Reversible tractor beam can repel and attract objects

Dr Vladlen Shvedov (left) and Dr Cyril Hnatovsky (right) with a magnified projection of the "hollow" laser beam (Photo: Stuart Hay, ANU)

We're still a far cry from Star Trek's ship-towing and repelling technology, but laser physicists at the Australian National University (ANU) have built a tractor beam that can repel and attract objects. The beam moved spherical glass shells one fifth of a millimeter in diameter across a distance of up to 20 centimeters (7.87 in), which is around 100 times further than previous experiments at this scale, using only a single hollow laser beam that's bright around the edges and dark in its center.

The ANU researchers previously developed a similar device that moves very small particles over long distances using an optical vortex that created something called photophoretic force, which pushes the particle into a dark hollow in the center of the beam as the momentum of the photons drives it forward.

This new technique builds on the previous study, with energy from the laser heating the surrounding air particles to create hotspots on one side of the glass shell's surface. The heat drives air particles away, causing the tiny shell to recoil – thus propelling it in the opposite direction. And by altering the polarization of the laser beam on the fly, switching for instance from axial (shaped like a star) to azimuthal (like a ring), the researchers can cause the shell to change direction or stop.

The beam could have broad applications in the real-world, the researchers believe, as a means of controlling or sampling atmospheric pollution, and it could be scaled up to work over several meters. "Our lab just was not big enough to show it," explained co-author Dr Vladlen Shvedov.

It may not sound like much, but this is a big step forward in the quest to develop optical tractor beams like those from science fiction. Long-distance optical tractor beams were previously restricted only to theory – a concept that in essence sees particles, the environment, or an object's electromagnetic field manipulated by "negative forces" from lasers or Bessel beams.

"Demonstration of a large-scale laser beam like this is a kind of Holy Grail for laser physicists," said co-author Wieslaw Krolikowski. Now that the concept's been proven, the agonizing (and possibly futile) wait for something truly large scale, like the Starship Enterprise towing another ship to safety or the portal-creating, cube-moving Aperture Science Handheld Portal Device used in the game Portal, begins.

A paper describing research appeared in the journal Nature Photonics.

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