Sept. 30, 1993
Vol. 13, No. 3

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    Yu's research puts spotlight on polymer chemistry

    When light passes through any substance, such as glass or water, it appears to bend--the refractive properties of the material change the angle of the light. But some materials interact with light in more complex ways, as light energy interacts with the molecules of the materials themselves.

    A photorefractive material is one in which the refractive index changes as light strikes it. Inorganic compounds that exhibited these properties were discovered nearly 30 years ago. But these materials, although they exhibited astonishing physical properties, were expensive and difficult to make and therefore had little practical application.

    More recently, scientists have been trying to synthesize organic polymers that exhibit photorefractive properties. In 1990, scientists at IBM made the first photorefractive polymers, but there were serious obstacles to their commercial viability. These materials were made by "doping" small molecules into a polymer as it was forming, but problems with phase separation made it difficult to synthesize a material that was uniform in its structure and properties. In addition, these polymers exhibited a photorefractive effect only when a large external electrical field was applied.

    Now Luping Yu, Assistant Professor in Chemistry, has synthesized a novel photorefractive organic polymer with two distinct advantages over previous materials--it works in the absence of an external electrical field, and all of the functional groups are inherently part of the structure of the polymer. Yu presented the results of his research Aug. 23 at a meeting of the American Chemical Society in Chicago.

    Yu's colleagues said his research represents the "cutting edge" of photorefractive materials science and is a step toward using the polymers in telecommunications, three-dimensional storage of optical data and signal processing.

    Yu's approach to making the compounds was conceptually different from that of previous researchers. He began by designing the material so that all the components required to make the material act as a photorefractor--a charge-generating species, a charge-conducting species, an electron trap and a non-linear optical chromophore--are part of the structure of the polymer itself.

    Yu makes use of the conjugated polymeric backbone in three ways: as charge-generator, conductor and structural backbone of the material.

    When light from a laser beam is passed through the compound, it "excites" an electron in the charge-generator to "let go" of its parent molecule. The charge-conductor then carries that free electron along or among the backbones of the polymer. The electron trap then captures the electron, creating a separation of charge that induces a change in the refractive index of the polymer.

    This is the first time that anyone has seen this kind of phenomenon in the absence of an external electrical field in polymeric materials, Yu said.

    "I think we've got the best results so far in this area, both conceptually and in the physical properties of the material," he said. "The difficult thing is to have an initial concept, and we have that."

    Yu said the next step is to improve the optical properties and the response time of the material.

    --Diana Steele