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The One Where NASA Asks for a Tractor Beam

Image Credit: Greg Williams 
David Ruffner wasn't thinking about tractor beams-- until NASA called. 

Last year, a team of researchers at NASA set out to work with scientists to develop a beam of light that could draw objects back along its length.  Affixed to a space-probe, the tractor beam would remotely collect extraterrestrial atmospheric and planetary particles and transport them back to the space-probe for analysis. Now, scientists at New York University have put science-fiction into practice with a device that can pull particles of sand, plastic, and even molecules and cells up a conveyor belt of light.

David Grier is an optical and soft condensed matter physicist at New York University and an expert on the forces that light can exert on matter. When NASA called, he and his graduate student, David Ruffner, decided to tackle a tractor beam technique that was recently proposed by theoretical physicists, but never demonstrated in the laboratory.

When you shine a laser pointer at a slide, it forms a little spot. The tractor beam that Grier and Ruffner pursued used a beam that forms a bulls eye instead of a spot. The beam is called a Bessel beam.

Figure 1: Reconstruction of the two Bessel beams
interfering to produce individual packets of light (red)
along the beam axis. Image Credit: David Ruffner

The scientists overlapped two Bessel beams so that when the beams combined, they formed an interference pattern of bright and dark spots along the beam axis. (Figure 1) 

The shifting intensity of light along the axis produces a force, called an optical gradient force, which becomes stronger with higher contrast between the bright spots and dark spots.

As a particle crosses the beam axis, this force draws it into the areas of greater intensity (the bright spots). By shifting the pattern of bright spots, the scientists can direct this force up or down the length of the beam and draw objects along the beam. 

Any object with a dipole moment feels this force. Magnetic dipoles have north and south poles, electric dipoles have ends of greater positive and negative charge. Grains of sand, plastic spheres, water molecules, proteins and DNA all have dipole moments.

Ruffner noted that, unlike other tractor beam techniques, the Bessel tractor beam works with almost any object that has a dipole moment, making it a widely applicable tool.

In the experiments published in Physics Review Letters on October 19, 2012,  Ruffner and Grier sent the beam through a drop of liquid that contained spherical silica (sand) particles, one-thousandth of a millimeter wide. The particles were drawn into the light packets by the force of the changing light intensity. Once the particles were trapped, the scientists pulled the chain of light--and the trapped particles-- back towards the laser source. Using a normal DVD player, Ruffner and Grier recorded the motion of the particles and reconstructed their movement in three-dimensions (see video below).

Video reconstruction of particles moving in opposite directions on two Bessel tractor beams.  Image Credit: David Ruffner
Right now, the new tractor beam can move objects about 30 micrometers and Ruffner thinks it might be possible to get around a millimeter with their beam. "I was actually kind of skeptical going into this project but it turns out that Bessel tractor beams really do a pretty good job," he said, "but to get to the kilometers NASA mentioned, that's a harder problem." In the meantime, Ruffner is teeming with ideas for how scientists may be able to use the beam-- from microfluidic "lab on a chip" circuits to air and dust collection for environmental testing.

"You get sort of excited," said Ruffner, "you could do something like in Star Trek-- it's very challenging and possibly impossible, but it's cool to work on a problem like that."

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