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Tech

Behold the Long-Range, Reversible Tractor Beam

Researchers have successfully created a laser beam that can move objects macroscopic distances.
Image: Elena Schweitzer/Shutterstock

It's a classic sci-fi image: some spaceship has nearly escaped the enemy fleet, only to be snatched back by a tractor beam. As if grabbed by a photonic lasso, the helpless ship is pulled back into the enemy's clutches.

Using lasers to move stuff is more common than you'd think. They're often used to scoot around the microscopic objects involved in the studies of biology, physical chemistry, and condensed matter physics. At small enough scales, "touching" an object in the conventional sense becomes difficult, like trying to pick a dandelion with a front-end loader. Instead, we move things using light.

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The tractor beam version of this—in which objects are drawn inward by a beam rather than just pushed around, or "tweezed" between laser chopsticks—already exists experimentally, but at very small scales.

Now, a global team of researchers based at universities in the United States, Australia, and Qatar has developed a tractor beam capable of pulling gold-coated spheres of glass (admittedly only a fifth of a millimeter in diameter) over distances as far as tens of centimeters.

Their work is published in the latest edition of Nature Photonics and describes the successful development of a "long-range polarization-controlled optical tractor beam." In addition to pulling an object inward, the team reports, "by varying the polarization state of the beam we can stop the spheres or reverse the direction of their motion at will."

The researchers swapped the usual beams of light with ones shaped like a doughnut: a laser beam with a hollow core

The laser chopsticks mentioned above are known as optical tweezers. The basic idea is to transfer the momentum carried by individual light particles (photons) to some object. But current tweezing techniques are pretty limited.

For one, it's only possible to move an object around within the beam's "focal plane"—the spot of light it would make on some surface. Moreover, these schemes require a vacuum or liquids with large amounts of thermal conductivity. This is simply because any intervening material, whether a liquid or gas, works to block the particles.

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The new study describes an elegant solution that involves both the geometry of the beam and the polarization of the particles within it. The researchers, led by Vladlen Shvedov of the Australian National University, swapped the usual beams of light with ones shaped like a doughnut: a laser beam with a hollow core. And instead of light, they used the heat of the laser to move the glass particles.

The particle being manipulated by the tractor beam is targeted so it's within the central "dark" core, and the direction the particle moves in depends on the polarization of the laser beam.

As the beam moves in one direction, it oscillates in another; this is polarization. If you were to take some giant rubber band and give it a smack, the energy would travel along the band toward the other end, but it would also be vibrating up and down.

Usually we'd have something like radial polarization, where the oscillation is at a right angle to the laser beam, pointing inwards or outwards. You can see that in the left-hand figure in the diagram below.

Image: Shvedov et al

There's another possibility, however. This is azimuthal polarization. As you can see in the right-hand figure, the polarization is still occurring at right angles to the beam, but the directions are all coordinated such that it looks like the beam is rotating around some axis.

The result is a sort of spiral or vortex. Instead of the beam's energy hitting the glass spheres from the usual direction (the light source), it's hitting them more on the dark side (away from the beam's source), pushing them in the opposite direction—like a tractor beam attracting something towards its source.

As the study concludes, "The presented tunable optical beam can transport absorbing particles in air over macroscopic distances (that is, tens of centimetres)."