Spectroscopy and high efficiency laser operation of high purity Yb3+-doped Lu2O3 grown by the heat exchanger method

This work presents a clear improvement of the crystal growth by the heat exchanger method (HEM) leading to longer fluorescence lifetime and higher laser efficiency of this material. Polycrystalline boules of 40 mm diameter and 25 mm length with monocrystalline regions of up 5 cm³ have been grown with 6N LU₂O₃ and 5N Yb₂O₃ in a 40 mm diameter high purity Re-crucible under H₂/N₂ atmosphere. The fluorescence lifetime of the obtained crystals has been determined with the pinhole-method to exclude radiation trapping. In contrast to previous growth experiments no concentration quenching occurred up to the highest grown doping concentration of 5%. Additionally, the ratio of parasitic visible green fluorescence of Er³⁺ to the infrared emission was reduced by a factor of 7 compared to the crystals grown in previous experiments, which is a further indicator for less rare-earth impurities. An increase of more than 30% in cw laser efficiency of Yb:Lu₂O₃ promises likewise increased efficiencies in thin disk laser experiments planned for the near future.