Low-phonon BaF 2: Ho 3+, Tm 3+ doped crystals for 3.5–4 μm lasing

Abstract

We study new pumping and sensitization scheme of 3.9 μm mid-IR laser transition ( 5I 5 → 5I 6 of Ho 3+) in the BaF 2 low-phonon crystal doped by Ho 3+ and co-doped by Tm 3+. It includes direct energy transfer from Tm 3+ to Ho 3+ governing by the 3H 4 → 3H 6; 5I 8 → 5I 5 cross-relaxation that gives ninefold increase of absorption of pumping light. We measure and analyze the lifetimes of all participated levels of Ho 3+ and Tm 3+ at low impurity concentrations, as well as energy transfer kinetics of the 3H 4 level of Tm 3+ and the 5I 6 level of Ho 3+ vs. acceptor concentration. We analyze the drawback of sensitization scheme, i.e. a self-quenching of Tm 3+ excitation governing by the 5H 4 → 3F 4; 3H 6 → 3F 4 cross-relaxation. Nevertheless, this drawback compensates by fast depletion of the 5I 6 terminal laser level with energy transfer governing by the 5I 6 → 5I 8; 3H 6 → 3H 5 cross-relaxation. As a result 3.9 μm laser transition of Ho 3+ in BaF 2: Ho 3+: Tm 3+ is not self-terminated. Moreover, efficient depletion of Ho 3+ terminal laser level with energy transfer to the initial level of 3.6 μm laser transition of Tm 3+ may cause cascade lasing in the mid-IR spectral range at two wavelengths of two different ions (Ho 3+ and Tm 3+).