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

摘要

We discuss the upconversion luminescence efficiencies of phosphors that generate red, green, and blue light. The phosphors studied are single crystals and powders co-doped with Er(sup 3+) and Yb(sup 3+), and with Tm(sup 3+) and Yb(sup 3+). The Yb ions are pumped near 980 nm; transfers of two or three quanta to the co-doped rare earth ion generate visible luminescence. The main contribution embodied in this work 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 as the pumping of Er(sup 3+) ions into a bottleneck (long-lived state)- the (sup 4)I(sub 13/2) metastable level, making them unavailable for further excitation transfer. 32 refs., 5 figs., 3 tabs.