The tractor beam, a science fiction mainstay, is inching closer to reality. Two theoretical papers in Physical Review Letters demonstrate how small objects could be pulled towards a laser beam or set of laser beams that strike the objects at wide angles. The trick is in tuning the beams so that the light mostly scatters off in the forward direction where the object’s shadow would normally be.
In the far-off future, tractor beams may reel in space ships, but a more modest goal now is to use them to manipulate molecules and cells. The main hurdle is dealing with the forward push that light normally gives when it reflects off or is absorbed by an object. One way to counter this momentum transfer is to induce an electrical force in the object with a highly focused laser beam. However, this sort of tractor beam only drags objects a short distance.
An alternative strategy is to stimulate a small object with light in such a way that it reradiates the light mostly in the forward direction. The object—in order to compensate the change in the light’s momentum—will be continuously drawn back towards the light source. Sergey Sukhov and Aristide Dogariu of the University of Central Florida considered how this might work if a beam consisting of a combination of plane-wave components with different propagating vectors, is structured to match the size and shape of any object placed in this cross fire. The object will scatter light in several different directions, but the scientists show for the case of a random cluster of tiny spheres that the phase and polarization of each beam can be adjusted to maximize forward scattering and thus produce an inward pull.
A similar idea is explored by Andrey Novitsky and Cheng-Wei Qiu from the Technical University of Denmark and the National University of Singapore, respectively. They use a single Bessel beam, whose cross section of concentric rings doesn’t spread out as it propagates. The authors find that a spherically shaped object can be pulled backwards by a Bessel beam if the object is made from a material with certain electric and magnetic properties. – Michael Schirber