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The mechanism of the Yb 3ϩ →Er 3ϩ energy transfer as a function of the donor and the acceptor concentration was investigated in Yb 3ϩ -Er 3ϩ codoped fluorozirconate glass. The luminescence decay curves were measured and analyzed by monitoring the Er 3ϩ ( 4 I 11/2 ) fluorescence induced by the Yb 3ϩ ( 2 F 5/2 ) excitation. The energy transfer microparameters were determined and used to estimate the Yb-Er transfer rate of an energy transfer process assisted by excitation migration among donors state ͑diffusion model͒. The experimental transfer rates were determined from the best fitting of the acceptor luminescence decay obtained using a theoretical approach analog to that one used in the Inokuti-Hirayama model for the donor luminescence decay. The obtained values of transfer parameter gamma ͓␥͑exp͔͒ were always higher than that predicted by the Inokuti-Hirayama model. Also, the experimental transfer rate, ␥ 2 (exp), was observed to be higher than the transfer rate predicted by the migration model. Assuming a random distribution among excited donors at the initial time (tϭ0) and that a fast excitation migration, which occurs in a very short time (t Ӷ␥ Ϫ2 ), reducing the mean distance between donor ͑excited͒ and acceptor, all the observed results could be explained.

The mechanism involved in the Tm 3ϩ ( 3 F 4 )→Ho 3ϩ ( 5 I 7 ) energy transfer and Tm 3ϩ ( 3 H 4 , 3 H 6 ) →Tm 3ϩ ( 3 F 4 , 3 F 4 ) cross relaxation as a function of the donor and acceptor concentrations was investigated in Tm-Ho-codoped fluorozirconate glasses. The experimental transfer rates were determined for the Tm→Ho energy transfer from the best fit of the acceptor luminescence decay using an expression which takes into account the Inokuti-Hirayama model and localized donor-to-acceptor interaction solution. The original acceptor solution derived from the InokutiHirayama model fits well the acceptor luminescence transient only for low-concentrated systems. The results showed that a fast excitation diffusion that occurs in a very short time (tӶ␥ Ϫ2 ) reduces the mean distance between an excited donor (D*) and the acceptor ͑A͒. A localized donor-to-acceptor interaction takes place, leading to an exponential decay of donors as an average of the microscopic rate equation solution of each D* -A pair separated by distance R that contributes in addition to the Inokuti-Hirayama solution. The observation that the experimental transfer rates were always much bigger than the one predicted by the diffusion model, in which the energy transfer process is assisted by excitation migration among donors state, reinforces the existence of a fast excitation diffusion among donor ions before the energy transfer to acceptor already observed in Yb:Er:ZBLAN. The fast excitation diffusion effect was observed to dominate both the Tm→Tm cross relaxation and Tm→Ho energy transfer ions from 3 H 4 and 3 F 4 thulium states, respectively.