Microsphere kinematics from the polarization of tightly focused nonseparable light

TL;DR

This paper demonstrates a novel method for 3D microsphere position sensing using polarization analysis of tightly focused nonseparable light, extending previous vector beam sensing techniques to the nonparaxial regime.

Contribution

It introduces a polarization-based position sensing method for dielectric microspheres in the nonparaxial regime, expanding the application of structured light in optical tweezers.

Findings

Successful experimental and theoretical demonstration of 3D position sensing.

Polarization measurements alone enable precise tracking of microsphere position.

Extends back focal plane interferometry using polarization analysis.

Abstract

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.