Energy transfer in samarium-doped calcium tungstate crystals

Results of an extensive investigation of the optical properties of CaW${\mathrm{O}}_4$: ${\mathrm{Sm}}^{3+}$ are reported. Absorption, fluorescence, and excitation spectra and pulsed fluorescence measurements were obtained at temperatures from below 8 K to room temperature on an undoped single crystal and crystals containing samarium concentrations ranging between 0.01% and 1.0%. An empirical energy-level diagram is determined for trivalent samarium in this host and the existance of several different nonequivalent crystal-field states is established. It is found that host-sensitized energy transfer to the samarium occurs in varying degrees from all four of the lowest excited states of calcium tungstate and the efficiency of transfer is different for different crystal-field sites. The ratios of the integrated fluorescence intensities and decay times were measured and from these the temperature and doping concentration dependences of the energy transfer rate were found for excitation in the different host excited states. A model is proposed to explain the results which is based on energy transfer from self-trapped excitons at low temperatures and thermally activated exciton hopping migration to activator-induced host traps at high temperatures. This model predicts the correct dependences for all of the observed data. Quantitative estimates indicate that exchange is somewhat stronger than dipole-dipole interaction for both migration and transfer and the exciton diffusion coefficient is found to be on the order of ${10}^{-7\mathrm{}}$ ${\mathrm{cm}}^2$ ${\sec }^{-1\mathrm{}}$.