Upconversion properties of SrF2:Yb3+,Er3+ single crystals

Abstract

Synthesis of new upconversion (UC) materials that can convert near-infrared excitation light into visible emission light with high efficiency has crucial importance for energy and bio-applications. Herein, SrF₂ single crystals with a doping concentration of 2 mol% Er³⁺ and 2–8 mol% Yb³⁺ were synthesized and the optical properties were studied. The absorption cross-section of the different doping ions was calculated. To identify the most efficient and brightest material absolute excitation power dependent UC photoluminescence quantum yields (ϕ_UC) and brightness values were determined. In addition, excitation characteristics and luminescence lifetimes were analysed to understand the changes in population pathways and possible quenching mechanisms. A dominant two-photon population behaviour of the red and green emission bands was observed for all investigated doping concentrations of Yb³⁺ and Er³⁺. The ϕ_UC value of 6.5% measured using 230 W cm⁻² of 976 nm excitation for SrF₂ crystal co-doped with 2 mol% Er³⁺ and 3 mol% Yb³⁺. These findings broaden the scope of efficient UC materials apart from the record UC material β-NaYF₄:Yb³⁺, Er³⁺ and provide benchmark values for cubic-phase and especially SrF₂- nano and micrometer sized materials.