  We present terahertz (THz) metasurfaces based on aligned rectangular graphene
patches placed on top of a dielectric layer to convert the transmitted linearly
polarized waves to circular or elliptical polarized radiation. Our results lead
to the design of an ultrathin broadband THz quarter-wave plate. In addition,
ultrathin metasurfaces based on arrays of L-shaped graphene periodic patches
are demonstrated to achieve broadband cross-polarization transformation in
reflection and transmission. The proposed metasurface designs have tunable
responses and are envisioned to become the building blocks of several
integrated THz systems.
