Backward Pulling Force from a Forward Propagating Beam

TL;DR

This paper demonstrates that under certain conditions, a forward propagating light beam can exert a backward pulling force on a particle, expanding the capabilities of optical micromanipulation through interference of radiation multipoles.

Contribution

It introduces a method to achieve backward pulling forces using interference of multipoles in particles with specific beam conditions, a novel approach in optical manipulation.

Findings

Backward scattering force can pull particles towards the source.

Maximization of forward scattering via multipole interference is key.

Negative optical force occurs when multipoles are simultaneously excited.

Abstract

A photon carries a momentum of, so one may anticipate light to "push" on any object standing in its path via the scattering force. In the absence of intensity gradient, using a light beam to pull a particle backwards is counter intuitive. Here, we show that it is possible to realize a backward scattering force which pulls a particle all the way towards the source without an equilibrium point. The underlining physics is the maximization of forward scattering via interference of the radiation multipoles. We show explicitly that the necessary condition to realize a negative (pulling) optical force is the simultaneous excitation of multipoles in the particle and if the projection of the total photon momentum along the propagation direction is small (as in some propagation invariant beams), attractive optical force is possible. This possibility adds "pulling" as an additional degree of freedom to optical micromanipulation.