'Tractor beam' is possible with lasers, say scientists

image captionThe idea hinges on the use of a laser whose beam is of a specific shape

A laser can act as a "tractor beam", drawing small objects back toward the laser's source, scientists have said.

It is known that light can provide a "push", for example in solar sails that propel spacecraft on a "wind of light".

Now, in a paper on the Arxiv server, researchers from Hong Kong and China have calculated the conditions required to create a laser-based "pull".

Rather than a science fiction-style weapon, however, the approach would only work over small 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; this new force, the authors propose, would be one continuous pull toward the source.

And it relies on directly impinging on an object, making it distinct from an approach demonstrated in 2010 by Australian researchers whose trapping worked by heating air around a trapped particle.

The trick is not to use a standard laser beam, but rather one known as a Bessel beam, that has a precise pattern of peaks and troughs in its intensity.

Seen straight-on, a Bessel beam would look like the ripples surrounding a pebble dropped in a pond.

If such a Bessel beam were 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" back toward the source.

The properties of Bessel beams are not entirely new to science; in 2006, Washington State University physicist Philip Marston proposed using sound waves to achieve a similar result, and the current work invokes many of the ideas first put forward by Farid Mitri, now at Los Alamos National Laboratory in the US, in a 2009 paper in Journal of Physics A.

'Radical idea'

"Light can indeed pull a particle," the authors wrote, "...and 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."

Ortwin Hess at Imperial College London called the work - which has not yet been peer-reviewed - as "fascinating", saying that it "takes a radical idea forward".

"It's a bit like a boat moving through water," Professor Hess told BBC News. "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."

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

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

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