Oliver S. Wenger, Daniel R. Gamelin, Hans U. Güdel, A.V. Butashin and Alexander A. Kaminskii
Site-selective optical spectroscopy and upconversion mechanisms of the new laser material BaLu₂F₈:Er³⁺
Phys. Rev. B 60, 5312-5320 (1999)                 

Abstract: Crystals of BaLu₂F₈ doped with Er^{3 +} 1.0 and 4.5 % were studied by high-resolution optical absorption, luminescence, and excitation spectroscopy in the temperature range 10-300 K with particular focus on the role of the two crystallographically different sites for the dopant ions in this host, referred to here as Er³⁺ "A" and "B." Efficient green upconversion luminescence was observed at all temperatures between 10 and 300 K under both excitation into ⁴I_11/2 around 980 nm and into ⁴I_9/2 around 800 nm. For both excitation wavelengths excited state absorption (ESA) and energy transfer upconversion (ETU) were found to be active and could, in the case of ⁴I_11/2 excitation, be identified on the basis of time-resolved measurements. At 10 K, the ⁴I_11/2 and the ⁴I_9/2 excited states shows completely different behavior: whereas with ⁴I_9/2 excitation the Er³⁺ "A" and "B" sites act independently of each other, a directional Er^{3 +} "B" "A" energy transfer is observed upon ⁴I_11/2 excitation, leading to predominantly Er³⁺ "A" ETU luminescence. In contrast, upconversion luminescence excited via the ⁴I_11/2-ESA mechanism mainly occurs from Er³⁺ site "B." Measured oscillator strengths of the observed f-f transitions are modeled using a Judd-Ofelt calculation, from which relevant ESA oscillator strengths are estimated.