03 March 2011

More Star Trek physics

First it was cloaking devices, now it seems that tractor beams are theoretically possible. In a paper available at arXiv, Physicists at Fudan University Shanghai show how a beam of photons might be used to draw particles towards the source of the beam. According to the abstract,
A photon carries a momentum of hk, 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.

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