Though the lab looks bare, nestled in the basement of the Chester F. Carlson Center for Imaging Science (CAR, 76) is something microscopic that’s very big. So big, in fact, that the results could enable space travel powered by the sun and manipulate the very building blocks of life.
Alexandra Artusio-Glimpse, an Imaging Science graduate student, explained the discovery with a smile. She and other researchers on the project have successfully built and tested an optical wing — a small device that generates lift from the energy of a laser beam.
“Light has momentum,” she explained. If you hit a stationary golf ball with a club, the ball goes into motion; this is because some of the force of movement (momentum) from your swing is transferred to the ball. The same principle applies to particles of light, called photons. Because photons have momentum, light can transfer energy.
In Artusio-Glimpse’s experiment, tiny plastic semi-cylindrical rods, thinner than a strand of human hair, are immersed in water and hit with a beam of laser light. Initially, the rods simply rotate in the water, but soon find stability and begin move forward and up.
This works because of the rod’s carefully crafted shape. When a beam of light hits the rod, the rod’s shape bends it, much like the way a prism or the lens of a magnifying glass does, and forces it out of the bottom of the rod. This is where Newton’s Third Law — every action has an equal and opposite reaction — comes into play. The light hitting the rod gives it a forward push, and the light exiting downwards propels the rod upwards.
The project is credited to a team of four researchers at RIT. Grover Swartzlander, a professor of Physics and Imaging Science, headed the experiment. Timothy Peterson, a student pursuing a master’s degree in Computer Science, modeled what would happen. Alan Raisanen, associate director of RIT’s Semiconductor and Microsystems Fabrication Lab, was responsible for making the tiny rods. Artusio-Glimpse took the experimental measurements.
The roots of the experiment began when Swartzlander and other researchers at a symposium at Sandia National Laboratory started pondering the momentum of light and the force obtained when that momentum is changed. If that’s true for forces such as kinetic energy, they reasoned, why not with light? “Can it exhibit the same lift as an air foil, an aerodynamic wing?” Swartzlander, appearing on a Canadian science podcast, recalled thinking. “When you do science, no matter what you calculate, you’re always surprised by the experimental result.” And no one expected this result.
Creating movement from light’s momentum isn’t new. NASA tried to use a similar technology for moving in a vacuum late last year with technology known as a “solar sail.” The theory was that momentum from light would push the sail forward in the same way that wind pushes a sailboat forward. The momentum of photons would reflect off the material and then bounce backwards, imparting twice the momentum forward and resulting in forward motion. Unfortunately, the experiment ended inconclusively when the sail went missing. Unlike the optical wing, a solar sail can only travel in one direction.
Artusio-Glimpse said that a solar sail could be made out of billions of optical wings arranged in vast arrays. One array would control right and left movement, while another would control up and down movement. This way, the sail could move in any direction. With large enough arrays, it might even be possible to make flying carpets or saucers.
But perhaps a more practical use would be to improve so-called optical tweezers, which are focused laser beams that trap and move particles by moving the beam. Already, lasers control particles that move small biological structures like DNA and single cells. But with optical lift, the “tweezers” could be easier to use and more precise. This could greatly simplify microbiological research and could be used to manually manipulate genetic code.
Swartzlander said the results of the experiment were “almost like the first stages of what the Wright brothers did.” The ability to use light as a mechanic for flight needs to be further tested in air with other materials and other types of light. Optical lift then, clearly has a bright future ahead of it.