Spotlighting the power of crystal light - photorefractive crystals

Science News, Nov 5, 1988 by Ivars Peterson

Spotlighting the power of crystal light

The unexpected happens when you shine laser light into a crystal of barium titanate. At first, the beam passes right through the material. But within seconds, a second beam emerges from the crystal, heading straight back into the incoming beam and rapidly growing in strength.

This curious property is one mark of a remarkable group of materials known as photorefractive crystals. Long considered little more than a physics novelty, photorefractive crystals are starting to find their way into a variety of sensors and instruments. In one instance, they play a role in detecting minute vibrations; in another, they're used for tracking and reproducing the motion of a robot arm. These applications are among several reported this week in Santa Clara, Calif., at the Optical Society of America annual meeting.

"The field is wide open," says Jack Feinberg of the University of Southern California in Los Angeles. "In the beginning, we were concentrating on the physics of what was going on. Now people are starting to let their minds roam a little."

The photorefractive effect is caused by traces of impurities and minor crystal defects, which supply the crystal with extra electrical charges trapped within the material. Laser light dislodges charges, forcing them to drift and then become trapped again. This electrical rearrangement warps the crystal lattice, distorting and scattering the light beam. When the illuminating light is turned off, the charges stay put, in effect "remembering" the light pattern.

Photorefractive crystals provide a way of storing hologrpahic images without the bother and delay associated with photographic plates. An image-bearing beam and a reference beam illuminate the crystal. The two beams interfere, canceling each other in some areas and reinforcing each other elsewhere, to create a complicated intensity pattern. Another beam can then "read" the stored pattern, recreating the original image. If set up correctly, the system even compensates for the distorting effects of any medium through which the light beam travels, cleaning up the image.

Feinberg and his colleagues have used this holographic effect for detecting tiny vibrations of rough surfaces. Unlike shiny mirros, rough surfaces generally scatter light in all directions. By using a barium titanate crystal, Feinberg can holographically process the scattered light to produce a clean signal and dramatically improve the chances of optically detecting surface deflections of less than an angstrom.

Dana Z. Anderson and his collaborators at the University of Colorado in Boulder use holographic encoding to record a mechanical arm's movements. Light shining through an optical fiber attached to a robot arm generates a characteristic speckle pattern depending on the arm's position. A lithium niobate photorefractive crystal records the speckle pattersn associated with a sequence of movements. REading the stored patterns allows the arm to repeat its movements.