Energy transfer and upconversions in cubic ${\mathrm{Cs}}_2{\mathrm{NaYCl}}_6{:\mathrm{E}\mathrm{r}}^{3+}$ and ${\mathrm{Cs}}_2{\mathrm{NaErCl}}_6$

Upconversion by two-ion energy transfer, two-photon absorption (TPA), and stepwise absorption processes has been reported for ${\mathrm{Er}}^{3+}$ in cubic ${\mathrm{Cs}}_2{\mathrm{NaYCl}}_6$ crystal. Blue emission from the ${}^2{G}_{9/2}$ and green emission from the ${}^4{S}_{3/2}$ state have been observed under the pulsed excitation of ${}^4{F}_{9/2}$ in the red. Using a rate equation analysis of the time behavior of the upconverted signal, energy pathways have been determined. On the basis of the time evolution and the concentration dependence of the blue emission a two-ion energy-transfer process has been proposed. TPA in the ${}^2{H}_{9/2}$ state $(E>\mathrm{}36\mathrm{}000{\mathrm{cm}}^{\mathrm{-}1})$ is also observed. However, a resonant one-photon absorption in the ${}^4{S}_{3/2}$ state at low temperatures greatly complicates and modifies the TPA spectrum. Fluorescence and excitation spectra associated with ${}^2{H}_{9/2}$ and ${}^4{I}_{9/2}$ states, and fluorescence lifetimes of various $f^n$ states involved in upconversion processes are also reported.