An up-conversion luminophore with high quantum yield and brightness based on BaF2:Yb3+,Er3+ single crystals

Abstract

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 BaF₂ single crystals with nominal concentrations of Yb³⁺ (2–15 mol%)/Er³⁺ (2 mol%) as well as Yb³⁺ (3 mol%)/Er³⁺ (2–15 mol%). The highest ϕ_UC value of 10.0% was demonstrated for the BaF₂:Er³⁺ (2 mol%) and Yb³⁺ (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 ⁴S_3/2 (ϕ_DS ≤ 4%) and ⁴F_9/2 (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 ⁴S_3/2 state for all investigated compositions. In addition, we observed an unusually strong contribution of the Er³⁺: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 BaF₂ crystals (240 cm⁻¹).