Tuning the Polarization State of Light via Retardation with a Microstructured Surface

We report in this letter an approach to tune efficiently the phase difference of light in two orthogonal directions, $\Delta \phi $, by controlling the time retardation with a microstructured surface made of L-shaped metallic patterns. The $\Delta \phi $ can be linearly tuned accurately from -180 degree to 180 degree by changing the frequency of incident light. Particularly the amplitudes in two orthogonal directions are identical so that the polarization state always locates on a meridian of Poincar\'e sphere. Near field measurement confirms that there is indeed time retardation between the oscillations in the orthogonal directions of the L-shaped patterns. This approach offers a new way in manipulating the polarization state of light.