Tunable optical meta-surface using graphene-coated spherical nanoparticles

Graphene-coated spherical nanoparticles are proposed as the unit cells of a single negative (SNG) meta-surface at infrared frequencies. To derive the effective permittivity of the meta-surface, each graphene layer is represented as a thin shell with complex conductivity calculated by the Kubo formulas. Later, based on the modified Mie Lorenz coefficients of the particle, electric polarizability of the unit cell is obtained, to be used in the Clausius–Mossotti effective medium formulation. The proposed structure has lower optical losses in comparison to its noble metal counterparts and it is tunable by varying the bias voltages of graphene shells. Moreover, material and geometrical parameters of the particles along with the filling fraction of the meta-surface are other parameters for the control of the optical response in the target frequency. A deep sub-wavelength tunable perfect reflector is considered as an instance. Two possible applications of the designed reflector are in lenses and semiconductor laser cavities, as a promising replacement to Bragg reflectors.