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Not Pulling Your Leg: Tractor Beams May Be Possible

14 November 2011 5:59 pm
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Egmason/Wikimedia Commons

Bizarre beam. A Bessel beam is made by overlapping ordinary light waves that travel at an angle (left). The entire beam propagates forward with an intensity profile (right) that remains constant along the length of the beam, and new calculations suggest it can act as a small-scale tractor beam.

It's not nearly powerful enough to pull in the Millennium Falcon, but four groups of physicists have independently come up with the same basic idea for a real-life tractor beam. The laser beam the groups have dreamed up could drag an object the size of only a grain of salt or smaller, but experts say it could provide a new tool for manipulating tiny objects such as cells.

"It's new, it's exciting, and who knows what it might be good for?" says Juan José Saénz, a theorist at the Autonomous University of Madrid, who was not involved any of the papers but whose own group was also working on the idea. "It's fascinating that four or five groups are at the same time coming out with the same idea."

It's not news that light can move an object. Since the 1980s, scientists have pulled small objects around using so-called optical tweezers, which can grab hold of tiny objects such as cells and pull them around. And in the past half-decade, physicists have used the tiny force of light to set nanometer-scale beams and cantilevers aquiver—or to still their motion.

Physicists have identified a few basic ways that light can flex its muscle. Like a blast of water from a hose, a light beam exerts a tiny forward push when it hits something. Light can also pull an object, using something called the gradient force. This force comes about because the electric and magnetic fields in the light polarize the material in the object, and the polarized object can then reduce its energy by moving to where the light is most intense.

The tractor beam would work in a new way. In this case, the light would pull an object toward the source of the beam even though the beam has the same intensity all along its length. The trick is to use a special type of laser beam. In an ordinary beam, each photon moves in the direction of the beam, so when a photon bounces directly back from an object, it imparts the largest possible push. However, physicists can generate a beam by overlapping light waves that make an angle relative to the desired direction (see figure). The overlapping waves produce a forward-moving beam known as a Bessel beam whose intensity remains constant along its length. But each photon is now moving at an angle relative to the beam. So when one bounces off an object, it exerts a smaller forward push.

Nevertheless, the beam is still pushing, and to overcome that push, physicists need to rely on another bit of physics. Again, the light will polarize the material in the object electrically and magnetically. The polarized object will then radiate and redirect the light. By adjusting the material properties of the object and the polarizations and synchronization of the individual light waves in the beam, physicists can make the object radiate more light forward along the beam than backward toward its source. The radiated light then acts like a reverse thruster, overcoming the already-reduced forward push of the beam and driving the object back toward its source.

The details of the various groups' proposals differ. For example, Andrey Novitsky of the Technical University of Denmark in Lyngby and colleagues in Singapore consider a single spherical particle in a Bessel beam and study primarily how the size and material properties of the particle must be adjusted to produce an attraction, as they reported online 10 November in Physical Review Letters. "In general, there is no strict limitation on the parameters" to achieve an attraction, Novitsky says. In July, Jack Ng of the Hong Kong University of Science and Technology in Kowloon and colleagues reported a similar analysis in Nature Photonics. (In February, the two papers were posted within a day of each other on the arXiv preprint server.)

In contrast, Aristide Dogariu and Sergey Sukhov, physicists at the University of Central Florida in Orlando, take a slightly different tack. Instead of using a simple sphere and varying its material parameters and size, they begin with an arbitrary object—a random collection of 160 spheres—and show that they can always adjust the synchronization and polarization of the individual waves in the beam to create an attraction, as they also reported online 10 November in Physical Review Letters. "Once we have the shape, we can design and manipulate the beam to get the pull you want," Dogariu says.

In 2006, Philip Marston of Washington State University, Pullman, described a similar pull using a Bessel beam of sound waves. In September, he and Likun Zhang published an analysis in Physical Review E connecting the acoustic and optical phenomena.

So far, none of the teams has actually created a tractor beam. So can it really work? Probably, Marston says. "I think somebody will demonstrate a pulling force," he says. Saénz says that pulling a simple sphere as Chan and Novitsky describe should be easier to implement, but Sukhov and Dogariu's approaches could have more general applications, perhaps in manipulating biological specimens.

As for pulling a starship, don't hold your breath. The dragged object must be smaller than the length scale over which the light waves remain orderly and "coherent," Saénz says. So to feel the tug, the Millennium Falcon would have to be shrunk to less than a millimeter in length. And really, how much of Han Solo's ego could you fit into such a tiny ship?

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