The Impact of BiF3 Doping on the Yb3+ to Yb2+ Reduction during the LiYE4: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+ (pi-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 (similar to 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.