A pupil mask to reduce the lateral spot size of a highly focused radially polarized light field

We demonstrate experimentally that the lateral spot size of a highly focused radially polarised light field can be decreased by use of a specially designed pupil mask. The pupil mask is fabricated using a cellophane sheet which is birefringent and hence behaves like a commercial half waveplate. The diameter of the mask is such that, the central portion of the beam passes through the mask and hence it's phase is altered. This means, the outer portion of the beam remains radially polarized while that transmitted through the cellophane has a polarization order number -1. As a result, the focusing beam comprises of beams of polarization order number +1 and -1.This results into the partial cancellation of the tranverse component in one direction.The radially polarized light field is generated using a liquid crystal.