Observation of Anti-Stokes Fluorescence Cooling in Thulium-Doped Glass

We report the first observation of anti-Stokes fluorescence cooling in a thulium-doped solid with pump excitation at 1.82μm<λ<1.97μm. At a pump wavelength of 1.9μm and incident average power of ~3W, a Tm³⁺:ZBLANP ( ZrF₄-BaF₂-LaF₃-AlF₃-NaF-PbF₂) sample cooled to -1.2^∘C from room temperature for a single pass of the pump beam. This corresponds to an absorbed pump power of ~40mW and a peak temperature change per absorbed power of ~-30^∘C/W from room temperature.