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Chinese researchers build a working, nanoscopic tractor beam

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Chinese researchers build a working, nanoscopic tractor beam

Tractor beams are one of those sci-fi staples that, when you get right down to it, are actually pretty hard to wrap your head around: it's a force that projects outward, and yet it pulls an object inward. Crazy as it is, Chinese researchers may have just figured out how to do just that.

Before you look to China for all your future cow abduction conspiracy theories, the beam that the researchers at Fudan University in Shanghai have going works on the nanoscopic level. How'd they do it? Well, in the simplest terms, they turned a laser beam inside out, which, according to Popular Science, "should generate a backward pulling force from a forward traveling stream of photons."

Of course, like any awesome technology in its infancy, there's still a laundry list of "buts." For one, the conditions whereby the beam operates flawlessly are rather particular, and even the smallest deviation from the beam's optimal environment could throw it off. That, and at its tiny scale, you won't see China tractorin' anything all that large — unless this technology upscales surprisingly well. That said, it could be an amazingly useful tool on a smaller scale, especially with, say, medical nanotech.

*ttp://dvice.com/archives/2011/02/chinese-researc.php

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How To Turn A Laser Into A Tractor Beam

Physicists work out how to generate a backward pulling force from a forward propagating beam

kfc 02/28/2011


A photon has a small momentum which it can impart to anything it hits, as Arthur Compton and Peter Lebedev discovered at the beginning of the last century. We now know that photons can be used to push anything from electrons to solar sails.

Today, Jun Chen from Fudan University in China and a few pals demonstrate the counterintuitive result that photons can pull things too. In other words, they've worked out how to generate a backward pulling force from a forward propagating beam.

Chen and buddies say this is possible when the system meets two conditions. First, it works only for beams in which the momentum in the direction of propagation is small, as is the case for beams that merely glance off an object. Second, the photons must simultaneously excite several multipoles within the particle, which scatter the beam.

If the scattering angle is just right, the total momentum in the direction of propagation can be negative, meaning the particle is pulled back towards the source and the light becomes a tractor beam.

This must not be confused with various "optical tweezer" type mechanisms in which particles trapped in a beam follow the intensity gradient of the light. In this case, the particles always reach some point of equilibrium where the intensity reaches a maximum.

Chen and co's new force works when there is no gradient. Given the chance, their tractor beam will pull a particle all the way back to the source.

That's a handy additional tool in the nanomanipulator's box of tricks. "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," say Chen and co.

This is a theory paper so there's one piece of the puzzle left to fit. All they have to do now is demonstrate that their tractor beam works.

Ref: arxiv.org/abs/1102.4905: Backward Pulling Force From A Forward Propagating Beam

*ttp://www.technologyreview.com/blog/arxiv/26448/
 
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