The Impact of BiF3 Doping on the Yb3+ to Yb2+ Reduction during the LiYF4:Yb3+ Crystal-Growth Process

Here, we report on the opportunity to suppress Yb3+ to Yb2+ dopant-ion reduction in LiYbxY1−xF4 mixed crystals during growth processes, using the Bridgmen–Stocbarger technique in graphite crucibles in vacuum. This was carried out by the additional doping of the LiF-YF3-YbF3 powder mixture with 1% of BiF3 additive. The crystals of LiY0.8Yb0.2F4 and LiY0.8Yb0.2F4 with BiF3 doping in the charge, were grown. The spatial distribution of the spectral-kinetic properties of Yb3+ and Yb2+ ions along the grown crystalline-boules were studied. It was established that the Yb2+ concentration rises during the LiY0.8Yb0.2F4 crystal-growth processes without the BiF3 additive: the absorption coefficient of Yb2+ (π-polarization) at 340 nm rises from 0 (at the beginning of the boule) to 2.5 cm−1 (at the end of the boule). In contrast, the undetectable absorption of Yb2+ along the crystals grown from the BiF3 doped melt was displayed. The luminescence-decay time of Yb3+ decreases from 3.7 to 1.8 ms from the beginning to the end of the LiY0.8Yb0.2F4 boule grown from the BiF3 undoped melt, and stays constant (~3.7 ms) along the samples grown with BiF3. Here we demonstrate a positive effect of BiF3 doping on the optical homogeneity of LiYF4:Yb3+ crystals.