Laser Cooling of Solids Advances

Michael D. Wheeler

Since 1995, when a group of scientists discovered how to cool solids with laser light, the prospect of improving the technique has enthralled many researchers. Now a group from the University of New Mexico here and from Los Alamos National Laboratory in Los Alamos, N.M., has reported that it has efficiently cooled solid glass with infrared laser light.

When an atom absorbs energy from a laser, it reaches an excited state and, upon relaxing, releases the energy in the form of luminescence, thermal energy or both. Materials that luminesce typically release less energy than they absorb, a phenomenon known as Stokes shifting.

To cool, the research team employed anti-Stokes luminescence, in which the energies of the emitted photons are higher than those absorbed. By repeating this process, the solid material sheds more and more thermal energy, leaving it cooler than its original state.

The team, led by Mansoor Sheik-Bahae, Chad W. Hoyt and Richard I. Epstein, selected thulium-doped ZBLANP as a target, a heavy fluoride glass similar to the ytterbium-doped samples used in the 1995 experiments. A 6.2-W optical parametric oscillator with a tunable range of 1.75 to 2.05 µm served as the laser source.

Hoyt said the researchers hoped that the Tm:ZBLANP would outperform its predecessor. In fact, the glass cooled twice as efficiently.

Commercial coolers

For a solid to cool, the dopant atoms must give up almost all of their energy as fluorescence, since any nonradiative processes can quickly overwhelm cooling. This particular combination of glass and dopant has the attractive radiative properties essential for cooling. The group observed a quantum efficiency of greater than 99 percent, opening the prospect of manufacturing an optical, all-solid-state cooler for commercial use.

"These cooling systems – glass, crystal and semiconductor – have great potential for commercial applications," Hoyt said. "They have begun to realize optical refrigerators: compact, solid-state, vibration- and electromagnetic-interference-free coolers. " Of particular interest: The optical coolers may be integrated with semiconductor-based devices, such as detectors.

Details of the team's work appeared in the Oct. 23 issue of Physical Review Letters