting the magnetic field strength, one can realize the dynamic switching between a dual-mode (odd and even states) topological waveguide and a trivial waveguide within a specific frequency range. Microwave experiments further verify the occurrence of such topological transitions. This study expands current strategies for controlling light transport in MOPCs and establishes a practical platform for developing topological photonic devices with multiple tunable degrees of freedom.