Graphene-based meta-coupler for direction-controllable emission of surface plasmons.

Gradient metasurfaces have offered a promising approach to achieve high-efficiency conversion between surface plasmons (SPs) and propagating waves (PWs), and hence have found numerous applications in photonics. However, the available SP-PW couplers lack flexibility for active control, which limits their use in practice. Graphene-based meta-couplers are proposed to realize dynamical SP-PW conversion by providing a tunable phase shift to the scattering SPs by means of chemical potential modulation of graphene. In-plane/out-of-plane SP-PW conversions are demonstrated with graphene ribbon/block-based meta-couplers. Converting SPs to single or two beams of PWs with variable radiation angles is realizable by varying the chemical potential of graphene without re-optimizing the structural parameters.