  The unique gate-voltage dependent optical properties of graphene make it a
promising electrically-tunable plasmonic material. In this work, we proposed
in-situ control of the polarization of nanoantennas by combining plasmonic
structures with an electrostatically tunable graphene monolayer. The tunable
polarizer is designed based on an asymmetric cross nanoantenna comprising two
orthogonal metallic dipoles sharing the same feed gap. Graphene monolayer is
deposited on a Si/SiO2 substrate, and inserted beneath the nanoantenna. Our
modelling demonstrates that as the chemical potential is incremented up to 1 eV
by electrostatic doping, resonant wavelength for the longer graphene-loaded
dipole is blue shifted for 500 nm (~ 10% of the resonance) in the mid-infrared
range, whereas the shorter dipole experiences much smaller influences due to
the unique wavelength-dependent optical properties of graphene. In this way,
the relative field amplitude and phase between the two dipole nanoantennas are
electrically adjusted, and the polarization state of the reflected wave can be
electrically tuned from the circular into near-linear states with the axial
ratio changing over 8 dB. Our study thus confirms the strong light-graphene
interaction with metallic nanostructures, and illuminates promises for
high-speed electrically controllable optoelectronic devices.
