Nonreciprocal and collimated surface plasmons in drift-biased graphene metasurfaces

We explore the unusual non-reciprocal and diffraction-less properties of\nsurface plasmon polaritons propagating in drift-biased graphene-based\nmetasurfaces. We show that applying a drift-current on a graphene sheet leads\nto extremely asymmetric in-plane modal dispersions from terahertz to infrared\nfrequencies, associated with plasmons with low-loss (high-loss and ultra-high\nconfinement) traveling along (against) the bias. Strikingly, truly\nunidirectional wave propagation is prevented by the intrinsic nonlocal response\nof a graphene, a mechanism that shapes the energy flow over the surface. We\nalso show that highly-directive hyperbolic plasmons completely immune to\nbackscattering propagate obliquely along the drift in nanostructured graphene.\nFinally, we discuss how spin-orbit interactions can be exploited in this\nplatform to efficiently launch collimated plasmons along a single direction\nwhile maintaining giant non-reciprocal responses. Our findings open a new\nparadigm to excite, collimate, steer, and process surface plasmons over a broad\nfrequency band.\n