LASER COOLING

Researchers at the Los Alamos National Laboratory (Los Alamos, NM) have demonstrated the principle of Ooptical refrigeration,O which may someday serve as the basis for cooling spacecraft electronics and detectors as well as superconductive circuits. The anti-Stokes fluorescence phenomenon occurs when a material excited by light at one frequency radiates energy by fluorescence at a shorter wavelength (higher frequency) and thus with greater energy. The result is a net energy loss per photon, effe

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LASER COOLING

Optical refrigeration cools solids

Researchers at the Los Alamos National Laboratory (Los Alamos, NM) have demonstrated the principle of Ooptical refrigeration,O which may someday serve as the basis for cooling spacecraft electronics and detectors as well as superconductive circuits. The anti-Stokes fluorescence phenomenon occurs when a material excited by light at one frequency radiates energy by fluorescence at a shorter wavelength (higher frequency) and thus with greater energy. The result is a net energy loss per photon, effectively cooling the material (see Fig. 1). Tim Gosnell, head of the laboratory that conducted the experiments, says light "soaks up some of the vibrational, or heat, energy of the object, then carries away the excess energy. We?ve discovered how to use laser light to excite an object to special quantum states in which it can trap thermal vibrations but can't create them."

The experiments demonstrate what is probably the first continuous solid-state cooling technique to be shown since Peltier discovered thermoelectric cooling in 1834. The Los Alamos group beamed 1020-nm light from a ytterbium-fiber laser pumped by a 980-nm diode laser into rare-earth-doped (1% Yb3+) heavy-metal fluoride (ZBLANP) glass. Ytterbium ions fluoresce in a single band about a mean wavelength of 995 nm, a frequency band much higher than the possible thermal vibration frequencies, precluding any heating. "These coolers are basically solid-state lasers running backwards," says team member Richard Epstein. "Thus we can draw on existing technology."

While the cooling power produced was only around 1% of the laser power input, woefully inefficient for domestic refrigeration or air conditioning applications, it would still be adequate for cooling superconductive microelectronics to cryogenic temperatures. Based on these initial experiments, the researchers have postulated the Los Alamos solid-state optical refrigerator (LASSOR) for cooling instruments to at least liquid nitrogen temperatures of 77 K (see Fig. 2). The next goal is to demonstrate cooling at 100 K, which is still below thermoelectric-cooler operating levels, sometime in the next 12 months.

Compact with no moving parts, the cryocoolers may eventually see use in desk-top computers. But with current high-powered diode-laser lifetimes on the order of 14 months, a spacecraft LASSOR would need more than eight diode-pumped fiber lasers to ensure functioning over ten years on long-duration or deep-space missions. This portion of the cooler could benefit from recently developed aluminum-free diode-laser materials having longevity more than the duration of such missions (see Laser Focus World, Nov. 1995, p. 36). These materials may also have sufficient wavelength flexibility to eliminate the fiber-laser conversion stage.

Rick DeMeis


FIGURE 1. Fluorescence cooling occurs when the rare-earth-ion electrons in the upper levels of a broad, or manifold, ground-energy state are photon pumped to lower levels of the excited-state manifold. The populations of these energy levels absorb heat in distinct quantums of vibrational energy, or phonons, to return to thermal equilibrium. The ions then fluoresce; these photons, which are more energetic than the pump photons, revert back to the initial ground state.
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FIGURE 2. Proposed cryocooler will be about 3 cm in both height and diameter, with a ZBLANP:Yb3+ cooling element. For greatest efficiency, pump-laser wavelength should be 3 cm in both height and diameter, with a ZBLANP:Yb3+ cooling element. For greatest efficiency, pump-laser wavelength should be as long as possible and be completely absorbed in the yttrium-doped material. Dielectric end mirrors trap pump-laser radiation. Most of the fluorescence (dotted lines) passes through the concentric side radiation shields.
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