Optical forces and torques in non-uniform beams of light

TL;DR

This paper explores how spin angular momentum in elliptically polarized light beams influences optical forces and torques, demonstrating these effects experimentally with colloidal particles in optical tweezers.

Contribution

It provides experimental evidence of how spin angular momentum affects forces and torques in non-uniform light beams, including force field mapping via Brownian vortex circulation.

Findings

Clusters circulate deterministically around the beam axis.

Single spheres exhibit stochastic Brownian vortex circulation.

Spin angular momentum contributes to both forces and torques without conversion.

Abstract

The spin angular momentum in an elliptically polarized beam of light plays several noteworthy roles in optical traps. It contributes to the linear momentum density in a non-uniform beam, and thus to the radiation pressure exerted on illuminated objects. It can be converted into orbital angular momentum, and thus can exert torques even on optically isotropic objects. Its curl, moreover, contributes to both forces and torques without spin-to-orbit conversion. We demonstrate these effects experimentally by tracking colloidal spheres diffusing in elliptically polarized optical tweezers. Clusters of spheres circulate deterministically about the beam's axis. A single sphere, by contrast, undergoes stochastic Brownian vortex circulation that maps out the optical force field.