An up-conversion luminophore with high quantum yield and brightness based on BaF2_{2}:Yb3+^{3+},Er3+^{3+} single crystals

Up-conversion (UC) of near-infrared radiation to visible light has received much attention because of its use in the conversion of solar radiation, luminescence thermometry, biosensing, and anti-counterfeiting applications. However, the main issue hindering the successful utilization of UC is the relatively low quantum efficiency of the process. In order to design new UC systems with high quantum yield (ϕ$_{UC}$) values, we synthesized two series of co-doped BaF$_{2}$ single crystals with nominal concentrations of Yb$^{3+}$ (2–15 mol%)/Er$^{3+}$ (2 mol%) as well as Yb$^{3+}$ (3 mol%)/Er$^{3+}$ (2–15 mol%). The highest ϕ$_{UC}$ value of 10.0% was demonstrated for the BaF$_{2}$:Er$^{3+}$ (2 mol%) and Yb$^{3+}$ (3 mol%) sample under 490 W cm$^{-2}$ of 976 nm excitation. To study the natural limit of UC efficiency, quantum yield values upon direct excitation (ϕ$_{DS}$) of the $^{4}$S$_{3/2}$ (ϕ$_{DS}$ ≤ 26%) levels were measured. Comparison of experimental values of quantum yields to the ones obtained using Judd–Ofelt theory reveals strong quenching of the $^{4}$S$_{3/2}$ state for all investigated compositions. In addition, we observed an unusually strong contribution of the Er$^{3+}$:4I$_{9/2}$ excited state to both UC and down-shifting luminescent processes. This contribution becomes possible due to the very low maximum phonon energy of BaF$_{2}$ crystals (240 cm$^{-1}$)