Low-phonon BaF2: Ho3+, Tm3+ doped crystals for 3.5-4 μm lasing

We study new pumping and sensitization scheme of 3.9 μm mid-IR laser transition (5I5 → 5I6 of Ho3+) in the BaF2 low-phonon crystal doped by Ho3+ and co-doped by Tm3+. It includes direct energy transfer from Tm3+ to Ho3+ governing by the 3H4 → 3H6; 5I8 → 5I5 cross-relaxation that gives ninefold increase of absorption of pumping light. We measure and analyze the lifetimes of all participated levels of Ho3+ and Tm3+ at low impurity concentrations, as well as energy transfer kinetics of the 3H4 level of Tm3+ and the 5I6 level of Ho3+ vs. acceptor concentration. We analyze the drawback of sensitization scheme, i.e. a self-quenching of Tm3+ excitation governing by the 5H4 → 3F4; 3H6 → 3F4 cross-relaxation. Nevertheless, this drawback compensates by fast depletion of the 5I6 terminal laser level with energy transfer governing by the 5I6 → 5I8; 3H6 → 3H5 cross-relaxation. As a result 3.9 μm laser transition of Ho3+ in BaF2: Ho3+: Tm3+ is not self-terminated. Moreover, efficient depletion of Ho3+ terminal laser level with energy transfer to the initial level of 3.6 μm laser transition of Tm3+ may cause cascade lasing in the mid-IR spectral range at two wavelengths of two different ions (Ho3+ and Tm3+).