By Jon Cartwright
Physicists in the United States have demonstrated the optical analogue of an aerofoil--a "lightfoil" that generates lift when passing through laser light.
The demonstration, which comes more than a century after the development of the first airplanes, suggests that lightfoils could one day be used to maneuver objects in the vacuum of outer space using only the Sun's rays. "It's almost like the first stages of what the Wright brothers did," says lead author Grover Swartzlander, a physicist at the Rochester Institute of Technology, New York, whose study appeared December 5 in Nature Photonics. (Scientific American is part of Nature Publishing Group.)
The principle of a lightfoil is similar to that of an aerofoil: both require the pressure to be greater on one side than the other, which generates a force, or lift, in that direction. With an aerofoil, the pressure difference arises because air must pass faster over the longer, curved side to rejoin the air passing underneath.
With the lightfoil, the pressure comes from light rather than air. Such "radiation pressure" was theorized by physicists James Clerk Maxwell and Adolfo Bartoli in the late 19th century, and exists because photons impart momentum to an object when they reflect off or pass through it. It is the reason, for example, that comet tails always point away from the Sun--the Sun's rays push them that way.
Grover and his colleagues predicted that radiation pressure could generate lift in a lightfoil having performed computer analyses to learn how light rays refract and reflect as they enter different shaped objects. Success came in the form of a semi-cylindrical rod, which showed from the analyses that a large portion of incident light rays should leave in a perpendicular direction. The side where they leave would experience the greatest radiation pressure and, therefore, lift.
To test this prediction, Grover and his group dropped plastic semi-cylindrical rods, just a few micrometers in length, into water. When they shone a laser beam onto the rods from beneath, the rods moved upwards--thanks to the direct levitating force of the laser--but they also moved to the side. It was this latter, perpendicular motion that, the researchers claim, proves the existence of optical lift.
One application of the lightfoil would be to control the direction of space vehicles that rely on radiation pressure for thrust, such as the experimental solar-sail spacecraft LightSail-1, which the Planetary Society, a public space organization based in Pasadena, Calif., is planning to launch later this year. The lightfoil concept could also be used to power micromachines, or transport particles in liquids.
For these last applications, Matt Eichenfield, a physicist who specializes in nanoscale optical and mechanical systems at the California Institute of Technology in Pasadena, believes it would be more useful if the lift could be realized for any transparent object. This might be possible, he says, if the problem were considered backwards, so that it is the shape of the laser beam, rather than the object, that is tailored.
Eichenfield adds, however: "It's an interesting effect. And it's key, as this field of nanomechanics combined with optics becomes more important, that we revisit the simplest phenomena, as they've done here."