Upconversion luminescence of fluoride phosphors SrF$_{2}$ : Er, Yb under laser excitation at 1.5 $\mu$m

Upconversion luminescence of fluoride phosphors SrF$_{2}$ : Er, Yb co-precipitated from aqueous solutions is studied. Under excitation of Er$^{3+}$ ions to the ${}^{4}I_{13/2}$ level, upconversion luminescence spectra are recorded in the visible and near-IR spectral regions, which correspond to the ${}^{4}G_{11/2}\to{}^{4}I_{15/2}$, ${}^{2}H_{9/2}\to{}^{4}I_{15/2}$, ${}^{4}F_{5/2}\to{}^{4}I_{15/2}$, ${}^{4}F_{7/2}\to{}^{4}I_{15/2}$, ${}^{2}H_{11/2}\to^{4}I_{15/2}$, ${}^{4}S_{3/2}\to^{4}I_{15/2}$, ${}^{4}F_{9/2}\to{}^{4}I_{15/2}$, ${}^{4}I_{9/2}\to{}^{4}I_{15/2}$, and $^{4}I_{11/2}\to{}^{4}I_{15/2}$, electronic transitions of Er$^{3+}$ ions and to the ${}^{2}F_{5/2}\to{}^{2}F_{7/2}$ transition of Yb$^{3+}$. The absolute quantum yields of visible upconversion luminescence under excitation in the region of 1.5 $\mu$m are determined for SrF$_{2}$ : Er, Yb phosphors with different concentrations of Er$^{3+}$ and Yb$^{3+}$ ions and analyzed comparatively with the corresponding values for SrF$_{2}$ : Er phosphor. A study of chromaticity coordinates showed that the yellow-green upconversion luminescence of SrF$_{2}$ : Er, Yb phosphors is characterized by color temperatures of 3037–3526 K at an incident power density of 850 W/cm$^2$.