Circular polarization before and after the sharp focus for linearly polarized light

We consider sharp focusing of a linearly polarized light beam. Using the Richards-Wolf formalism, we show that before and after the focal plane there are cross-section regions in which the polarization is circular (elliptical). When passing through the focal plane, the direction of rotation of the polarization vector is reversed. If before the focus the light is left-hand circular polarized at a definite region of the beam cross-section, then exactly at the focus the polarization becomes linear at this cross-section region, before becoming right circular polarized after the focus at this region. This effect allows linearly polarized light to be used to rotate weakly absorbing dielectric microparticles around their center of mass.