Off-axis dipole forces in optical tweezers by an optical analog of the {Magnus} effect

TL;DR

This paper demonstrates that circular dipoles in optical tweezers experience off-axis forces analogous to the Magnus effect, leading to spin-dependent displacements of trapped atoms, with potential applications in controlling atomic motion.

Contribution

It introduces a novel optical analog of the Magnus effect, showing how spin and orbital angular momentum coupling causes off-axis forces in optical tweezers.

Findings

Circular dipoles deflect focused laser beams.

Off-axis displacement of atoms up to λ/2π.

Force depends on dipole spin and magnetic field direction.

Abstract

It is shown that a circular dipole can deflect the focused laser beam that induces it, and will experience a corresponding transverse force. Quantitative expressions are derived for Gaussian and angular tophat beams, while the effects vanish in the plane-wave limit. The phenomena are analogous to the Magnus effect pushing a spinning ball onto a curved trajectory. The optical case originates in the coupling of spin and orbital angular momentum of the dipole and the light. In optical tweezers the force causes off-axis displacement of the trapping position of an atom by a spin-dependent amount up to $\lambda/2\pi$, set by the direction of a magnetic field. This suggests direct methods to demonstrate and explore these effects, for instance to induce spin-dependent motion.