Energy Transfer Under Strong Pumping in High-Concentration Rare Earth Doped Laser Crystals

The weak interaction between isolated Er ions, and between Er and Pr ions, has been quantitatively measured in the host crystal BaY_2F _8. Fluorescent decay data from samples doped with 1%-100% Er and 0.05%-0.5% Pr show the decay rate is linearly proportional to the acceptor (Pr) concentration for all samples, and linearly proportional to the donor (Er) concentration for 5% Er or less, as predicted by the standard dipole-dipole coupling model. For Er concentrations greater than 20%, the decay has a much stronger dependence on the Er concentration, indicative of a strong short range coupling mechanism (higher order multipole or superexchange) operating between nearest neighbor ions. The dipole-dipole interaction strengths for Er-Er and Er-Pr transfer have been measured for the ^4{rm I }_{13/2}, ^4 {rm I}_{11/2}, and ^4{rm F}_{9/2 }<=vels, and the exciton diffusion rate is calculated as a function of the Er concentration. The upconversion interaction rate between excited Er ions has also been determined for 5%-100% Er:BaY _2F_8. The results show behavior similar to previous measurements of upconversion in Er:LiYF_4: the ^4 {rm I}_{11/2} upconversion strength increases with Er concentration, and the ^4{rm I}_{13/2} upconversion strength saturates at roughly 20% Er. The measured ^4{rm I} _{11/2} upconversion strengths have the same Er dependence as the Er-Pr transfer, indicating that the increase at high concentrations is due to the same short range interaction mechanism. The ^4 {rm I}_{13/2} saturation is thought to be due to a bottleneck for Er-Er transfer due to the poor overlap of the Er ^4{ rm I}_{13/2} emission and excited state absorption. The ratio of ^4 {rm I}_{13/2} to ^4{rm I}_{11/2 } upconversion strengths is greater in BaY _2F_8, indicating this may be a better host for an upconversion laser. Attempts to confirm this by directly measuring the laser slope efficiency for different crystals in the same laser configuration, indicate that BaY_2F_8 may be a better host, but the maximum slope efficiency was much less than expected for efficient recycling of energy by ^4{rm I}_ {13/2} upconversion. (Copies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617-253-5668; Fax 617-253-1690.).