University of New Mexico researchers have surpassed themselves in a laser-based cooling project. Professor Mansoor Sheik-Bahae (et al.) and other researchers from the University of Pisa, Italy and the Los Alamos Institute created the world’s first all-solid-state cryocooler, that can be used from cooling infrared sensors to superconductors.
The method is also known as “optical refrigeration” – the process of lowering a solid’s temperature by shining a laser light on it. The heat is dissipated from the material via the fluorescence that follows the laser absorption.
Laser cooling is not a new idea. Back in 1995, Los Alamos scientists succeeded obtaining a 1 degree cooling of a solid, and then a -65°C, starting from room temperature. Then, the usage of extremely pure materials and the learning of the physics of luminescent crystals led the scientists to build a system that can go even below what is possible with classic thermoelectric (Peltier) coolers.
“We obtained cooling down to 155 Kelvin (equivalent to -118 C or -180 F) from room temperature using optical refrigeration in a crystal containing Ytterbium ions. Ytterbium is an element from a group in the period table known as the rare-earths that are extremely efficient in their fluorescence, an essential requirement for optical refrigeration” said Sheik-Bahae. “We expect that material research may soon lead to temperatures dipping below 77K, the boiling point of liquid nitrogen, and in the future, maybe as low as 10K will be possible.”
To achieve their results, the scientists enhanced cooling efficiency by exploiting resonances in the absorption spectrum, growing pure crystals, delicate thermal load management and by trapping laser light in an optical cavity.
“We tune high power lasers to excite sharp resonances of Ytterbium ions sitting in a fluoride crystalline host,” explained Denis Seletskiy, one of the graduate students that performed the experiments at UNM. “We trap laser light by careful alignment of the optical cavity mirrors inside of a high vacuum chamber. A specially designed and coated sample chamber allows us to minimize parasitic heat load from the environment.
“We infer crystal temperature using a technique we developed that allows to measure temperature without making a contact with the sample, further avoiding unnecessary parasitic heat load on the sample. Combination of all of these ideas and tricks has allowed us to reach 155K, breaking ‘Peltier barrier.’
“We’ve set the bar high or low in this case,” said Sheik-Bahae. “We feel 100K is within reach and also 77K, the melting point of liquid nitrogen. In the end, it is primarily materials science that is allowing this breakthrough. Reaching those temperatures is achievable using high purity crystals.”
The team will continue their collaboration with researchers from the University of Pisa, Italy, and will further develop small-scale cryocoolers that will definitely help superconductors get in touch with real-life applications and improve the energy efficiency of theoretically any heat-wasting device, from your phone to the grid lines, where needed.
The red line that needs to be drawn from these researchers’ experiments is that they succeeded breaking the -170°C barrier, thus opening new horizons for the above-mentioned superconductor science.