Oliver S. Wenger, Daniel R. Gamelin, Hans U. Güdel, Andrei V. Butashin and Alexander A. Kaminskii
Site-selective yellow to violet and near-infrared to green upconversion in BaLu₂F₈:Nd³⁺
Phys. Rev. B 61, 16530-16537 (2000)                 

Abstract: 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 ⁴G_5/2 and ²G_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 ⁴F_5/2 and ²H_9/2 excited states around 800 nm. By means of time-resolved spectroscopy an excited-state absorption originating from the long-lived ⁴F_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.