Site-selective yellow to violet and near-infrared to green upconversion in BaLu2F8:Nd3+

Single crystals of BaLu2F8 doped with 0.6% Nd3+ 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 F-4(5/2) and H-2(9/2) excited states around 800 nm. By means of time-resolved spectroscopy an excited-state absorption originating from the long-lived F-4(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 Nd3+ 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 BaLu2F8:0.6% Nd3+ and BaLu2F8:1% Er3+ with respect to the two crystallographically distinct rare-earth-dopant sites are discussed.