Physics

Physicists Develop Reversible Laser Tractor Beam Functional Over Long Distances

October 21, 2014 | by Lisa Winter

Photo credit: Dr Vladlen Shvedov (L) and Dr Cyril Hnatovsky adjusting the hollow laser beam in their lab. Credit: Stuart Hay, ANU

Spaceships in movies and TV shows routinely use tractor beams to tow other vessels or keep them in place. Physicists have been hard at work trying take this technology from science fiction to reality. Significant process has recently been made by a team who have developed a laser tractor beam able to attract and repel particles about 100 times further than has been previously achieved. The lead author of the paper, published in Nature Photonics, is Vladlen Shvedov at Australian National University in Canberra.

Other recent tractor beams have used acoustics or water, but this one uses a single laser beam to control tiny particles about 0.2 millimeters in diameter. The tractor beam was able to manipulate the particles from a distance of 20 centimeters, shattering previous records. Despite this incredible distance, the researchers claim it is still on the short end of what is possible for this tractor beam technique.

“Because lasers retain their beam quality for such long distances, this could work over meters. Our lab just was not big enough to show it,” Shvedov said in a press release.

Previous laser tractor beams have focused on using photons to bombard particles and use that momentum to propel them forward; an unstable method which has only been moderately successful. The tractor beam from ANU uses a hollow laser beam to surround the particle, and heat from the beam manipulates the air and can move the particle forward, hold it still, or even pull it in reverse.

Image: (a) Unstable conditions are created for particles with Gaussian beams. (b) Hollow laser beams create stable conditions that can be manipulated. (c) Diagram of glass particle within the hollow laser beam. Credit: Shvedov et al., Nature Photonics

“Demonstration of a large scale laser beam like this is a kind of holy grail for laser physicists,” senior author Wieslaw Krolikowski commented.

The particles used in the study were tiny gold-plated hollow glass spheres. The laser beam interacts with the particles and generates a hotspot. When particles in the air bump into the hotspot, they heat up and blast away. The force from the air blasting away then pushes the particle in the opposite direction.

In addition to being used across long distances, this method also gives the team an unprecedented level of control over where the particle goes. By controlling the position of the hotspot by changing the polarization of the laser, the researchers can control which direction the heated air will push the particle. It can easily be pushed, pulled, or held in one spot just by changing the laser’s polarization.

“We have devised a technique that can create unusual states of polarization in the doughnut shaped laser beam, such as star-shaped (axial) or ring polarized (azimuthal),” co-author Cyril Hnatovsky added. “We can move smoothly from one polarization to another and thereby stop the particle or reverse its direction at will.”

Sadly, even if the tractor beam is successfully tested in a larger space, it won’t be used to reel in enemy spacecraft like in the movies (not that this is a huge problem for humanity right now). The researchers note that this kind of tractor beam could be very useful for physicists who would need to control individual particles, and it could possibly even be used to manipulate pollutants in the air.

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