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 $1.82\mu \mathrm{m}<\lambda <1.97\mathrm{}\mu \mathrm{m}$. At a pump wavelength of $1.9\mu \mathrm{m}$ and incident average power of $\sim 3\mathrm{W}$, a ${\mathrm{Tm}}^{3+}$:ZBLANP ( ${\mathrm{ZrF}}_4-{\mathrm{BaF}}_2-{\mathrm{LaF}}_3-{\mathrm{AlF}}_3-\mathrm{NaF}-{\mathrm{PbF}}_2$) sample cooled to $-1.2\mathrm{}°\mathrm{C}$ from room temperature for a single pass of the pump beam. This corresponds to an absorbed pump power of $\sim 40\mathrm{mW}$ and a peak temperature change per absorbed power of $\sim -30\mathrm{}°\mathrm{C}/\mathrm{W}$ from room temperature.