Energy transfer processes in Yb3+ and Tm3+ ion-doped fluoride crystals

Energy transfer processes from ${\mathrm{Yb}}^{\text{3+}}$ to ${\mathrm{Tm}}^{\text{3+}}$ ions have been investigated in various fluoride microcrystals to examine the application limit of a simple rate equation model and to obtain methods for characterizing the host materials as to energy transfer. It has been shown that the energy transfer coefficients from donor ${\mathrm{Yb}}^{\text{3+}}$ to acceptor ${\mathrm{Tm}}^{\text{3+}}$ ions differ by nearly one order of magnitude even for fluoride lattices that have similar luminescence characteristics. The energy transfer coefficients increase monotonically with ${\mathrm{Yb}}^{\text{3+}}$ concentration at low ${\mathrm{Yb}}^{\text{3+}}$ concentrations, while they take nearly constant values at high ${\mathrm{Yb}}^{\text{3+}}$ concentrations. The present experimental results were compared with computer simulation results as well as related analyses. It is shown that the apparently complicated behaviors are consistently explained by a microscopic model. As an example of the application of the present results, the optimization of the infrared emission of ${\mathrm{Tm}}^{\text{3+}}$ ions is shown.