  Recently, it was shown that vector beams can be utilized for fast kinematic
sensing via measurements of their global polarization state [Optica 2(10), 864
(2015)]. The method relies on correlations between the spatial and polarization
degrees of freedom of the illuminating field which result from its nonseparable
mode structure. Here, we extend the method to the nonparaxial regime. We study
experimentally and theoretically the far-field polarization state generated by
the scattering of a dielectric microsphere in a tightly focused vector beam as
a function of the particle position. Using polarization measurements only, we
demonstrate position sensing of a Mie particle in three dimensions. Our work
extends the concept of back focal plane interferometry and highlights the
potential of polarization analysis in optical tweezers employing structured
light.
