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 focal plane there are cross-section regions in which polarization is circular (elliptical). When passing through plane, direction rotation vector reversed. If focus left-hand at definite region beam cross-section, then exactly becomes linear this region, becoming right region. This effect allows to be used rotate weakly absorbing dielectric microparticles around their center mass.