Upconversion-pumped luminescence efficiency of rare-earth-doped hosts sensitized with trivalent ytterbium

Abstract

We discuss the upconversion luminescence efficiencies of phosphors that generate red, green, and blue light. The phosphors studied are single crystals and powders codoped with ${\mathrm{Er}}^{3+}$ and ${\mathrm{Yb}}^{3+}$ and with ${\mathrm{Tm}}^{3+}$ and ${\mathrm{Yb}}^{3+}.$ The Yb ions are pumped near 980 nm; transfers of two or three quanta to the codoped rare-earth ion generate visible luminescence. The main contribution is the quantitative measurement of this upconversion efficiency, based on the use of a calibrated integrating sphere, determination of the fraction of pump light absorbed, and careful control of the pump-laser-beam profile. The green phosphors are the most efficient, yielding efficiency values as high as 4%, with the red and blue materials giving 1%–2%. Saturation was observed in all cases, suggesting that populations of upconversion steps of the ions are maximized at higher power. Quasi-cw modeling of the intensity-dependent upconversion efficiency was attempted; input data included level lifetimes, transition cross sections, and cross-relaxation-rate coefficients. The saturation of the Yb,Er:fluoride media is explained by the pumping of ${\mathrm{Er}}^{3+}$ ions into a bottleneck (long-lived state), that is, the ${}^4{I}_{13/2}$ metastable level, making them unavailable for further excitation transfer.