Up conversion processes in yttrium-lithium-flouride crystals co-doped with erbium and ytterbium ions

We report on studies of the up-conversion process in YLiF 4 single crystals co-doped with Er 3+ and with Yb 3+ . Er 3+ has a well known complicated energy level system within the 4f shell which gives rise to the up- conversion process. Yb 3+ with a broad absorption band int eh regime 940 nm 3+ system thereby increasing the quantum efficiency of the up- conversion process considerably. We have performed steady state and time-resolved measurements of emission and absorption spectra as well as luminescence excitation spectra following either excitation with a light emitting diode, a laser diode or a pulsed dye laser. We have tried to trace some of the channels for energy transfer, relaxation and up-conversion and to optimize the system for an optimum up-conversion efficiency. At optimum conditions we can estimate quantum yields for up-conversion of RI to green light in the order of about 2.5 to 3 percent. Three-step up- conversion to UV emission is also observed. These observation suggest that YliF 4 single crystal co-doped with Er 3+ and with Yb 3+ may be considered as suitable candidates for diode-laser-pumped displays and for solid state laser system int eh visible and near UV region.