Near-infrared to visible upconversion in ${\mathrm{Er}}^{3+}$-doped ${\mathrm{Cs}}_3{\mathrm{Lu}}_2{\mathrm{Cl}}_9,$ ${\mathrm{Cs}}_3{\mathrm{Lu}}_2{\mathrm{Br}}_9,$ and ${\mathrm{Cs}}_3{\mathrm{Y}}_2{\mathrm{I}}_9$ excited at 1.54 μm

A detailed study of upconversion processes in ${\mathrm{Cs}}_3{\mathrm{Er}}_2{X}_9$ $(X=\mathrm{C}\mathrm{l},\mathrm{}\mathrm{B}\mathrm{r},\mathrm{}\mathrm{I})$ crystals and in the diluted systems ${\mathrm{Cs}}_3{\mathrm{Lu}}_2{\mathrm{Cl}}_9:1\%$ ${\mathrm{Er}}^{3+},$ ${\mathrm{Cs}}_3{\mathrm{Lu}}_2{\mathrm{Br}}_9:1\%$ ${\mathrm{Er}}^{3+},$ and ${\mathrm{Cs}}_3{\mathrm{Y}}_2{\mathrm{I}}_9:1\%$ ${\mathrm{Er}}^{3+}$ is presented. Efficient two-, three-, and four-step upconversion excitation along the sequence ${}^4{\overrightarrow{I_{15/2}}}^4{\overrightarrow{I_{13/2}}}^4{\overrightarrow{I_{9/2}}}^4{\overrightarrow{S_{3/2}}}^2{H}_{9/2}$ leading to luminescence throughout the visible and near UV is demonstrated using a 1.54-μm excitation wavelength. This stepwise excitation is possible due to the low phonon energies and, consequently, the significantly longer lifetime of the ${}^4{I}_{9/2}$ intermediate state in these systems relative to oxides and fluorides. The absorption and upconversion luminescence intensities increase along the isostructural series $X=\mathrm{Cl},$ Br, I as a result of the decreasing energy of the electric-dipole allowed $4f-5d$ transitions and, thus, their increasing influence on the parity forbidden $4f-4f$ transitions. The excitation mechanisms in the chloride systems are investigated by time-resolved spectroscopy and the respective dynamics is studied by a rate-equation model. In the diluted sample ${}^4{\overrightarrow{I_{9/2}}}^4{S}_{3/2}$ excited-state absorption plays a major role and occurs within 3 ${\mathrm{cm}}^{\mathrm{-}1}$ of the ground-state absorption, whereas the dynamics in the concentrated system is dominated by energy-transfer upconversion (ETU) in all excitation steps. Of the 35 most likely ETU processes, eight are found to contribute significantly to the excitation mechanisms in the concentrated system. The excitation pathways leading to red luminescence from ${}^4{F}_{9/2}$ are also partly resolved.