How researchers were able to use a laser to refrigerate liquids

Researchers from the University of Washington have figured out how to concentrate beams of laser light for the purpose of cooling liquids.

The study, which is published in the Proceedings of the National Academy of Sciences journal, details the use of an infrared laser to cool water by about 36 degrees Fahrenheit. 

“Typically, when you go to the movies and see Star Wars laser blasters, they heat things up. This is the first example of a laser beam that will refrigerate liquids like water under everyday conditions,” said senior author Peter Pauzauskie, UW assistant professor of materials science and engineering. “It was really an open question as to whether this could be done because normally water warms when illuminated.”

In terms of actual, real-world usage, this new solution could be used to “point cool” small areas with a tiny point of light. A good for-instance — microprocessors could benefit from this technology, wherein a laser beam is used to cool specific components in computer chips, to prevent the chip itself from overheating, and to also enable more efficient processing.

Furthermore, bio-engineers could use the laser beam to cool a portion of a cell as it divides or repairs itself. Doing this would slow down the rapid processes that occur when this sort of change takes place, and give researchers a better opportunity to understand how everything is working.

“There's a lot of interest in how cells divide and how molecules and enzymes function, and it's never been possible before to refrigerate them to study their properties,” said Pauzauskie. “Using laser cooling, it may be possible to prepare slow-motion movies of life in action. And the advantage is that you don't have to cool the entire cell, which could kill it or change its behavior.”

In terms of achieving the actual breakthrough, the team chose to go with an infrared light because they pinpointed biological applications as being the chief beneficiary of this technology (visible light is known to damage cells). Using a material often found in commercial lasers, they were able to essentially run the laser phenomenon backwards; that is, they illuminated a single microscopic crystal (the aforementioned “material”) suspended in water with infrared laser light. This excited a unique glow that has a bit more energy than the amount of light absorbed. And as the high-energy glow grew, it carried heat away from both the crystal and the surrounding water.  

Interesting solution, but there’s nothing new to it — yet. You see, this laser-refrigeration process was actually first demonstrated two decades ago at Los Alamos National Laboratory. The difference between then and now is the fact that when it was first demonstrated, it was done in vacuum conditions. Today’s breakthrough marks the first time it’s ever been achieved in liquids.

Now, while growing laser crystals is both time consuming and financially draining, the team was able to demonstrate a low-cost hydrothermal process that allows for the manufacturing of the crystal in a quicker, cheaper way which, in turn, makes it more scalable than current processes. 

As if that wasn’t already enough achievements to put into one paper, the team also created an instrument that uses a laser trap to literally “hold” the single nanocrystal surrounded by liquid in a chamber for the purpose of illuminating it with the infrared laser. As far as determining whether the liquid is cooling, this instrument projects the particle’s “shadow” in such a way so as to allow the researchers the ability to observe very small changes in its motion — as the surrounding liquid begins to cool, the trapped particle starts to slow down. This allows the team to very clearly observe and record the refrigerating effect. The crystal has also been designed to change from its blue-green coloring to a reddish-green as it cools, similar to a built-in color thermometer. 

“The real challenge of the project was building an instrument and devising a method capable of determining the temperature of these nanocrystals using signatures of the same light that was used to trap them,” said lead author Paden Roder.

To date, the team has only demonstrated the cooling effect using a single nanocrystal — exciting numerous ones would require a lot more laser power. Looking ahead, they hope to improve upon its efficiency, with the goal being to one day develop cooling technology that might be used to enable high-power lasers for manufacturing, telecommunications, and / or defense applications. 

“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,” he said. “We are interested in the ideas other scientists or businesses might have for how this might impact their basic research or bottom line.”

Via the University of Washington