Transfer and back transfer processes in Yb3+-Er3+ codoped fluoroindate glasses

The kinetics of forward and backward energy transfer processes in fluoroindate glasses double doped with Yb3+ and Er3+ has been studied. It was possible to excite selectively Yb3+ ions and detect emission due only to these ions or combined with the emission coming from the Er3+ ions. At low concentration of Yb3+ (0.1 mol %) the emission decay of these ions is nonexponential when there is back transfer from Er3+ ions; the dynamics is well described using the fluorescence "transfer function" model. Moreover, the evolution of the emission from the I-4(11/2) (Er3+) level is in good agreement with the behavior predicted by the model. The critical radii obtained from the fitting for forward and backward energy transfer are compared with those calculated with the Dexter formula. When the Yb3+ concentration is increased, the migration among these ions is very important and the emission decay of the Yb3+ ions is nearly exponential with the lifetime of these ions. This behavior is explained considering that, at high Yb3+ concentration and due to the migration, the transfer from Yb3+ to Er3+ ions is restricted to very closed couples of ions with high back transfer probability. The limitations of the fluorescence "transfer function" model associated with the correlation effect in the excitation of Er3+ ions by energy transfer are discussed. (C) 1999 American Institute of Physics. [S0021-8979(99)00814-2].