SATURATION BY RESONANT UP-CONVERSION IN TB-DOPED PHOSPHORS

We have investigated the influence of second order energy loss processes in Tb-doped phosphors on the saturation behavior under cathode ray excitation. A model has been developed assuming resonant electric dipole interaction between excited activators to be the dominant loss mechanism at excitation densities per pulse below 10(-2) J/cm2. The loss process encompasses up-conversion of 5D(J)-excitons (J = 3,4) into highly excited 4f- and 5d-states with subsequent nonradiative decay back to the 5D(J)-states. Experimentally, the absorption cross-sections between the excited states are accessed by a dedicated excited state absorption (ESA) technique for phosphor powders. The cross-sections for transitions from the excited 5D(J)-states to the lower 7F(J)-multiplet are obtained from calibrated emission spectra. It is concluded that the different saturation behavior of the investigated Tb-doped phosphors (YAG, YAGaG, LaOBr, and Y2SiO5) can be explained on the basis of our model.