Oxyfluoride glass-ceramics to be used for laser cooling were firstly proposed by Nemova and Kashyap [
14], and glass-ceramics containing PbF
2 nanocrystals were also investigated experimentally by members of our group through measuring the temperature changes as a function of the pump wavelength [
15]. The results showed that glass-ceramics have a higher cooling figure-of-merit than their glassy counterparts. The calorimetric results on the quantum efficiency of the samples are in line with the results obtained using an integrating sphere, showing an above 90% quantum efficiency for all the samples measured. It has also been shown for the first time, that an optically-cooled sample can be an accurate reference for quantum efficiency measurements. The low phonon energy of these materials, in addition to their low background absorption and high photoluminescence quantum yield (PLQY) makes them an ideal candidate for laser cooling applications [
16]. The optical and photoluminescence properties of oxyfluoride glasses and ultra-transparent glass-ceramics (SiO
2-Al
2O
3-CdF
2-PbF
2-YbF
3) were investigated in 2016 [
17]. All the samples showed the PLQY close to unity (100%). The detailed study on those samples with different Yb
3+ concentrations showed that the 2 mol% Yb
3+ doped oxyfluoride glass with its high PLQY, its low maximum phonon energy and low background absorption is the most promising candidate for laser cooling and solid-state laser applications besides serving as a reference to calibrate the instruments for PLQY measurements [
18]. Pump power dependence studies have revealed a linear increase in the PLQY and a decrease in the lifetime with increasing pump power. Also a decrease in lifetime was observed for the Yb
3+ excited level with increasing Yb
3+ concentration. Taking advantage of both the ability of the lead-cadmium oxyfluoride glass system to form transparent and durable nano glass-ceramics with promising optical properties, a new fiber material for laser cooling applications was proposed [
19]. Theoretical as well as experimental calculations showed that optical refrigeration would be achievable from these samples provided that 95% (segregation ratio) of Yb
3+ ions are incorporated into the fluoride nanocrystals.