Effect of rare earth impurities on fluorescent cooling in ZBLAN glass

Anti-Stokes excitation of ytterbium-doped fluorozirconate glasses can result in a cooling effect. This process requires a maximum quantum efficiency for Yb3+ emission and therefore the smallest concentrations of impurities. In this respect, the effect of Er3+ and Tm3+ ions, which are often associated with Yb3+, is studied by evaluating the heating power generated by the excitation of these ions through energy transfers. For this purpose, glasses with composition (in mol%) 56−x% ZrF4, 28% BaF2, 4% AlF3, 7% NaF, 2.5% LaF3, 2.5% YbF3, x% ErF3, with x: 0, 0.02, 0.1, 0.5, were prepared by conventional methods. In the case of Er3+, the energy transfer rate populating the 4F7/2 state from Yb3+ 2F5/2 state by upconversion has first been experimentally determined. Then, using multiphonon non-radiative relaxation parameters and Judd–Ofelt theory, a complete rate equation model (up to Er3+ 4F7/2 state) has been solved for the Er3+–Yb3+ system. For an excitation at 1010 nm, the calculated ratio between cooling and heating powers shows that for <1017 ions/cm3, erbium has a negligible effect even at excitation power densities of 25 kW/cm2. For this ion, further purification of the glass should not be useful. At 1018 ions/cm3, the heating power due to erbium is still a few percent of the cooling power but the upconverted emission may be used to evaluate the glass temperature by measuring the ratio between 2H11/2→4I15/2 and 4S3/2→4I15/2 emission intensities. At larger concentrations, cooling is reduced. Energy transfer to Tm3+ ions, which is largely non-resonant, is estimated to be very small compared to other processes so that these ions have a negligible effect on cooling.