Optical refrigeration of solids requires crystals with exceptional qualities. Crystals with external quantum efficiencies (EQE) larger than 99% and background absorptions of 4×10-4cm-1 have been cooled to cryogenic temperatures using non resonant cavities. Estimating the cooling efficiency requires accurate measurements of the above mentioned quantities. Here we discuss measurements of EQE and background absorption for two high quality Yb:YLF samples. For any given sample, to reach minimum achievable temperatures heat generated by fluorescence must be removed from the surrounding clamshell and more importantly, absorption of the laser light must be maximized. Since the absorption coefficient drops at lower temperatures the only option is to confine laser light in a cavity until almost 100% of the light is absorbed. This can be achieved by placing the crystal between a cylindrical and spherical mirror to form an astigmatic Herriott cell. In this geometry light enters through a hole in the middle of the spherical mirror and if the entrance angle is correct, it can make as many round trips as required to absorb all the light. At 120 K 60 passes and 150 passes at 100K ensures more than 95% absorption of the laser light. 5 and 10% Yb:YLF crystals placed in such a cell cool to sub 90K temperatures. Non-contact temperature measurements are more challenging for such a geometry. Reabsorption of fluorescence for each pass must be taken into account for accurate temperature measurements by differential luminescence thermometry (DLT). Alternatively, we used part of the spectrum that is not affected by reabsorption.
Aram Gragossian, Junwei Meng, Mohammadreza Ghasemkhani, Alexander R. Albrecht, Mauro Tonelli, and Mansoor Sheik-Bahae, "Enhanced cooling of Yb:YLF using astigmatic Herriott cell (Conference Presentation)," Proc. SPIE 10121, Optical and Electronic Cooling of Solids II, 1012101 (Presented at SPIE OPTO: February 01, 2017; Published: 20 April 2017); https://doi.org/10.1117/12.2254623.5397967385001.
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