It seems like we're getting closer to the Star Trek future by leaps and bounds these days. We recently learned that a NASA scientist has set his sights on building a warp drive, and now news has come in that a team of physicists have built their own tractor beam in the lab. Now, it's nothing to get too excited over — they used an optical beam to pull a 30 micrometer silica sphere suspended in water — but the demonstration proved that a tractor beam can actually exist.
We've known about the power of Bessel beams for some time now, including their potential to act as a kind of tractor beam. This optical phenomenon was first discovered by Friedrich Bessel over a hundred years ago, and has since been applied by microbiologists as a kind of tweezer.
But up until this point no one has been able to prove that a Bessel beam can actually function as a kind of tractor beam. So, in an effort to make it work, physicists David Ruffner and David Grier sought to harness a rather unique property of Bessel beams.
Specifically, these laser beams are capable of reconstructing themselves on the opposite side of an object. Subsequently, the theory is that these beams can pull an object back towards the stream of light — not unlike a tractor beam.
So, in an effort to create this ‘optical conveyor,' Ruffner and Grier adjusted the periodic intensity of the Bessel beam's axis so that it could optically trap the micrometer silica sphere. Then, by changing the beam's relative phase, the trapped object was selectively moved both upstream and downstream along the conveyor. Unlike previous (failed) experiments, they were able to do this by multiple lenses that could slightly bend the beams and cause them to overlap — what caused a strobe effect behind the particle, which provided the required energy to draw the object back towards the source.
And not only that, the physicists say that the same method can be used to combine multiple Bessel beams to create a series of overlapping optical conveyors — what could result in the bidirectional transport of microscale objects in three dimensions.
But as PhysOrg reports, the tiny tractor beam may not be scalable — at least not for the foreseeable future. The creation of a space-based tractor beam like the one portrayed in Star Trek would require a tremendous amount of energy — enough to destroy the object that it's trying to pull in. But that said, the breakthrough indicates that a similar device might be possible by using a less energy intensive energy source.
The Ruffner and Grier paper was published in Physical Review Letters and can be found here. Alternately, you can read a version of it at Grier's homepage.
Images: CBS, University of St. Andrews, New York University via PhysOrg.