  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.
