Calcium difluoride (CaF2) is an industrially important material for high power optical devices due to its high melting point, stoichiometric composition, low background absorption coefficient, and large optical transparency window. Solid state laser refrigeration has recently been reported with single crystals of cubic CaF2 (fluorite crystal structure, space group Fm-3m) doped with trivalent ytterbium ions. In this presentation we present the recent design, hydrothermal synthesis, and characterization of ytterbium-doped CaF2 microspheres with diameters ranging from 500 nm to 4 micrometers. Raman spectroscopy reveals the presence of point defects that compensate for trivalent ytterbium ions that reside at divalent calcium ion sites in the crystal lattice. Near-infrared optical luminescence spectroscopy is used to demonstrate that CaF2 microspheres undergo solid state laser refrigeration under ambient conditions. These materials are anticipated to have significant applications in experimental single molecule biophysics, radiation-balanced microlasing, and the future development of optomechanical quantum sensors.
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