  Linear birefringence, as implemented in wave plates, is a natural way to
control the state of polarization of light. We report on a general method for
designing miniature planar wave plates using surface plasmons. The resonant
optical device considered here is a single circular aperture surrounded by an
elliptical antenna grating. The difference in short and long axis of each
ellipses introduces a phase shift on the surface plasmons which enables the
realization of a quarter wave plate. Furthermore, the experimental results and
the theoretical analysis show that the general procedure used does not
influence the optical coherence of the polarization state and allows us to
explore completely the surface of the unit Poincare sphere by changing only the
shape of the elliptical grating.
