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

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.