Understanding particle trajectories by mapping optical force vortices

Abstract

Vortices can be found in many types of fields, from fluid velocity fields to optical fields. In optical manipulation, we are interested in the optical force field arising from the interaction between the beam and the particle. There are several mechanisms that can lead to circulation of particles around an optical trap including transfer of angular momentum from the beam to the particle or oscillations due to a lack of damping in the dynamical system. Understanding of the creation and behaviour of vortices occurring in optical fields can provide means for creating interesting rotational dynamics for particles held in structured light fields. Here, we describe the mechanisms that can lead to circulation in an optical trap. In particular we describe how the force field vortices can be found in different trapping configurations and we discuss the relationship between force vortices, optical vortices and Brownian vortices.