  Tuning the macroscopic dielectric response on demand holds potential for
actively tunable metaphotonics and optical devices. In recent years, graphene
has been extensively investigated as a tunable element in nanophotonics.
Significant theoretical work has been devoted on the tuning the hyperbolic
properties of graphene/dielectric heterostructures, however, until now, such a
motif has not been demonstrated experimentally. Here we focus on a
graphene/polaritonic dielectric metamaterial, with strong optical resonances
arising from the polar response of the dielectric, which are, in general,
difficult to actively control. By controlling the doping level of graphene via
external bias we experimentally demonstrate a wide range of tunability from a
Fermi level of EF = 0 eV to EF = 0.5 eV, which yields an effective
epsilon-near-zero crossing and tunable dielectric properties, verified through
spectroscopic ellipsometry and transmission measurements.
