Hold for SCIENCE embargo: 4 P.M. Eastern Time, Thursday, July 24, 1997
Media Contact: Warren R. Froelich, (619) 534-8564, [email protected]
UCSD CHEMISTS CREATE POLYMER "MIRROR" FOR A VARIETY OF "TIME REVERSAL" OPTICAL TASKS
Reflect, for a moment, on a different kind of mirror that, unlike a conventional looking glass, could restore clarity to an image distorted by a pane of frosted glass or a greasy lens.
It would be like projecting an image through a coke bottle, without disturbing the outcome.
Such a mirror, capable of performing this and other feats of optical trickery, may soon be available with the help of a new plastic created by scientists at the University of California, San Diego.
The plastic, when assembled in a multi-layer organic polymer likened to a club sandwich, is the first material of its kind to complete such optical tasks, all with potential implications for communications and image processing. Before now, the ability to bring clarity to distorted traveling light waves, including reflected images, has only been found in a few highly expensive inorganic crystals.
"People have been interested in using crystals for a long time, but they've been too expensive and hard to deal with," said W. E. Moerner, professor of chemistry and biochemistry at UCSD.
"Now, we can produce the same special features with a material that's both simple to create and cheap to manufacture," he added. "We have passed a crucial milestone in the potential of these materials."
Moerner and Anders Grunnet-Jepsen, a postdoctoral fellow, along with undergraduate chemistry student Courtney L. Thompson, described the new "photorefractive" polymer, its construction and potential uses, in the current issue of the journal Science.
Polymers capable of the "photorefractive" effect were first discovered by Moerner and colleagues at IBM in 1991. Here, illumination by light or laser beams causes electrical charges to move within the material, altering its index of refraction--the optical characteristic that affects the speed with which light passes through a material. When white light passes through raindrops, prisms or decorative crystal, for example, simple differences in the indexes of refraction result in rainbows of color. When two laser beams cross within a photorefractive material, they create a pattern similar to a hologram that changes the optical properties of the material.
One property of photorefractive materials, called "phase conjugation," can actually enhance the quality of laser images and optical communications. When a laser beam passes through a distorting medium, such as decorative crystal or turbulent air, the resulting transmitted beam is blurred by the distortion. If you have a phase conjugating mirror, when the blurred image is reflected off such a mirror and then sent back through the same distorting medium, the image is restored to its original clarity.
Scientists have been exploring this phenomenon, popularly referred to as "time reversal," for a variety of optical applications since the 1960s, when it was first discovered by scientists at Bell Laboratories. Besides the transmission of complete undistorted images through optical fibers and the atmosphere, researchers have envisioned the technology's potential for lensless imaging in photolithography, screening of manufactured parts for defects, and the tracking of distant objects such as satellites.
However, progress has been hindered by the limited availability and high cost of the crystals, a few thousand dollars each, which have made them practical for only a few niche applications.
Since Moerner's original discovery at IBM, several research groups have been experimenting with a variety of polymer recipes to tailor for desired photorefractive effects. First, the material must be "electro-optic"--its index of refraction changes in the presence of an electric field. It must also contain combinations of atomic or molecular impurities that can "donate" negative or positive electrical charges (electrons or positively charged "holes") and others that will "trap" them after they move in response to light.
For their latest polymer, the UCSD researchers started with a chemical called polyvinyl carbazole (PVK), first used in photocopying machines, to serve as a charge transporting medium. They then added an organic material called PDCST that has an index of refraction that can be altered by an electric field. To increase the material's sensitivity to longer light wavelengths, the researchers sprinkled in small amounts of C60, otherwise known as buckminsterfullerene. Finally, a plasticizer (BBP) was added to make the material soft at room temperature.
To test for photorefractivity, the researchers first performed what's called a "two-beam coupling" experiment. Here, two laser beams of equal energy are made to intersect inside the material, creating a holographic pattern or grating. If in the process, one beam gains energy at the expense of the other, that's a clear signature of a photorefractive effect. In a way, the material may be thought of as an optical transistor.
The results were encouraging.
"Normally, all materials have loss so when you send a laser beam through a material, even glass or something that looks clear or transparent, on the other side the beam will be a little weaker," said Moerner. "This material showed a large gain in energy for one of the two beams."
One measure of how efficiently a sample can amplify an optical signal is called the beam-coupling gain coefficient. The first photorefractive polymer at IBM had a coefficient of 0.33 cm-1. The latest version boasts a net gain coefficient greater than 200 cm-1. By comparison, values typically measured in the best inorganic crystals are near 30 to 40 cm-1.
To see if laser signals could be boosted even further, the UCSD researchers prepared two-layered structures of the polymer, sandwiching the layers together between glass slides attached to small electrodes. A small laser beam directed into this two-layered stack resulted in a 400 percent amplification of the signal, a significant gain. When an additional layer was added, somewhat like a club sandwich--and an extra trick referred to as the "moving grating technique" was used--the signal amplification soared by 50,000 percent.
"No other organic polymer has shown this large of a gain," said Moerner. "These types of gains are allowing us to see some neat effects only seen before in the crystals."
Among other things, the researchers have created a "self-pumped phase conjugating mirror." This was accomplished by placing two concave mirrors on either side of their two-layered structure, building an optical cavity in which light waves transmitted through their polymer were allowed to oscillate over time, without needing any additional outside source of energy.
"Each time the beam goes through the material, it's amplified a little bit more in this cavity," said Grunnet-Jepsen. "At the speed of light, in one second, it's going to oscillate several millions of times. And each time, it gains a little bit of energy."
The UCSD researchers noted that their polymer not only demonstrated high gain factors in energy, they've also proved to be highly stable, lasting at least six months so far.
"This is significant because other people have made high-performance materials, but they haven't demonstrated high stability, nor have they demonstrated these new physical features," said Grunnet-Jepsen.
The research was supported by a grant from the U.S. Air Force Office of Scientific Research.
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