Nonreciprocal Terahertz Propagation via InSb Topological Photonic Crystals

Here, we demonstrate that applying an external magnetic field to break time‐reversal symmetry (TRS) induces topological phase transitions and topologically protected unidirectional edge state propagation in photonic crystals. We investigate nonreciprocal propagation in a square array of indium antimonide (InSb) rods embedded in air at terahertz frequencies. In photonic crystals, a modest external magnetic field applied to semiconductor rods creates gyroelectric (magneto‐optical) anisotropy. This anisotropy breaks the TRS, which lifts the degeneracy at the Dirac‐like point, opening a new nontrivial bandgap. The resulting edge modes supported by photonic crystals exhibit nonreciprocal propagation and remain immune to backscattering, even when encountering large obstacles, 90° sharp bends, and structural defects. The propagation direction of this state is determined by the magnetic field. A novel design approach for nonreciprocal terahertz topological devices is proposed, leveraging the strong correlation between the unidirectional edge mode properties and the Voigt effect. These findings offer promising potential for developing and fabricating advanced nonreciprocal terahertz topological devices.