Time-resolved fluorescence-line narrowing and energy-transfer studies in a Eu3+-doped fluorophosphate glass

The optical properties of Eu3+-doped fluorophosphate glasses of composition (in mol%) 60 NaPO3-15BaF(2)-(25-x)YF3-xEuF(3) (x=0.5, 2, 5, 10, 15, 29, and 25) have been investigated in the 4.2-300 K temperature range by using optical absorption spectroscopy and time-resolved resonant laser-induced fluorescence line narrowing. From the room-temperature absorption spectra Judd-Ofelt parameters have been obtained and used to calculate the spontaneous emission probabilities from the D-5(0) state. The spectral features of the time-resolved fluorescence line-narrowed D-5(0)-->F-7(0,1) emission spectra under resonant excitation at different wavelengths along the F-7(0)-->D-5(0) transition as a function of concentration and temperature reveal the existence of energy migration between discrete regions of the inhomogeneous broadened spectral profile. From the concentration and time dependence of the average rate of excitation transfer, the electronic mechanism ruling the ion-ion interaction can be identified as a dipole-dipole energy transfer process. At low temperatures the average transfer rate parameter slightly depends on wavelength showing a temperature independent behavior. Above 77 K the weak dependence of the transfer rate on excitation wavelength (weak dependence on energy mismatch) together with its T-3 temperature dependence point to a transfer mechanism consistent with a two-site nonresonant two-phonon assisted process. The estimated average crystal field strength grows monotonically with the F-7(0)-->D-5(0) energy suggesting a large variation in the local environment of EU(3+) ions in these glasses. The slight increase with concentration of the D-5(0) fluorescence decays together with their single exponential character suggest that the transfer process may be fast enough to drive the system of excited centers to thermal equilibrium.