| Theoretical and experimental research in cooling thulium-doped heavy metal fluoride glass is presented. The first observation of anti-Stokes fluorescence cooling in a thulium-doped material is reported. Cooling to 24 K below ambient with a multiple pump-pass scheme is attained, as well as results yielding ∼2.2 W of absorbed power and a cooling power of ∼73 mW. Single-pass cooling results agree with a simple model for anti-Stokes fluorescence cooling that includes considerations of quantum efficiency and parasitic heating mechanisms. Issues relating to a practical optical refrigerator are examined, including a general model for the effects of multiple pump passes. Spectroscopic studies of Tm3+:ZBLANP—including Fourier-transform infrared absorption, photoluminescence excitation, and fluorescence measurements—allow low-temperature cooling behavior to be predicted. Such modelling indicates the feasibility of cooling to below 150 K. |
