Light has been put to work generating the same force that makes airplanes fly, a study appearing online December 5 in Nature Photonics shows. With the right design, a uniform stream of light has pushed tiny objects in much the same way that an airplane wing hoists a 747 off the ground.
Researchers have known for a long time that blasting an object with light can push the object away. That’s the idea behind solar sails, which harness radiation for propulsion in space, for instance. “The ability of light to push on something is known,” says study coauthor Grover Swartzlander of the Rochester Institute of Technology in New York.
Light’s new trick is fancier than a boring push: It created the more complicated force called lift, evident when a flow in one direction moves an object perpendicularly. Airfoils generate lift; as an engine propels a plane forward, its cambered wings cause it to rise.
Lightfoils aren’t about to keep an Airbus aloft for the time it takes to fly from JFK to LAX. But arrays of the tiny devices might be used to power micromachines, transport tiny particles or even enable better steering methods on solar sails.
Optical lift is “a really neat idea,” says physicist Miles Padgett of the University of Glasgow in Scotland, but it’s too early to say how the effect might be harnessed. “Maybe it’s useful, maybe it’s not. Time will tell.”
That light can have this unexpected lift effect started with a very simple question, Swartzlander says: “If we have something in the shape of a wing and we shine light through it, what happens?” Modeling experiments told the researchers that an asymmetrical deflection of light would create a surprisingly stable lift force. “So we thought we’d better do an experiment,” Swartzlander says. “Because this just looks too pretty.”
The researchers created tiny rods shaped kind of like airplane wings — flat on one side and rounded on the other. When these micron-sized lightfoils were immersed in water and hit with 130 milliwatts of light from the bottom of the chamber, they started to move up, as expected. But the rods also began moving to the side, a direction perpendicular to the incoming light. Tiny symmetrical spheres didn’t exhibit this lift effect, the team found.
Optical lift is different from the aerodynamic lift created by an airfoil. A plane flies because air flowing more slowly under its wing exerts more pressure than the faster-moving air flowing above. But in a lightfoil, the lift is created inside the object as the beam shines through. The shape of the transparent lightfoil causes light to be refracted differently depending on where it goes through, which causes a corresponding bending of the beam’s momentum that creates lift.
These lightfoils’ lift angles were about 60 degrees, the team found. “Most aerodynamic things take off at very gradual angles, but this has a very striking, very powerful lift angle,” Swartzlander says. “You can imagine what would happen if your airplane took off at 60 degrees — your stomach would be in your feet.”
As the rods lift, they shouldn’t stall out, the paper predicts. “The subtlety is that it actually self-stabilizes,” Padgett says. “It twists a little bit one way, and you think, ‘Oh dear, it’ll stop working,’ then the light rotates it back again.”
Swartzlander says he hopes to ultimately test the lightfoils in air, too, and try different shapes and materials with various refractive properties. In the study, the researchers used ultraviolet light to generate the lift, but other kinds of light would work just as well, Swartzlander says. “The beautiful thing about this is that it would work as long as you have light.”
Video: Riding a beam of light, a tiny particle thousandths of millimeters in size is pushed sideways by the same force that keeps airplanes aloft. Credit: Swartzlander et al.
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