Nonreciprocal and collimated surface plasmons in drift-biased graphene metasurfaces

We explore the unusual nonreciprocal and diffractionless properties of surface plasmon polaritons propagating in drift-biased graphene-based metasurfaces. We show that applying a drift current on a graphene sheet leads to extremely asymmetric in-plane modal dispersions, from terahertz to infrared frequencies, associated with plasmons with low loss (high loss and ultrahigh confinement) traveling along (against) the bias. Strikingly, truly unidirectional wave propagation is prevented by the intrinsic nonlocal response of a graphene, a mechanism that shapes the energy flow over the surface. We also show that highly directive hyperbolic plasmons completely immune to backscattering propagate obliquely along the drift in nanostructured graphene. Finally, we discuss how spin-orbit interactions can be exploited in this platform to efficiently launch collimated plasmons along a single direction while maintaining giant nonreciprocal responses. Our findings offer a paradigm to excite, collimate, steer, and process surface plasmons over a broad frequency band.