Surface plasmons are collective electron excitations in metallic systems, and the associated electromagnetic wave usually has the transverse-magnetic polarization. On the other hand, spin waves are spin excitations perpendicular to the equilibrium magnetization and are usually circularly polarized in a ferromagnet. The direct coupling of these two modes is challenging due to the difficulty of matching electromagnetic boundary conditions at the interface of magnetic and nonmagnetic materials. Here, we overcome this challenge by utilizing the linearly polarized spin waves in antiferromagnets (AFM) and show that a strong coupling between AFM magnons and surface plasmons can be realized in a hybrid two-dimensional (2D) material/AFM structure, featuring a clear anticrossing spectrum at resonance. The coupling strength, characterized by the gap of anticrossing at resonance, can be tuned by electric gating on 2D materials and probed by measuring the two reflection minima in the reflection spectrum. Further, as a potential application, we show that plasmonic modes can mediate the coupling of two well-separated AFMs over several micrometers, featuring symmetric and antisymmetric hybrid modes. Our results may open a platform to study antiferromagnetic spintronics and its interplay with plasmonic photonics.

Surface plasmons in two-dimensional (2D) electron systems have attracted great attention for their promising light-matter applications. However, the excitation of a surface plasmon, in particular, transverse-electric (TE) surface plasmon, remains an outstanding challenge due to the difficulty to conserve energy and momentum simultaneously in the normal 2D materials. Here we show that the TE surface plasmons ranging from gigahertz to terahertz regime can be effectively excited and manipulated in a hybrid dielectric, 2D material, and magnet structure. The essential physics is that the surface spin wave supplements an additional freedom of surface plasmon excitation and thus greatly enhances the electric field in the 2D medium. Based on widely used magnetic materials like yttrium iron garnet and manganese difluoride, we further show that the plasmon excitation manifests itself as a measurable dip in the reflection spectrum of the hybrid system while the dip position and the dip depth can be well controlled by an electric gating on the 2D layer and an external magnetic field. Our findings should bridge the fields of low-dimensional physics, plasmonics, and spintronics and open a novel route to integrate plasmonic and spintronic devices.