Abstract

This work unveils the fundamental limits of linear and nonreciprocal plasmonic metasurfaces in terms of isolation and loss. The proposed bounds are related to surface waves and only depend on the nonreciprocal material employed within the metasurface, thus being independent of geometrical considerations and the presence of other materials. We apply these fundamental limits to explore two different platforms, namely drift-biased and magnetically-biased graphene metasurfaces. For each platform, we first analytically derive the upper bounds in terms of graphene conductivity. Then, we explore devices proposed in the literature and benchmark their response against their upper bounds. Results highlight that drift-biased hyperbolic metasurfaces exhibit outstanding performance in the mid-infrared region, whereas magnetically-biased devices are better suited for the low terahertz band. More broadly, our bounds allow to quickly assess the performance of nonreciprocal plasmonic metasurfaces with respect to their fundamental limit, thus streamlining the device design process and preventing that significant efforts are dedicated to marginal performance improvements. The proposed bounds pave the way toward the development of quasi-optimal nonreciprocal metasurfaces, with important applications in sensing, imaging, communications, and nonlinear optics, among many others.