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.

中文翻译：

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回旋导体和各向同性导体之间界面处的可调谐单向表面等离子体激元

单向传播的电磁波在自然界中很少见，但由于它们在无反向散射的光学信息处理装置中的潜在应用而备受追捧。理论上表明，回旋金属和各向同性金属的不同体光带拓扑结构可以在它们的界面上形成受拓扑保护的单向表面等离子体激元（SPP）。在这里，我们通过实验确定了太赫兹频率的这种界面模式。通过定制的光栅耦合器发射界面 SPP，获得的远场光谱数据揭示了与理论预测高度一致的强烈非互易 SPP 色散。此处研究的界面 SPP 的方向性可以通过改变外场或调整大块材料的金属特性来灵活调节。主动可调单向 SPP 的实验实现为开发基于拓扑保护界面波的纳米光子信息处理设备奠定了基础。