Achieving nonreciprocal light propagation is of fundamental importance in photonic devices and systems. Nonreciprocal effects are typically obtained using bulky magneto-optical materials externally biased by a static magnetic field. Notably, it was recently demonstrated that some of these magnetically-biased systems with a broken-time reversal symmetry have nontrivial topological properties and support unidirectional backscattering immune chiral edge modes. Nevertheless, the required external magnetic bias, together with the relatively weak gyrotropic responses achievable at optical frequencies, makes the integration of such elements in nanophotonic systems extremely difficult. Because of this, there has been recently a great effort in the development of magnetic-free solutions that give nonreciprocal responses and are fully compatible with modern highly-integrated photonic systems.
Here we propose a novel route to achieve magnetic-free nonreciprocal subwavelength light propagation. We theoretically demonstrate that by biasing a graphene sheet with a direct electric current it is possible to break the Lorentz reciprocity and have a broadband regime of unidirectional propagation of surface plasmons. Remarkably, we prove that the drift-current biasing also enables enhancing the propagation length of the graphene plasmons. Furthermore, it is shown that the surface plasmons supported by the graphene sheet with a drift current are protected against backscattering from obstacles and imperfections, similar to the “one-way” topologically protected chiral edge modes supported by topological photonic systems. We believe that these findings may open a new and exciting opportunity towards the full integration of nonreciprocal components in nanophotonic systems.
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