  Plasmonic crystals are a class of optical metamaterials that consist of
engineered structures at the sub-wavelength scale. They exhibit optical
properties that are not found under normal circumstances in nature, such as
negative-refractive-index and epsilon-near-zero (ENZ) behavior. Graphene-based
plasmonic crystals present linear, elliptical, or hyperbolic dispersion
relations that exhibit ENZ behavior, normal or negative-index diffraction. The
optical properties can be dynamically tuned by controlling the operating
frequency and the doping level of graphene. We propose a construction approach
to expand the frequency range of the ENZ behavior. We demonstrate how the
combination of a host material with an optical Lorentzian response in
combination with a graphene conductivity that follows a Drude model leads to an
ENZ condition spanning a large frequency range.
