Pump saturation effect of the Er$^{3+}$/Yb$^{3+}$ co-doped Y$_{2}$Ti$_{2}$O$_{7}$ phosphors containing nanocrystals

Er$^{3+}$ and Yb$^{3+}$ co-doped Y$_{2}$Ti$_{2}$O$_{7}$ phosphors containing nanocrystals were prepared by sol-gel method and annealed at 800 $^{\circ}$C for 1 h in air. The X-ray diffraction (XRD) and transmission electron microscope (TEM) images showed that the mean size of nano-crystals is about 40-50nm, which corresponds well with the calculated results by Scherrer equation. Photoluminescence spectra and its upconversion properties were investigated. The anomalous slopes of the fitted line in the log-log plots for upconversion emissions and the pump-saturation effect of near infrared (NIR) emission were observed in the nanocrystalline samples. The reasons for the highly efficient upconversion luminescence and pump-saturation effect of NIR emission were discussed by a theoretical model of practical Er$^{3+}$/Yb$^{3+}$ co-doped system based on the rate equations.