Topologically Protected Unidirectional Surface States in Biased Ferrites: Duality and Application to Directional Couplers

We demonstrate theoretically the existence of a unidirectional surface mode at the interface between a gyromagnetic medium (biased ferrite) and a magnetically opaque medium, occurring in the frequency gap of the bulk medium. The topological invariant of the gyromagnetic medium, the Chern number, is calculated analytically from the bulk dispersion bands, and duality is used to connect with the previously studied gyroelectric (magnetoplasma) case. The surface wave transmits electromagnetic energy without any backscattering or radiation, even in the presence of arbitrarily large physical perturbations at the interface. An application to a surface-plasmon directional coupler with perfect isolation is proposed.