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Optical refrigeration proves really cool.

Laser light, most often used to heat objects, can now cool them as well.

Richard I. Epstein, an astrophysicist at Los Alamos (N.M.) National Laboratory, and his colleagues have developed an "optical refrigerator." The new system, described in the Oct. 12 Nature, may someday offer computer makers a simple way to cool their machines' hardware, making possible faster and more powerful circuitry for computing and communicating.

Speaking metaphorically, Epstein likens the process to bathing a warm object in "cool light." Since heat arises from atomic motion, a carefully tuned laser can chill an object by dampening its molecular vibrations, causing the object to shed energy as fluorescent light.

To get a feel for the process, he says, think of "pouring light into an object, soaking up some of the vibrational energy," and then letting the fluorescence "carry the energy away."

"The fact that they can extract heat from something with light is interesting," says Clifford R. Pollock, an electrical engineer at Cornell University. "People usually use light to create heat rather than to remove it."

Pollock sees optical cooling as especially handy for applications in which vibrations cause problems, such as in cooling satellite circuitry.

In the Los Alamos Solid-State Optical Refrigerator--or LASSOR--a diode laser shines infrared light into fluoride glass impregnated with ytterbium ions. When "pumped" with photons, the glass glows, cooled with 2 percent efficiency. Though well below the cooling efficiencies of a kitchen refrigerator, the 2 percent energy conversion rate ranks well against other methods for freezing items to 77 kelvins, the temperature at which nitrogen liquefies.

The basic principle is "fairly simple," Epstein says. Though the idea for laser cooling dates back to 1929, scientists couldn't quite get the process to work until recently. The chief difference today, he says, has come from the recent development of compact, solid-state lasers and carefully crafted fiber optic materials, he says.

The LASSOR represents a spin-off of astrophysical research: The group had worked earlier on cooling space-based photon detectors, Epstein says. Now they will try to chill objects to various temperatures. "We want to make a rugged cooler with no moving parts," he says, "just a laser, a piece of glass, and a shell to absorb the fluorescence and shed the extra heat.

"We think the potential for this is phenomenal."
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Title Annotation:Science News of the Week; laser used to slow heated object's molecular vibrations
Author:Lipkin, Richard
Publication:Science News
Date:Oct 14, 1995
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