A photon has a small momentum which it can impart to anything it hits, as Arthur Compton and Peter Lebedev discovered at the beginning of the last century. We now know that photons can be used to push anything from electrons to solar sails.
Today, Jun Chen from Fudan University in China and a few pals demonstrate the counterintuitive result that photons can pull things too. In other words, they’ve worked out how to generate a backward pulling force from a forward propagating beam.
Chen and buddies say this is possible when the system meets two conditions. First, it works only for beams in which the momentum in the direction of propagation is small, as is the case for beams that merely glance off an object. Second, the photons must simultaneously excite several multipoles within the particle, which scatter the beam.
If the scattering angle is just right, the total momentum in the direction of propagation can be negative, meaning the particle is pulled back towards the source and the light becomes a tractor beam.
This must not be confused with various “optical tweezer” type mechanisms in which particles trapped in a beam follow the intensity gradient of the light. In this case, the particles always reach some point of equilibrium where the intensity reaches a maximum.
Chen and co’s new force works when there is no gradient. Given the chance, their tractor beam will pull a particle all the way back to the source.
That’s a handy additional tool in the nanomanipulator’s box of tricks. “This may open up new avenues for optical micromanipulation, of which typical examples include transporting a particle backward over a long distance and particle sorting,” say Chen and co.
This is a theory paper so there’s one piece of the puzzle left to fit. All they have to do now is demonstrate that their tractor beam works.
Ref: arxiv.org/abs/1102.4905: Backward Pulling Force From A Forward Propagating Beam
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