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Dimmer lasers brighten the photon's future.

In efforts to create useful gadgets from the semiconductor layer cakes called multiple quantum well structures, researchers have already fashioned miniature lasers and optical switches that route information riding on light beams.

Now, in a significant advance, scientists at Purdue University in Lafayette, Ind., have decreased the intensity of laser light at which one such material allows light itself to change the rate at which it travels through the layers.

These so-called photorefractive materials have the ability to store holograms internally as a pattern of electrical charges. The patterns can hold a tremendous amount of information about an image projected into the crystal with laser light. This makes photorefractive materials ideal for use in the emerging technology of optical computing - controlling light with light.

Previous photorefractive materials required laser intensities 10 times greater than those demanded by the Purdue device. The new material's higher sensitivity to light bodes well for the use of photorefractive materials in optical computing, says physicist David D. Nolte, co-investigator on the Purdue project. "It means that the power requirements are way down, that you can use very low-power, low-cost, safe, compact laser diodes," he says.

Alastair Glass, a researcher at AT&T Bell Laboratories in Murray Hill, N.J., and co-holder of the patent on the material, characterizes the improvement in sensitivity as a welcome, though not unexpected, advance. These multilayer crystals, he says, "look like they're going to be the material of choice for image processing."

This is the first time researchers have demonstrated experimentally the low power requirements of the material, Nolte says. The purdue team describes its results in the September JOURNAL OF THE OPTICAL SOCIETY OF AMERICA.

In an ordinary material - glass, for example--intersecting beams of photons pass right through each other unperturbed. Photorefractive materials, however, allow researchers to modulate one light beam with another. In the device tested at Purdue, ultrathin semiconductor layers--the multiple quantum wells--confine photons, enhancing certain optical properties of the material.

The Purdue researchers demonstrated significant modulation at intensities comparable to the illumination in a dimly lit room. Moreover, the device redirects an unprecedented 10 percent of one beam's energy into the other. Nolte says this dual result -- a high degree of modulation at a relatively low laser intensity -- is a "world record" for photorefractive materials.

In their experiments, the researchers place the device at the intersection of two laser beams. The beams interfere with each other, generating a pattern of bright and dark fringes. Electric charges move into these fringes and form a holographic impression of the information projected in the laser beams.

The laser beams also interact with the hologram. This allows one beam to control another: A change in the light entering the crystal changes its optical properties, which in turn affect the behavior of the other beam.

Nolte says these photorefractive materials --whose applications include holographic memories and robot vision--are drawing the attention of other researchers. He cites well-attended sessions on the subject at last week's meeting of the Optical Society of America, held in Albuquerque, N.M. "More and more people are getting interested in these devices," he says.
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Title Annotation:lasers lights' intensity can be decreased, and rate of traveling through layers can be changed
Author:Pendick, Daniel
Publication:Science News
Article Type:Brief Article
Date:Oct 3, 1992
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