We study new pumping and sensitization scheme of 3.9 mu m mid-IR laser transition (I-5(5) -> I-5(6) 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 H-3(4) -> H-3(6); I-5(8) -> I-5(5) 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 H-3(4) level of Tm3+ and the I-5(6) level of Ho3+ vs. acceptor concentration. We analyze the drawback of sensitization scheme, i.e. a self-quenching of Tm3+ excitation governing by the H-5(4) -> F-3(4); H-3(6) -> F-3(4) cross-relaxation. Nevertheless, this drawback compensates by fast depletion of the I-5(6) terminal laser level with energy transfer governing by the I-5(6) -> I-5(8); H-3(6) -> H-3(5) cross-relaxation. As a result 3.9 mu m 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 mu m laser transition of Tm3+ may cause cascade lasing in the mid-IR spectral range at two wavelengths of two different ions (Ho3+ and Tm3+). (c) 2009 Elsevier B.V. All rights reserved.
