The World’s First All Solid State Cryo-Cooler

Optical refrigeration of rare-earth-doped solids is based on anti-Stokes fluorescence, where emitted photons have higher mean energy than the absorbed photons and approximately unity external quantum efficiency. The extra energy is supplied by absorption of thermal phonons of the solid, which leads to cooling. The past few years have witnessed tremendous progress in optical refrigeration, reaching temperatures as low as 90 K in Yb3+:YLF crystals. This milestone paves the way for the operation of compact, reliable, and vibration-free all-solid-state optical cryocoolers. Currently, only mechanical coolers (such as Stirling refrigerators) can reach cryogenic temperatures (<123 >K), all these mechanical coolers are involving mechanical compressors and cryogenic liquids like nitrogen or helium have the drawbacks of microphonic noise and limited lifetime due to mechanical wear. In our work, we demonstrate the world’s first all solid state vibration free optical cryo-cooler, by attaching the cooling crystal to a double 90-degree kink sapphire thermal link and a copper cold finger. With this design, we have cooled a silicon temperature sensor to 134.9 K. In the setup, the thermal link is bonded to the crystal by a thin layer of glue and the kinks with frosted arm help leak out the fluorescence, which prevent the fluorescence from reaching the cold finger to generate heat. A clamshell, coated with low emissivity material, encloses the cooling crystal, thermal link, and cold finger to efficiently absorb the emitted fluorescence from the crystal and reduce the radiative heat load. By using different clamshell and thermal link geometry designs, we have improved the efficiency of fluorescence absorption and rejection, with the help of optical simulation program, we have address the problem of light trapping.