A single gradientless light beam drags particles as “tractor beam”

Usually a light beam pushes a particle when the photons act upon it. We investigate the optical forces by non-paraxial gradientless beams and find that the forces can drag suitable particles all the way towards the light source. The major criterion of realizing the backward dragging force is the strong non-paraxiality of the light beam, which contributes to the pulling force owing to the momentum conservation. The non-paraxiality of the Bessel beam can be manipulated to possess dragging force along both radial and longitudinal directions, i.e., “tractor beam” with stable trajectories is achieved.