Microscopic treatment of upconversion in Nd3+-doped samples

We deal with the analysis of fluorescence decay of metastable manifolds of rare-earth ions in the presence of upconversion processes, with attention focused on Nd3+-doped crystals. In the literature this phenomenon is usually studied by means of rate equations or microscopic treatment. Here we show that only the second approach is correct in our experimental conditions, i.e., when the population dynamics is fast in comparison with the typical migration time tau(0), and tau(0) is considerably longer than the radiative lifetime. We studied the population dynamics after pulsed-laser excitation of some Nd3+-doped crystals, namely, Bay(2)F(8):Nd3+ at 3.75%, LiYF4:Nd3+ at 1.05%, and KLa(MoO4)(2):Nd3+ at 5.3%. We observed that the rate-equation formalism cannot reproduce the experimental data, therefore we used a microscopic treatment that gave much better results. From this analysis, after reaching the saturation regime, we were able to determine the donor-acceptor transfer constant Cd. for the samples under investigation in an unconventional way. (C) 2003 Optical Society of America.