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Effect of impurities on cooling efficiency in fluoride crystals

We present a characterization of the optical refrigeration properties of two 5% Ytterbium-doped YLF crystals. Measurements showed different cooling efficiencies for the samples, despite the same concentration of dopant. We carried out a spectroscopic study on the crystals devoted to the identification of foreign contaminants inside them. These searches determined the presence of Erbium and Holmium impurities in both the cooling samples. We attributed the reduction of the cooling efficiency in one of the crystals to an increased amount of these contaminants.