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  • Laser Cools Liquid for First Time

Photonics.com
Nov 2015
SEATTLE, Nov. 24, 2015 — A near-infrared laser has been used to cool water by more than 10 °C in what is believed to be the first instance of laser cooling in a liquid.

A team of researchers from the University of Washington used a tunable CW laser to trap an up-converting yttrium lithium fluoride nanocrystal suspended in water. Heat was transferred out of the crystal by blue-shifted anti-Stokes photons.

To determine whether the liquid was cooling, the instrument projected the particle's shadow in a way that allowed the researchers to observe minute changes in its motion, a method called cold Brownian motion analysis.

As they are cooled by a laser, nanocrystals emit a reddish-green glow that can be seen by the naked eye.
As they are cooled by a laser, nanocrystals emit a reddish-green glow that can be seen by the naked eye. Courtesy of Dennis Wise/University of Washington. 

As the surrounding liquid cooled, the trapped particle slowed down, allowing the team to clearly observe the refrigeration effect. They also designed the crystal to change from a bluish-green to a reddish-green color as it cooled, like a built-in color thermometer.

Laser refrigeration was first demonstrated in vacuum conditions at Los Alamos National Laboratory in New Mexico in 1995, but it has taken nearly 20 years to demonstrate the process in liquids.

So far, the team has only demonstrated the cooling effect with a single nanocrystal, as exciting multiple crystals would require more laser power. UW professor Peter Pauzauskie said the laser refrigeration process is energy intensive — irradiance on the order of 1 MC/cm2 was needed to trap one crystal — and future work will seek to improve its efficiency.

The researchers identified myriad potential applications. The cooling technology could help prevent overheating in higher-power lasers used for manufacturing, telecommunications or defense applications.

The discovery could also help industrial users point-cool tiny areas with focused light. Microprocessors, for instance, might someday use a laser beam to cool specific components in computer chips to prevent overheating and enable more efficient information processing.

"Few people have thought about how they could use this technology to solve problems because using lasers to refrigerate liquids hasn't been possible before," said former doctoral student Paden Roder, who now works at Intel Corp. "We are interested in the ideas other scientists or businesses might have for how this might impact their basic research or bottom line."

The research, published in the Proceedings of the National Academy of Sciences (doi: 10.1073/pnas.1510418112), was funded by the U.S. Air Force Office of Scientific Research, National Science Foundation, Lawrence Livermore National Laboratory and Pacific Northwest National Laboratory.


GLOSSARY
laser cooling
A process and method by which manipulation and orientation of a given number of directed laser beams decreases the motion of a group of atoms or molecules such that their internal thermodynamic temperatures reach near absolute zero.  The !997 Nobel Prize in Physics was awarded to Steven Chu, Claude Cohen-Tannoudji and William D. Phillips for the development of methods to cool and trap atoms with laser light. 
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