Anti-Stokes fluorescence cooling has been demonstrated in a number rare earth doped materials. Ytterbium doped
oxides and fluorides, such as ZBLAN, YLF, and YAG, were the first materials to exhibit cooling.1,2,3 These materials
were originally developed as laser gain media and fluorescence cooling was eventually incorporated into the 1μm lasers
to reduce detrimental thermal loading.4 Anti-Stokes cooling can offset quantum defect heating allowing laser power to be scaled to very high average powers.
Since the early work in ytterbium, fluorescence cooling has been demonstrated in both erbium and thulium doped
materials.5,6 These materials were also initially developed as lasing media and their fluorescence cooling could be used to increase laser powers at 1.5μm and 2.0μm. In this study we examine the radiative efficiency of holmium and ask the question, “Can anti-Stokes fluorescence cooling be extended beyond 2μm?”