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 Er3+ and Yb3+ and with Tm3+ and Yb3+. 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 Er3+ ions into a bottleneck (long-lived state), that is, the 4I13/2 metastable level, making them unavailable for further excitation transfer.
© 1998 Optical Society of America
Ralph H. Page, Kathleen I. Schaffers, Phillip A. Waide, John B. Tassano, Stephen A. Payne, William F. Krupke, and William K. Bischel, "Upconversion-pumped luminescence efficiency of rare-earth-doped hosts sensitized with trivalent ytterbium," J. Opt. Soc. Am. B 15, 996-1008 (1998)