Ytterbium-doped KY3F10 as a promising material for optical cryocoolers

Abstract

Various rare-earth doped solids can be cooled by anti-Stokes fluorescence, but only a few, particularly ytterbium (Yb)-doped LiYF4 (YLF), showed the potential to reach the cryogenic temperature regime (below 123 K). We propose to adopt cubic Yb-doped KY3F10 (Yb:KYF) for reaching sub-100 K cooling temperatures. The temperature-dependent spectroscopy of Yb:KYF and the comparison with Yb:YLF indicate its high potential to achieve lower cooling temperatures. The calculated figure-of-merit of laser cooling of Yb:KYF is higher than that of Yb:YLF by a factor of five at 100 K. This is because Yb:KYF has a significantly shorter mean fluorescence wavelength of 991 nm compared to the value of 1004 nm for Yb:YLF at 100 K. We grew Yb:KYF crystals by the Czochralski method with varied growth parameters, and experimentally compared their laser cooling performance with an Yb:YLF also grown at our institute. We observed efficient laser cooling in the Yb:KYF crystals at room temperature. Laser-induced thermal modulation spectroscopy tests determined their external quantum efficiencies to be higher than 98.5% and background absorption coefficients to be as low as 1.0•10-4 cm-1. The minimal achievable temperature (MAT) of our best Yb:KYF sample was calculated to be ≈90 K, attractive to be used in optical cryocoolers.