In the latest issue of Optics Express, the group reports that when using a 90mW laser, their tractor beam in a box can produce about a micronewton of pulling force. The setup is deceptively simple. The scientists vapor-coated a sliver of glass with reflective gold, and then stuck a flake of cross-linked graphene to the other side. Then, they pointed blue, cyan, and green lasers at the flake of graphene. Lo and behold, it moved toward the laser emitter.
Graphene’s 3D molecular structure (Credit: iLexx/Getty Images)
The setup is built on established technology. Optical tweezers and solar sails also use light to move things around. However, optical tweezers usually confine themselves to objects the size of single molecules. Not so for this experiment, says the team.
“In previous studies, the light-pulling force was too small to pull a macroscopical object,” said research team member Lei Wang from China’s Qingdao University of Science and Technology. “With our new approach, the light pulling force has a much larger amplitude. In fact, it is more than three orders of magnitude larger than the light pressure used to drive a solar sail, which uses the momentum of photons to exert a small pushing force.”
Spooky Action at a Distance
The device works partly by way of graphene’s unique properties. Graphene is optically absorptive, meaning it retains some percent of the energy when photons hit it. It’s also a semiconductor and an effective heat pipe. So effective, the paper concludes, that when the scientists pointed the laser at the graphene sandwich, the graphene carried that energy right to the far side of the piece. Thermodynamics says that hot things emit more energy than cold things, all else being equal. In the lab environment, that differential heating was enough to make the object move.
(Video credit: Wang et al., 2023)
The researchers did their work in a tightly controlled low-pressure environment. This cut down on optical scattering that might have confounded the experiment. However, it also agrees with prior work suggesting that tractor beams might be most useful in space. When you’re moving things around with lasers, it turns out the laser is most effective when it isn’t also moving around random bits of stuff.
“Our technique provides a non-contact and long-distance pulling approach, which may be useful for various scientific experiments,” said Wang. “The rarefied gas environment we used to demonstrate the technique is similar to what is found on Mars. Therefore, it might have the potential for one day manipulating vehicles or aircraft on Mars.”
L. Wang, S. Wang, Q. Zhao, X. Wang, “Macroscopic laser pulling based on Knudsen force in rarefied gas,” Opt. Express, 31, 2, 2665-2674 (2023).
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