  In this work, we theoretically and experimentally study the conversion from a
circularly polarized plane electromagnetic wave into a linearly polarized
transmitted one using anisotropic self-complementary metasurfaces. For this
purpose, a metasurface design operable at sub-terahertz frequencies is proposed
and investigated in the range of 230-540 GHz. The metasurface is composed of
alternating complementary rectangular patches and apertures patterned in an
aluminum layer deposited on a thin polypropylene film. The term
\textit{self-complementary} implies that the pattern is invariant with respect
to its inversion (with some translation). Our study shows that the
translational symmetry of the metasurface results in unusual and useful
electromagnetic properties under illumination with circularly polarized
radiation beams. In particular, alongside with broadband circular-to-linear
conversion, the transmitted wave exhibits a frequency independent magnitude,
while its polarization angle gradually changes with frequency that paves the
way for new beam-splitting applications.
