  We show that polarization states of electromagnetic waves can be manipulated
easily using a single thin uniaxial crystal plate. By performing a rotational
transformation of the coordinates and controlling the thickness of the plate,
we can achieve a complete polarization conversion between TE wave and TM wave
in a spectral band. We show that the off-diagonal element of the permittivity
is the key for polarization conversion. Our analysis can explain clearly the
results found in experiments with metamaterials. Finally, we propose a simple
device to realize unidirectional transmission based on polarization conversion
and excitation of surface plasmon polaritons.
