Site-selective yellow to violet and near-infrared to green upconversion inBaLu2F8:Nd3+

Single crystals of ${\mathrm{BaLu}}_2{\mathrm{F}}_8$ doped with 0.6% ${\mathrm{Nd}}^{3+}$ were studied by high-resolution optical absorption, luminescence, and excitation spectroscopy in the 15–300 K temperature range. The two crystallographically distinct sites for the dopant ions present in this host are identified spectroscopically. Blue and violet upconversion luminescence was observed under pulsed excitation into the ${}^4{G}_{5/2}$ and ${}^2{G}_{7/2}$ multiplets in the yellow spectral range at all temperatures between 15 and 300 K. Green upconversion luminescence was obtained at 15 K after cw excitation into the ${}^4{F}_{5/2}$ and ${}^2{H}_{9/2}$ excited states around 800 nm. By means of time-resolved spectroscopy an excited-state absorption originating from the long-lived ${}^4{F}_{3/2}$ excited state was found to be responsible for yellow to violet upconversion. Near-infrared to green upconversion occurs via energy transfer involving two nearby excited ${\mathrm{Nd}}^{3+}$ ions doped into crystallographic sites of the same type. At 15 K both upconversion processes are site-selective, i.e., upconversion luminescence is observed exclusively from the initially excited site, and intersite energy transfer is only important above 30 K. The different photophysical properties of ${\mathrm{BaLu}}_2{\mathrm{F}}_8:0.6\%$ ${\mathrm{Nd}}^{3+}$ and ${\mathrm{BaLu}}_2{\mathrm{F}}_8:1\%$ ${\mathrm{Er}}^{3+}$ with respect to the two crystallographically distinct rare-earth-dopant sites are discussed.