Site-selective optical spectroscopy and upconversion mechanisms of the laser material BaLu2F8:Er3+

Crystals of BaLu2F8 doped with Er3+ 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 I-4(11/2) around 980 nm and into I-4(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 I-4(11/2) excitation, be identified on the basis of time-resolved measurements. At 10 K, the I-4(11/2) and the I-4(9/2) excited states shows completely different behavior: whereas with I-4(9/2) excitation the Er3+ "A" and "B" sites act independently of each other, a directional Er3+ 'B"-->"A" energy transfer is observed upon I-4(11/2) excitation, leading to predominantly Er3+ "A" ETU luminescence. In contrast, upconversion luminescence excited via the I-4(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. [S0163-1829(99)02028-7].