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

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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 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 w& 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

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