Multiphonon and energy transfer relaxation in charge compensated crystals

Abstract

Alkaline earth fluoride crystals doped with trivalent rare earth contain both isolated and clustered rare earths. Electronic nonradiative relaxation for isolated rare earths proceeds solely by multiphonon relaxation while for clustered rare earths, it proceeds by both efficient nonresonant energy transfer and multiphonon relaxation. We have measured the rates of all the relaxation processes for representative sites in CaF2, SrF2, and BaF2 in order to obtain a comprehensive understanding of relaxation in the materials. Exponential dependences of the multiphonon and energy transfer rates on energy gap have been found. The rates do not appear to scale simply with the lattice phonon energies but depend upon the details of the phonon distribution and coupling. Temperature dependence of the rates indicate a complex mixture of phonons may contribute to the observed relaxation.