The Optical Refrigerator Program at Los Alamos National Laboratory and the University of New Mexico is directed at developing the technology of optical refrigeration so that it can be deployed to cool radiation detectors and electronics to cryogenic temperatures in space and on Earth. Optical refrigeration is based on absorbing light on the low energy side of a material absorption spectrum and reemission at higher energies after equilibration with the thermal vibrations of the solid. Optical refrigerators, which have no moving parts or fluids, can have important advantages over existing technologies for small cryocoolers. Since these are compact solid-state devices, they are entirely free of vibrations. Additionally, since the cooling element is electrically isolated there is no electromagnetic interference. Optical refrigerators would be rugged, reliable, low-mass cryocoolers that would not adversely impact the electronic or mechanical component with which they are used. The improvements in materials and designs that we are investigating could make the efficiency and temperature range of optical refrigerators superior to those of small mechanical coolers.Summary of Research
Our team was the first to demonstrate laser cooling of a solid. In our first successful experiments, we cooled a small piece of ytterbium-doped glass by 0.3 C [1]+. We then constructed a prototype refrigerator that cooled a 6 g cylinder of this material by 48 C from room temperature with 25 mW of heat lift [15], and in following experiments we achieved 54 C of cooling (see first figure). We have designed and theoretically modeled coolers based on ytterbium-doped glass [10]. These coolers should be able to cool ~1 watt below 80 K with a net efficiency of ~ 3% (cooling/electrical power). In other experiments we verified that ytterbium-doped fluoride glass cools at temperatures as low as 100 K [8]. Since our initial demonstration of optical refrigeration, at least nine other research groups, including two in private industry, in five countries began working in this field.The performance of an optical refrigerator depends on the atomic structure and purity of the cooling material. We investigated a variety of ytterbium-doped materials that could be useful in optical refrigerators [11]. Trace elements, such as iron, copper and other rare-earth ions, quench ytterbium fluorescence and limit the refrigerator's performance. We used electrochemical purification to remove some impurities from the melts used for the glasses. By lowering the iron and copper contamination, we purified a material that initially had heated on optical pumping and turned it into one that cooled [4]; (these studies produced two Ph.D. theses in material sciences). Our subsequent experiments demonstrated optical refrigeration thulium-doped glass and for ytterbium-doped YAG crystals. Thulium-doped glass should achieve the same cooling at the ytterbium-doped glass with about half the laser power, thereby doubling the cooling efficiency [17, 20, 21]. The narrow, strong absorption lines in the ytterbium-doped crystal facilitate pumping and could allow cooling at lower temperatures [18]. We are currently investigating semiconductor-based optical refrigerators. These devices hold the promise of reaching 10 K in one stage from room temperature [19], and they would be much more compact than refrigerators that use rare-earth ions such as ytterbium and thulium (see second figure).
We were awarded the first U.S. patent for optical refrigeration [1]; this covers the general concept as well as several specific applications. Our second patent [2] covers improvements that can double the cooling efficiency of these devices. Our third patent [3] covers semiconductor-based optical refrigerators.+References refer to the items listed under Publications and Patents.