Light into Motion

David Pacchioli
June 01, 1996

Imagine remote control without wires or batteries, robots controlled by light. Kenji Uchino did. Now the idea Uchino hatched in a Tokyo karaoke bar in 1981 is on the verge of becoming an important 21st-century technology.

Uchino, professor of electrical engineering and director of Penn State's International Center for Actuators and Transducers, was sitting with a friend one evening long ago when the after-work talk came around to miniature robots, tiny devices that might someday be useful in advanced manufacturing and research. The friend, a precision-machine expert, explained that as these mechanisms get down to the one millimeter size, the weight and bulk of an electric lead becomes a problem. Do you suppose, he wondered, you could create a remote-controlled actuator that would bypass the need for electricity?

Uchino accepted the challenge. "Initially," he remembers, "I tried to use a solar battery combined with piezoelectric materials." Piezoelectric materials, he explains, convert electricity into mechanical energy—movement. When an electric current is run through piezoelectric ceramic, the ceramic changes size—it shrinks or expands. The speaker inside a telephone receiver or a computer is a layer of such ceramic bonded to a tiny disk of cheap metal, usually brass, and supplied with current at the right frequency to make it shrink and expand very rapidly—to resonate. In this case, Uchino hoped, the solar battery would provide the necessary power.

It wouldn't. A solar battery small enough to fit such a device could generate only one volt of electricity, Uchino found. "We needed one to two kilovolts." Around that time, however, he attended a meeting where a Russian physicist reported seeing a stronger photovoltaic (light into electricity) effect in certain ferroelectric materials. Uchino decided he would attempt to combine these ferroelectrics with his piezoelectrics (electricity into motion), creating a single material that would convert light directly into motion.

After a period of trial and error, he devised a crystalline compound made of lead, lanthanum, zirconium, and titanium, "doped" with tungsten. Called PLZT, this new material showed a large piezoelectric effect in response to near-ultraviolet light. Exactly how this so-called "photostriction" works remains unclear, but Uchino suspects that incoming particles of light (photons) excite electrons orbiting the tungsten atoms, and that the asymmetrical structure of the PLZT crystal allows these electrons to move slightly to one end of the molecule, the imbalance creating an electric voltage, which then triggers the material to change shape.

Whatever its mechanism, by pasting two tiny PLZT plates together and shining a purple light first on one side, then on the other, Uchino was able to make a device that flexed from side to side, its motion like that of a tiny leg. Attaching two such "legs" to a plastic "torso," he created a simple "light-driven micro walking machine," capable of inching its way across a tabletop.

More recently, Uchino has turned his attention (and his invention) to photo-acoustics. "The technology to transmit voice data—a phone call—at the speed of light through lasers and fiberoptics has been advancing rapidly," he explains. "But the end of the line—the ear speaker—limits the technology, since optical phone signals must be converted from light energy to mechanical movement via electrical energy."

To eliminate the hold-up, Uchino fashioned a resonator, similar to the tiny piezoelectric speaker found in today's telephones, except made of PLZT. When he flashed alternating pulses of light on either side of the device, he was able to make it vibrate at up to 80 times per second, just within the audible range for the human ear.

Now the task is to improve that frequency several-fold—bringing it high enough to accurately reproduce the human voice and create a viable "photophone" for commercial use. Such a device, once perfected, would clear the way for the optical communications of the future.

Kenji Uchino, Ph.D., is professor of electrical engineering and director of the International Center for Actuators and Transducers, 134 Materials Research Laboratory, Unviersity Park, PA 16802; 814-863-8035. Uchino's work on photoacoustic devices is supported by a grant from the Army Research Office.

Last Updated June 23, 2015