Multiphonon relaxation of mid-IR transitions of rare-earth ions in the crystals with fluorite structure

Fluorescence kinetics decay of the 4I9/2 initial level for the 4.6 μm laser transition of erbium in the fluoride crystals with fluorite structure (CaF2, SrF2, CdF2, BaF2, and PbF2) with short phonon spectra was measured at room temperature and 77 K. It was found that in the row of these crystals doped with erbium, the governing factor that influenced multiphonon relaxation (MR) rates is the number of phonons n, which is determined by the frequency of effective longitudinal optical phonon. A decrease of effective phonon frequency raises the number of phonons n. This decreases the MR rate and increases the fluorescence lifetimes for PbF2 and BaF2. The latter fact could be favorable in laser generation in the 4–5 μm spectral region where multiphonon relaxation usually dominates. Influence of the unit dimension of crystal lattice (rare-earth ion to ligand distance R0) on the MR rate is discussed.