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Optical refrigeration breaks record

15 Apr 2005

A prototype laser fridge cools to a record low temperature of 208 K.

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 (Applied Physics Letters 86 154107).

“Our goal is to develop laser-driven cryogenic refrigerators -- rugged, all solid-state devices which would be 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 IR [infrared] instruments on satellites or high-temperature superconductor electronics, for example.”

The principle of optical refrigeration is beautifully simple. 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 then there is a net energy loss. This is compensated by the absorption of thermal energy from the material and it gets colder.

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

The Los Alamos prototype optical fridge consists of a 8 mm long, 8 mm diameter cylinder of Yb-doped ZBLAN glass which is housed in a matchbox sized vacuum chamber. Both end-faces of the glass cylinder are coated with a dielectric mirror and one features a 1 mm diameter pinhole to admit the cooling laser beam.

When the team pumped it 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 2 h reached the record-breaking temperature of 208 K.

What’s more, Epstein is confident that even lower temperatures could be possible. “With the ytterbium-doped materials that we are using, I expect that we will be able to get below 100 K and theoretically 50 K is possible,” he told Optics.org. “The cooling of the rare-earth doped materials is limited by their purity [impurities quench the fluorescence]. We are setting up a glass-production facility to make ultrapure glass that should cool much better.”

Oliver Graydon is editor of Optics.org and Opto & Laser Europe magazine.

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