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US team is first to use laser to refrigerate water

17 November 2015

University of Washington (UW) researchers are the first to determine how to make a laser refrigerate water and other liquids under real-world conditions.

As they are cooled by the laser, the nanocrystals developed by the UW team emit a reddish-green 'glow' that can be seen by the naked eye (photo: Dennis Wise/University of Washington)

In a study published in the Proceedings of the National Academy of Sciences, the UW team used an infrared laser to cool water by about 36 degrees Fahrenheit — considered a major breakthrough in the field. The discovery could help industrial users 'point cool' tiny areas with a focused point of light.

The laser refrigeration process was first demonstrated in vacuum conditions at Los Alamos National Laboratory in 1995, but it has taken nearly 20 years to demonstrate this process in liquids.

The researchers suggest a variety of applications, including a laser beam to cool specific components in computer chips to prevent overheating, precise cooling a portion of a cell as it divides or repairs itself, or cooling a single neuron in a network — essentially silencing without damaging it — to see how its neighbours bypass it and rewire themselves.

The UW team chose infrared light for the cooling laser with biological applications in mind. They demonstrated that the laser could refrigerate saline solution and cell culture media that are commonly used in genetic and molecular research.

To achieve the breakthrough, the UW team used a material commonly found in commercial lasers but essentially ran the laser phenomenon in reverse. They illuminated a single microscopic crystal suspended in water with infrared laser light to excite a unique 'glow' that has slightly more energy than that amount of light absorbed. This higher-energy glow carries heat away from both the crystal and the water surrounding it.

Typically, growing laser crystals is an expensive, time-consuming process. The UW team demonstrated that a low-cost hydrothermal process can be used to manufacture a well-known laser crystal for laser refrigeration applications in a faster, inexpensive and scalable way.

The UW team also designed an instrument that uses a laser trap — akin to a microscopic tractor beam — to 'hold' a single nanocrystal surrounded by liquid in a chamber and illuminate it with the laser. To determine whether the liquid is cooling, the instrument also projects the particle’s 'shadow' in a way that allows the researchers to observe minute changes in its motion.

As the surrounding liquid cools, the trapped particle slows down, allowing the team to clearly observe the refrigerating effect. They also designed the crystal to change from a blueish-green to a reddish-green colour as it cools - creating a sort of 'chromatic thermometer'.

So far, the UW team has only demonstrated the cooling effect with a single nanocrystal, as exciting multiple crystals would require more laser power. The laser refrigeration process is currently quite energy intensive, and future steps will include looking for ways to improve its efficiency.

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