Excitation of surface-waves at terahertz frequencies on a suspended graphene sheet

Spatial dispersion effects upon local excitation of extrinsic plasmons in graphene micro-discs, is studied by a full-wave electromagnetic solver, in order to evaluate the effects of spatial dispersion when the infinite sheet is cut into various sizes and shapes at the microscopic scale. General considerations about the spatially dispersive anisotropic conductivity of graphene are used to select the conductivity response to be provided as input in a simulation of surface plasmons launched by the near field of a wire antenna, which is placed at sub-micrometric distances from a graphene micro-disk. The spatial dispersion of the graphene patch is shown to play a significant role not only in plasmon propagation free of external sources, but also in typical scanning probe microscopy configurations, recalling how a THz beam impinging on a metal atomic force microscope tip has been recently used to generate guided THz waves on graphene. Our analysis shows how spatial dispersion affects the excitation strength, or coupling, between the field sources and the surface plasmons.