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Free-electron light for probing interfaces.

After nearly a decade of development, free-electron lasers are beginning to live up to their promise as versatile, powerful tools for studying materials. Such lasers now operate at several institutions, and more advanced machines are already under construction.

At the free-electron laser center at Vanderbilt University in Nashville, Tenn., researchers have used intense infrared light precisely tuned to particular wavelengths to measure with high accuracy the discontinuity in the energy of conducting electrons at the interface between two dissimilar semiconductors. Norman H. Tolk and colleagues at Vanderbilt and the Federal Polytechnic Institute in Lausanne, Switzerland, report their findings in the Nov. 15 PHYSICAL REVIEW B.

"The experiment is the first application of a free-electron laser to interface work," the researchers say.

Conventional lasers work by exciting electrons in atoms to higher energy levels; the electrons then shed this extra energy as light of a particular wavelength. In contrast, free-electrons lasers generate light by harnessing beams of accelerated electrons already stripped from atoms. By adjusting electron energy and other parameters, researchers can tune the laser's output over a range of wavelengths.

Tolk and his co-workers studied the "conduction band discontinuity" between such pairs of semiconductors as gallium arsenide and gallium aluminum arsenide by irradiating the interface with infrared light from the Vanderbilt free-electron laser. To find the energy difference, the researchers decreased the wavelength of the incoming infrared light until they could detect an electrical current between the two semiconductors.

"The free-electron laser is basically the only high-intensity, tunable infrared source available for measuring that offset directly," says Vanderbilt physicist Alan V. Barnes. "It was a long-standing problem. People had been using lots of indirect methods to measure this, but most of those methods ... were very dependent on theory."

Such measurements aid researchers constructing and trying to understand better the behavior of layered semiconductors, which show potential as electronic devices and solid-state lasers. The Vanderbilt group is now extending these studies to other pairs of semiconductors and to more complicated interfaces.

"We're only a fraction of what's going on at the Vanderbilt Free-Electron Laser Center," Barnes notes. Researchers are exploring potential applications not only in materials science, but also in medicine, including laser surgery, and biophysics.
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Title Annotation:intense infrared lasers to measure differences in energy of conducting electrons
Author:Peterson, Ivars
Publication:Science News
Article Type:Brief Article
Date:Nov 21, 1992
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