Upconversion Luminescence of Fluoride Phosphors SrF2:Er,Yb under Laser Excitation at 1.5 μm
Upconversion luminescence of fluoride phosphors SrF2:Er,Yb co-precipitated from aqueous solutions is studied. Under excitation of Er3+ ions to the 4I13/2 level, upconversion luminescence spectra are recorded in the visible and near-IR spectral regions, which correspond to the 4G11/2 → 4I15/2, 2H9/2 → 4I15/2, 4F5/2 → 4I15/2, 4F7/2 → 4I15/2, 2H11/2 → 4I15/2, 4S3/2 → 4I15/2, 4F9/2 → 4I15/2, 4I9/2 → 4I15/2, and 4I11/2 → 4I15/2, electronic transitions of Er3+ ions and to the 2F5/2 → 2F7/2 transition of Yb3+. The absolute quantum yields of visible upconversion luminescence under excitation in the region of 1.5 μm are determined for SrF2:Er,Yb phosphors with different concentrations of Er3t+ and Yb3+ ions and analyzed comparatively with the corresponding values for SrF2:Er phosphor. A study of chromaticity coordinates showed that the yellow-green upconversion luminescence of SrF2:Er,Yb phosphors is characterized by color temperatures of 3037–3526 K at an incident power density of 850 W/cm2.