Dynamics of infrared-to-visible upconversion in ${\mathrm{Cs}}_3$${\mathrm{Lu}}_2$${\mathrm{Br}}_9$:1%${\mathrm{Er}}^{3+}$

Single crystals of ${\mathrm{Cs}}_3$ ${\mathrm{Lu}}_2$ ${\mathrm{Br}}_9$:1%${\mathrm{Er}}^{3+}$ were grown using the Bridgman technique. The low phonon energy of ≤190 ${\mathrm{cm}}^{\mathrm{-}1}$ strongly suppresses multiphonon relaxation in this host. For a series of excited states radiative relaxation is found to be dominant. As a consequence, ${\mathrm{Cs}}_3$ ${\mathrm{Lu}}_2$ ${\mathrm{Br}}_9$:1%${\mathrm{Er}}^{3+}$ shows strong visible upconversion luminescence from ${}_{}{}^2{}_{}{}^{}$ ${\mathit{H}}_{9/2}$ and ${}_{}{}^4{}_{}{}^{}$ ${\mathit{F}}_{7/2}$ when excited into the ${}_{}{}^4{}_{}{}^{}$ ${\mathit{I}}_{9/2}$ and ${}_{}{}^4{}_{}{}^{}$ ${\mathit{I}}_{11/2}$ multiplets in the near infrared, respectively. The upconversion dynamics for both excitations was studied in detail using time-resolved spectroscopy. Both excited-state absorption (ESA) and energy-transfer upconversion (ETU) were found to be active and could be identified on the basis of the upconversion transients. For the population of the ${}_{}{}^4{}_{}{}^{}$ ${\mathit{F}}_{7/2}$ and ${}_{}{}^4{}_{}{}^{}$ ${\mathit{S}}_{3/2}$ states both ESA and ETU can be observed simultaneously. The relative efficiency of ESA and ETU is determined by the energetic resonance of the respective processes. Whereas nonresonant ESA during the 10 ns laser pulse is strongly suppressed for energy mismatches >12 ${\mathrm{cm}}^{\mathrm{-}1}$, ETU is still efficient. An analysis of the upconversion transients shows that energy migration is dominant even at 1% ${\mathrm{Er}}^{3+}$ dotation. The excitations have excitonic character leading to a characteristic prolongation of the decays in the upconversion transients.