NIR and visible luminescence of Er3 +/Yb3+ co-doped fluorophosphate glasses with small additives of phosphates

A series of new fluorophosphate glasses co-doped with Er3+/Yb3+ions with composition 5 Ba(PO3)2–(90-x)(AlF3 –CaF2 –MgF2 –BaF2–SrF2)–5YbF3-xErF3, where x = 0, 0.1, 0.5, 1.0, 2.0, 3.0, 5.0 (mol.%) were developed. Impact of the Er3+ ions concentration on the luminescent properties was studied. Using Judd-Ofelt theory phenomenological intensity parameters Ωt (t = 2; 4; 6) were obtained. The stimulated emission cross-sections of 4I13/2→4I15/2 transitions were determined by the Fuchtbauer-Landenburg equation (FL) and the modified reciprocity method (MR). The upconversion spectra were measured as a function of the power density in the range 1.8–12.4 kW/cm2 and temperature in the range from 293 K to 693 K. It was found that the high intensity of UC luminescence of Er3+ ions and impurity Tm3+ ions occurs by the mechanism cooperative energy transfer (CET), which means the formation of excited clusters (dimers) of ytterbium. The sensitivity and temperature of the maximum sensitivity were evaluated by the fluorescence intensity ratio method from the measured up conversion spectra. The maximum value of absolute thermal sensitivity Smax of 40 × 10−4 K−1 at 693 K was obtained for the sample doped with 0.5 mol % of ErF3.