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Unidirectionally propagated electromagnetic waves are rare in nature but heavily sought after due to their potential applications in backscatter-free optical information processing setups. It was theoretically shown that the distinct bulk optical band topologies of a gyrotropic metal and an isotropic metal can enable topologically protected unidirectional surface plasmon polaritons (SPPs) at their interface. Here, we experimentally identify such interfacial modes at terahertz frequencies. Launching the interfacial SPPs via a tailored grating coupler, the far-field spectroscopy data obtained reveals strongly nonreciprocal SPP dispersions that are highly consistent with the theoretical predictions. The directionality of the interfacial SPPs studied here is flexibly tunable by either varying the external field or adjusting the metallic characteristics of the bulk materials. The experimental realization of actively tunable unidirectional SPPs sets the foundation for developing nanophotonic information processing devices based on topologically protected interfacial waves.

Magnetic-field-biased indium antimonide (InSb) is one of the most widely-discussed materials for supporting nonreciprocal surface plasmon polaritons (SPPs), which have recently been shown to be topological. In this work, we provide a critical assessment of InSb as a magneto-optical SPP platform, and show that it is only viable under a narrow set of conditions.