Solid-state laser cooling in Yb:CaF2 and Yb:SrF2 by anti-Stokes fluorescence

We report on the first example, to the best of our knowledge, of solid-state laser cooling in ytterbium-doped CaF2 and SrF2 crystals by anti-Stokes fluorescence. The crystals were grown by the Czochralski method in a fluorine-rich atmosphere to prevent the formation of divalent ytterbium ions. Using laser-induced thermal modulation spectroscopy (LITMoS), we find the cooling efficiencies for both crystals to be higher than 3% at room temperature. According to model calculations performed using temperature-dependent spectroscopic data, these crystals can be cooled to temperatures as low as 150 K when excited at around 1030 nm.