Laser-Based Cooler Refrigerates by 13 K

Kevin Robinson

Most of us envision lasers cutting through plate steel or vaporizing tissue, but they also can cool molecules and atoms. A team at the University of Queensland in Brisbane, Australia, hopes to use this phenomenon to create a laser-based solid-state cooling system.

Anton Rayner, a member of the project's research team, explained that laser cooling occurs when a molecule absorbs a low-energy photon and emits one of a higher energy, an effect known as anti-Stokes fluorescence. For cooling to occur, anti-Stokes fluorescence must be the only dominant process after a photon is absorbed. "Any other processes will cause heating," he said.

The researchers have demonstrated a functional laser-based cooler. They placed a 250-µm-diameter Yb:ZBLAN fiber into a vacuum chamber and coupled 1 W of 1015-nm light from a laser diode into the fiber, which began to cool by emitting 950-nm light. As the fiber cooled, the intensity of the 950-nm fluorescence dropped until the fiber gained heat from its surroundings as quickly as the laser could cool it -- a process that took about three minutes. At this point, the researchers measured the temperature of the fiber with a microthermocouple.

Currently, the setup cools by as much as 13 K, but Rayner said that increasing the laser power could improve this by a factor of five. However, he explained, because the saturation of the ytterbium transition restricts the cooling density, there are limits on scaling up the Yb:ZBLAN system. He said that the use of different materials should improve the performance by more than a factor of 10.

Target: Cooling better than 50 K

Although other groups have demonstrated las-er-based solid-state coolers, Rayner said that this is the first commercially viable prototype. Nevertheless, its performance must be improved to better than 50 K if it is to compete with thermoelectric coolers.

He said that three obstacles to commercial laser coolers remain: identifying more efficient cooling materials; producing robust, inexpensive high-power laser sources with good beam quality; and removing or, preferably, recycling the anti-Stokes fluorescence to reduce its unwanted thermal effects. The group plans to focus on the last challenge.

Laser coolers would be valuable in several fields. Because they have no moving parts, they should be longer-lived and more reliable than mechanical coolers, making them ideal for spacecraft. They can quickly adjust their cooling power, which would be useful for high-temperature superconductors that are highly sensitive to changes in temperature. And they could be integrated into microchips, creating onboard coolers free of moving parts and electrical connections.