A quantitative description of optical refrigeration in rare-earth doped solids in the presence of impurities is presented. The model includes the competition of radiative processes with energy migration, energy transfer to transition-metal ions, and multiphonon relaxation. The cooling efficiency is sensitive to the presence of both 3d metal ions with absorption in the near infrared and high-frequency vibrational impurities such as OH. A case study of ZBLAN:Yb3+ identifies Cu2+, Fe2+, Co2+, and OH as the most problematic species and establishes a 1-10 ppm upper limit for each of these impurities for a practical ZBLAN:Yb3+ optical cryocooler operating at
100-150 K to become feasible. The model results form the basis for an advanced strategy for the synthesis of high-purity ZBLAN:Yb3+ that exploits the potential of available purification techniques in an aqueous intermediate step. Such
high-performance ZBLAN:Yb3+ is expected to enable optical cryocoolers with ~1% overall efficiency at 120 K and find use in a wide range of applications that require highly reliable, noise-free, and vibration-free cooling of electronic and opto-electronic components.
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