March 3, 2011

Scientists Work To Create Laser ‘Tractor Beam’

The tractor beam -- a tool popularized in the realm of science fiction -- has been slowly making its way into the realm of reality, where beams of light are used to pull objects back toward the source of the light, BBC News reports.

In a paper published online, researchers from Hong Kong and China have analyzed and calculated the conditions required to create a tractor beam, or in reality, a laser-based "pull."

Unlike the ones popularized in science fiction -- most notably Star Trek -- the approach would only work over very small distances.

Last year, a team of Australian scientists used a hollow laser beam to move small particles through the air. Because the system requires air, it would not work in space, making it practically useless.

But in the latest development from the researchers in Hong Kong and China, a special laser, known as a Bessel beam, could slowly drag an object backward but only if the beam catches the object at an angle.

Far from being able to transport a vehicle from one location to another some 1,500 feet away or more, the laser-powered tractor beam can only work on smaller particles over much smaller distances.

The effect is different from that employed in "optical tweezers" approaches, in which tiny objects can be trapped in the focus of a laser beam and moved around, the new force, the authors suggest, would be one continuous pull toward the source.

The approach also relies on directly encroaching on an object, making it distinct from an approach demonstrated in 2010 by the Australian research.

Seen from a straight-on point-of-view, a Bessel beam would look like the ripples surrounding a pebble dropped into water. If such a Bessel beam was to encounter an object not head-on but at a glancing angle, the backward force can be stimulated.

As the atoms or molecules of the target absorb and re-radiate the incoming light, the fraction re-radiated forward along the beam direction can interfere and give the object a "push" toward the source.

"Light can indeed pull a particle," Jun Chen, Jack Ng, Zhifang Lin and C.T. Chan wrote in their paper on the subject. "This may open up new avenues for optical micromanipulation, of which typical examples include transporting a particle backward over a long distance and particle sorting," they added.

Ortwin Hess, of the Imperial College London told BBC News that the work, which has yet to be peer-reviewed, was "fascinating," and said it "takes a radical idea forward."

"It's a bit like a boat moving through water," he said. "In the eddies you generate as part of that forward movement, there are areas that literally seem to be pulling back."

"The ship has a shape, and you get these backward eddies at the side; in a similar way if you have a Bessel beam you have certain areas that do the same thing," Hess added.

However, the effect is only predicted to occur over a short distance, and that the effect first of all needs to be demonstrated in practice, he noted.

"As always with theory, if one doesn't obtain a theoretical argument that things are impossible for some reason, then it can happen," said Hess.


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