  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.
