Optical pulling force

A photon carries (h) over bark of momentum, so it may be anticipated that light will 'push' on any object standing in its path by means of the scattering force(1-3). In the absence of an 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 that 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, an attractive optical force is possible. This possibility adds 'pulling' as an additional degree of freedom to optical micromanipulation.