Graphene-based hyperbolic metamaterials for a tunable subwavelength dark hollow beam

Hyperbolic metamaterials have recently been widely investigated in nanophotonics systems. Here, we propose an alternating graphene/${{\rm SiO}_2}$SiO₂ multilayer structure as an anisotropic medium with hyperbolic dispersion. When in-plane and out-of-plane effective permittivity is negative and positive, respectively, the incident beam (transverse magnetic polarization wave) can be split into two subwavelength beams, and a dark hollow beam can be achieved for circularly polarized incidence. Also, the size of the dark hollow beam can be tuned by changing the Fermi level. Our method is believed to be used as a tunable optical tweezer for controlling molecules.