Mechanisms of the energy transfer between thulium ions in tungstate and molybdate crystals

In this work, we investigated mechanisms of the energy transfer in Tm : KY(WO4)2, Tm : KLu(WO4)2 and Tm:NaBi(MoO4)2 crystals. Room-temperature absorption and emission spectra were used to determine microparameters of energy migration among thulium ions in the 3H4 and 3F4 excited states in the frames of Förster – Dexter theory. Parameters of cross-relaxation 3H4 + 3H6 → 3F4 + 3F4 and energy migration were obtained via analysis of luminescence decay 3H4 → 3F4 with a hopping model. The parameters describing excitation migration between thulium ions in 3H4 state obtained by two methods were in good agreement. It has been shown that the dipole-dipole mechanism of interaction is responsible for the efficient cross-relaxation process in the crystals under study. The results indicate that the energy migration between 3H4 enhances the cross-relaxation at thulium content more than ∼1.3–1.5 at. % in these laser materials. The obtained values of the migration parameters CDD exceed the values of the cross-relaxation parameters CDA, and the energy transfer in these materials can be described with the hopping model. An efficient cross-relaxation process leads to the relatively high efficiencies of the systems based on these crystals under pumping at 0.8 µm. The dominant process of energy migration between thulium ions in 3F4 excited state makes tungstate and molybdate crystals good candidates for the Ho3+ co-activation for laser generation at 2.1 µm. Parameters obtained in this study can be used for mathematical modeling of laser characteristics.