Measurement of solid-state optical refrigeration by two-band differential luminescence thermometry

We present a non-contact optical technique for the measurement of laser-induced temperature changes in sol-ids. Two-band differential luminescence thermometry (TBDLT) achieves a sensitivity of 7 mK and enables a precise measurement of the net quantum efficiency of optical refrigerator materials. The TBDLT detects in-ternal temperature changes by decoupling surface and bulk heating effects via time-resolved luminescence spectroscopy. Several Yb3+-doped fluorozirconate (ZrF4–BaF2–LaF3–AlF3–NaF–InF3, ZBLANI) glasses fabri-cated from precursors of varying purity and by different processes are analyzed in detail. A net quantum effi-ciency of 97.39±0.01 % at 238 K (at a pump wavelength of 1020.5 nm) is found for a ZBLANI:1 % Yb3+ laser-cooling sample produced from metal fluoride precursors that were purified by chelate-assisted solvent extraction and dried in hydrofluoric gas. In comparison, a ZBLANI:1 % Yb3+ sample produced from commercial-grade metal fluoride precursors showed pronounced laser-induced heating that is indicative of a substantially higher impurity concentration. The TBDLT enables rapid and sensitive benchmarking of laser-cooling mate-rials and provides critical feedback to the development and optimization of high-performance optical cryocoole