Physicists Reach 91 K with Laser Cooling

by Aaron Hilf, University of New Mexico

A group of scientists at the University of New Mexico’s (UNM) Department of Physics and Astronomy are using lasers to advance optical refrigeration, a technique that reaches cryogenic temperatures without any moving parts. Led by Dr. Mansoor Sheik-Bahae, professor of physics and astronomy, the group is using laser light to chill a special type of crystal that can then be attached to devices requiring constant and reliable cooling, like infrared detectors on satellites.

“Right now, anything that cools other parts of a system has moving parts. Most of the time, there’s liquid running through it that adds vibrations which can impact the precision or resolution of the device,” explains Aram Gragossian, a research assistant in Sheik-Bahae’s lab. “But, when you have optical refrigeration, you can go to low temperatures without any vibrations and without any moving parts, making it convenient for a lot of applications.”

Earlier this year, Sheik-Bahae, along with collaborators at UNM and Los Alamos National Labs, reached the lowest temperatures ever recorded using an all-solid-state cryocooler—91 K or -296°F—a temperature that was previously only reached using liquid nitrogen or helium. The research was published in the journal (Nature) Scientific Reports.

“Here at UNM, we are the only group in the world that’s been able to cool to cryogenic temperatures with an all-solid-state optical cryocooler,” says Alexander Albrecht, one of the paper’s coauthors and research assistant professor at UNM.

“We are really on the cutting edge when it comes to solid-state laser cooling,” says Sheik-Bahae. “While achieving major milestones in the fundamental science aspect of this field, in parallel, we are making rapid advances in implementing this technology for real world applications. For example, in partnership with a NM startup (founded by one of the pioneers in the field, Richard Epstein), we are developing the world’s first all-solid-state cryocooler device.”

This latest achievement is the product of more than 20 years of work by researchers at UNM, the University of Pisa in Italy and New Mexico’s national laboratories. In 1995, researchers at Los Alamos accomplished cooling of about one degree. Since then, through vast improvements in the purity of synthetic crystals containing Ytterbium ions and the lasers being shot at them, UNM scientists have been able to continue cooling to colder and colder temperatures.

“We were able to identify what kinds of crystals can be used for this. And, in collaboration with Los Alamos National Labs and a few other universities, we finally found crystals that can cool to these extreme temperatures,” says Gragossian.

Moving forward, Sheik-Bahae and his team hope to continue working on the cutting edge of this technology to achieve even colder temperatures, something they believe will have a major impact in a variety of industries and applications.

One function of solid-state cryocoolers is to cool infrared focal plane arrays (cameras) that can be used for a variety of applications and are even being utilized to detect skin cancer in patients. The detectors must be able to read miniscule changes between healthy areas of skin and diseased areas, so having a cooling system that does not generate vibrations could be extremely useful.

Another application, currently in development from scientists at the National Institute of Standards and Technology (NIST) in Colorado, is cooling of delicate “reference cavities” for achieving super-stable lasers that will be used as high precision clocks in a myriad of metrology applications.