Graphene-based tunable Imbert-Fedorov shifts and orbital angular momentum sidebands for reflected vortex beams in the terahertz region.

Upon reflection, a light beam embedded with m-order orbital angular momentum (OAM) will undergo the Imbert-Fedorov (IF) shift, which induces OAM sidebands. The energies of the neighboring {-m-1} and {-m+1} sideband modes of the reflected beam are always equal. Controllable OAM sidebands are theoretically achieved by introducing a monolayer graphene in a three-layer structure composed of air, hexagonal boron nitride, and metal. By modulating the Fermi energy of graphene, the OAM-dependent IF shift can be tuned from positive to negative values, and the OAM sideband modes can be suppressed or enhanced, since the reflectivity for perpendicular and parallel polarizations vary with the Fermi energy. These findings provide an alternative method for the control of optical OAM in the terahertz region.