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Laser fridge comes of age

03 Jun 2005

Laser-powered cooling chambers that use light to remove heat could soon be a rival to conventional chillers. Oliver Graydon reports that the technology is maturing rapidly.

The development of compact laser-powered refrigerators that can cool to cryogenic temperatures has taken a leap forward, thanks to record-breaking research in the US. Scientists at Los Alamos National Laboratory in New Mexico have cooled a small bar of ytterbium-doped fluoride glass to a temperature of 208 K (-65 °C) using nothing but laser light - a new record for optical refrigeration.

In theory, by placing a small component, such as an infrared detector, in thermal contact with the glass, it too can be cooled. As a result, the technique could lead to a new breed of cooler that is cheap to make, powerful and highly reliable.

"Our goal is to develop laser-driven cryogenic refrigerators - rugged, all solid-state devices - that are compact and have no vibration," said Richard Epstein, leader of the Los Alamos research team. "The early applications for such laser refrigerators could be to cool infrared instruments on satellites, or high-temperature superconductor electronics, for example."

The principle of optical refrigeration is simple and is, effectively, a laser operated in reverse. A suitable material is pumped with laser light, which is absorbed and then emitted as fluorescence. However, if the energy of the fluorescence photons is slightly larger than the pump photons, there is a net energy loss. This is compensated for by the absorption of thermal energy (phonons) from the material, which gets colder.

"Our cooling fluid is light, which is used to suck entropy out of the glass," said Epstein. "You put a photon in and it gets absorbed and then re-emitted at a higher energy. Each time this happens, you remove heat that is equal to about 1% of the photon's energy."

For the scheme to work, it is important to use a material that has appropriately spaced energy levels. To date, the favourite choice is a fluoride glass called ZBLAN, which is doped with the rare earth element ytterbium.

The performance of the cooling is strongly dependent on the purity of the glass. Any impurities can potentially block the path of the "cooling" photons and cause the energy of the injected photon to be lost as heat instead. "If the excitation degrades as heat, then you get 100 times as much energy dumped into the glass than a single cooling-photon removes," said Epstein. "This is what we are fighting. If you do it at all wrong, then you get a heater, not a cooler."

The current Los Alamos prototype optical fridge consists of an 8 x 8 mm Yb-doped ZBLAN glass cylinder, housed in a matchbox-sized vacuum chamber. Both endfaces of the glass cylinder are coated with a dielectric mirror. One end features a 1 mm-diameter pinhole to admit the cooling laser beam.

When the cylinder was pumped with up to 11 W of 1.02 μm light from a diode-pumped Yb:YAG laser, the glass started to cool and, after about two hours, reached the record-breaking temperature of 208 K. However, Epstein is confident that lower temperatures can be reached by using purer glass.

"Theory suggests that, if we can decrease the impurities by a factor of between three and five, we should be able to get down to below 150 K," he told OLE. This would be an important achievement, as, according to Epstein, today's thermoelectric coolers that use the Peltier effect can't reach below 170 K. Instead, Stirling engines are needed to reach the low temperatures, but an optical cooler would have several advantages.

First, there is no vibration or electromagnetic noise, and coolers as small as 1 cm3 that feature laser-diode chips as a pump-source may be feasible. What's more, as there are no moving parts, the reliability of an optical cooler is limited purely by the lifetime of the pump laser, which could last tens of thousands of hours. In principle, the coolers should be easy and cost-effective to construct as there are no precise mechanical or electrical assemblies to worry about.

To push the technology further, the Los Alamos team is in the process of building its own $1 m (€0.8 m) glass-fabrication facility. This will allow it access to a regular supply of ultrapure glass. "It's hard to get samples with the reliability and purity we need. To motivate an external company to do this is more expensive than doing it ourselves," said Epstein. "There's a facility in Berne, Switzerland, that we've been using and we are going to copy the design of that. We should be up and running by November and producing glass that cools by the end of the year."

Theoretical calculations predict that temperatures as low as 50 K are achievable with ZBLAN glass. For even more demanding applications, it may be possible to get to 10 K by replacing the glass with a semiconductor sample with an appropriate energy-band structure. Neither the Los Alamos team nor anyone else in the field has yet managed to demonstrate cooling in semiconductors, but this is an active area of investigation.

Elsewhere in the US, Ball Aerospace has received financial support from NASA to further develop laser-cooling technology. The intention is to make compact, noise-free coolers in space applications, such as controlling the temperature of IR-detector arrays in scientific equipment.

To date, Ball has made a prototype optical refrigerator that is also based on Yb-doped ZBLAN glass. It succeeded in cooling a small load (an aluminium cylinder 10 mm in diameter, 6 mm in length and weighing 1.1 g) 15.6 °C below the temperature of its surroundings. The Yb-doped ZBLAN sample was pumped with 14 W of 1030 nm light from a Yb:YAG disc laser.

Ball believes that the result is highly significant and now has a roadmap to build smaller, more efficient coolers and test them in a variety of environments. "We have achieved a breakthrough in the proof of concept of an optical refrigerator in attaching the fluorescent element to a load," commented the firm in a presentation on the topic.

One thing is for sure, the work at Los Alamos and Ball Aerospace suggests that cryogenic laser coolers are not far away.

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