UP-CONVERSION-PUMPED LUMINESCENCE EFFICIENCY OF RARE-EARTH-DOPED HOSTS SENSITIZED WITH TRIVALENT YTTERBIUM

We discuss the upconversion luminescence efficiencies of phosphors tha t generate red, green, and blue light. The phosphors studied are singl e crystals and powders codoped with Er3+ and Yb3+ and with Tm3- and Yb 3+. The Yb ions are pumped near 980 nm; transfers of two or three quan ta to the codoped rare-earth ion generate visible luminescence. The ma in contribution is the quantitative measurement of this upconversion e fficiency, based on the use of a calibrated integrating sphere, determ ination of the fraction of pump light absorbed, and careful control of the pump-laser-beam profile. The green phosphors are the most efficie nt, yielding efficiency values as high as 4%, with the red and blue ma terials giving 1%-2%. Saturation was observed in all cases, suggesting that populations of upconversion steps of the ions are maximized at h igher power. Quasi-cw modeling of the intensity-dependent upconversion efficiency was attempted; input data included level lifetimes, transi tion cross sections, and cross-relaxation-rate coefficients. The satur ation of the Yb,Er:fluoride media is explained by the pumping of Er3ions into a bottleneck (long-lived state), that is, the I-4(13/2) meta stable level, making them unavailable for further excitation transfer. (C) 1998 Optical Society of America.