Energy transfer upconversion dynamics in YVO4:Yb3+,Er3+

A new approach to calculate the effective Ln-Ln energy transfer rates in a doped crystal matrices was proposed and successfully applied to the YVO4:Yb3+,Er3+- upconversion system. This approach is based on the known set of Ln-Ln distances obtained from the crystal structure of the matrix and the probabilities of their occurrence. The non-radiative energy transfer rates were calculated in the intermediate coupling scheme taking into account the shielding of the 4f electrons. The required partial (that is, forced electric dipole-only, lacking the dynamic coupling contribution) Judd-Ofelt Omega(lambda) parameters were calculated using the Simple Overlap Model (SOM). The spectral overlap factor F was estimated from optical absorption measurements. A simplified one level-one manifold energy level scheme was applied. The F-4(7/2), H-2(11/2) and S-4(3/2) states of Er3+ were treated as an effective level. The set of rate equations containing radiative, non-radiative and energy transfer processes was solved numerically using fourth-order Runge-Kutta method. Both continuous and pulsed excitation modes were simulated. The dependence of the upconversion intensity upon the pump power was found. The simulations resulted in good agreement with the experimental results and provided new insights into the dynamics of the upconversion process and a connection between the macroscopic properties and the microscopic description. (C) 2015 Elsevier B.V. All rights reserved.