The upconversion fluorescence spectra of Er<sup>3+</sup>/Yb<sup>3+</sup>: PLZT excited at 980 nm were measured from 10K to 320K. A
model for the dynamics of frequency upconversion process was proposed. Based on this model, the upconversion
luminescence intensities were fitted as a function of temperature and the temperature characteristics of upconversion
luminescence were discussed.
The absorption spectrum, upconversion spectrum and the fluorescence decay curve of Er<sup>3+</sup>/Yb<sup>3+</sup> co-doped KY(WO<sub>4</sub>)<sub>2</sub>
crystal were measured. The radiative transition rates were calculated by Judd-Ofelt theory. A model for the dynamics of
frequency upconversion process in Er<sup>3+</sup>/Yb<sup>3+</sup> co-doped KY(WO4)2 crystal was proposed. The Yb-to-Er energy transfer
rate and the upconversion coefficients were estimated by numerically solving the rate equations and fitting simulated
curve to the experimental data.