Photorefractive Polymers Perform Better Than Costly Inorganic Materials, UB Scientists Report

WASHINGTON, D.C. -- A new generation of inexpensive, organic polymers with special optical properties has surpassed the performance of the costly conventional materials they may one day replace, University at Buffalo scientists have found.

The new materials, called photorefractive polymers, could help bring optical information storage devices, such as optical computers, a little closer to reality.

The results of the work by the UB researchers, one of only a few groups in the world working on photorefractive polymers, was reported yesterday (Sunday, Aug. 21) at a meeting of the American Chemical Society in the Grand Hyatt Hotel.

"If these materials turn out to be as stable as inorganic materials are, then it will for the first time make photorefractive materials commercially useful," said Maciej E. Orczyk, Ph.D., research assistant professor in the UB Photonics Research Laboratory and a member of the team led by Paras Prasad, Ph.D., UB professor of chemistry and laboratory director.

Orczyk described photorefractivity as the ability of a material to change its index of refraction under the influence of light. A photorefractive material becomes operational when two beams of light pass through it and interact with each other. For example, when one beam carries the optical information, the other, stronger beam can transfer its energy to the first one, amplifying the optical signal.

The new photorefractive polymers engineered by UB researchers have significantly surpassed the performance of inorganic photorefractive materials as measured by two key parameters. These parameters are diffraction efficiency, a measure of how much light a material is able to deflect in different directions, and the beam-coupling gain coefficient, which describes how efficiently a material can amplify an optical signal.

Orczyk said that part of the difficulty in developing photorefractive polymers is that optimizing one important function in the material may negatively affect another.

He and Jaroslaw Zieba, Ph.D., a UB research assistant professor, and Bogdan Swedek, a UB graduate student, have engineered a material from separate molecules, each of which

contributes a necessary functionality.

Called PVK-TCP:C60-DEANST, the material is made of polyvinylcarbozole polymer matrix; C60 to make it photoconductive; TCP, a plasticizing agent, and DEANST molecules, which are responsible for the material's refractive-index changes under exposure to light.

Until the first sample was made by IBM in 1991, some scientists were skeptical that photorefractive polymers would ever be developed.

"Inorganic materials are crystalline, so they are highly ordered," explained Orczyk. "Organic polymers are fundamentally different, they're virtually disordered. This is, in part, why people couldn't believe that such a complex, multistep phenomenon could occur in polymers. Even after it was observed, few believed the effect would ever be considerably optimized."

Orczyk estimated that while a photorefractive inorganic sample now can cost as much as thousands of dollars, photorefractive polymers could eventually cost as little as just a few dollars.

Inorganic materials are made from crystals that must be grown under very controlled conditions and that may take many weeks. Polymers, on the other hand, are made from solutions, cast into films and dried, a process that generally takes just hours.

Applications for photorefractive materials include all-optical telecommunications, optical data storage and the transmission of computer images in real-time.

Photorefractive materials would allow for real-time processing of data because, unlike electronic materials, they holographically store whole images without having to digitize and then reconstruct them.

The UB scientists are working in cooperation with Laser Photonics Technology, Inc., a company specializing in development and technological applications of novel optical materials. It operates out of the University at Buffalo Foundation Inc. incubator adjacent to the UB campus in Amherst, N.Y.

The research is funded by the Air Force Office of Scientific Research and the National Science Foundation.