  In this paper, a broadband tunable polarization converter based on graphene
metasurfaces is proposed. This polarization converter works in the terahertz
(THz) frequency region, using the advantage of graphene characteristics to have
a tunable frequency response. The designed graphene-shaped periodic structure
on top of the substrate is utilized to convert the incident wave polarization
to the desired target in a flexible operational band in the THz frequencies.
The polarization conversion ratio is more than 0.85 in a wide range of
frequencies in the THz band from 4.86 to 8.42 THz (the fractional bandwidth is
54%). The proposed polarization converter is insensitive to the angle of the
incident wave up to 40{\deg}. Using graphene provides a tunable frequency
response without changing the geometry of the designed structure.
