NONLINEAR DYNAMICS OF COOPERATIVE UP-CONVERSION

We describe several new aspects of light-matter interactions for solid s in which interatomic coupling of impurity atoms plays a dominant rol e in population dynamics. We explore the implications of spatial coher ence in such multiatom interactions by introducing a density-matrix th eory of cooperative upconversion, focusing on pair systems for which a nalytic results can be obtained. We predict population pulsations in c oherent cooperative upconversion, enhanced quantum efficiency, enhance d energy transfer, and pair-mediated instabilities, not only in cooper ative upconversion media without extemal cavities but in upconversion lasers and conventional lasers in highly doped solids as well. These p redictions are compared with rate equation solutions and observations in lasers with inversions sustained by cooperative processes, particul arly the 2.8-mum Er laser. Rate equations fail to predict the observed steady-state instabilities of this laser, which are well reproduced b y density-matrix theory, furnishing evidence of weak coherent delocali zations in a rare-earth system.