Spectroscopy and fluorescence dynamics of (Tm3+,Tb3+) and (Tm3+,Eu3+) doped LiYF4 single crystals for 1.5-μm laser operation

Fluorescence dynamics of the ³H₄ and ³ F₄ energy levels of Tm³⁺ ions in LiYF₄ single crystals codoped with Tb³⁺ or Eu³⁺ ions were experimentally investigated and confronted with classical energy-transfer models: direct cross-relaxation-type energy transfers between donor D and acceptor A, where D=Tm³⁺ and A=Tb³⁺ or Eu³⁺ (static Forster-Dexter model extended by Inokuti-Hirayama) as well as energy migration between Tm³⁺ ions (hopping and diffusion models by Burshtein and Yokota-Tanimoto). The fluorescence decays were accurately described in the case of Tb³⁺ codoping because of resonant energy transfer, thus of more precise estimations of energy transfer parameters from the absorption and emission spectra. In the case of Eu³⁺ codoping, the Tm³⁺→Eu³⁺ energy transfers need to be phonon-assisted to compensate for the reduced overlap between the Tm³⁺ emission and Eu³⁺ absorption spectra and the models, though giving satisfactory results, appeared less powerful. Based on these results, optimized values for the dopant concentrations were finally derived to keep high the fluorescence quantum efficiency of ³H₄ and to shorten the lifetime of the ³F₄ energy levels of the Tm³⁺ ion in order to alleviate the problem of self-termination of the ³H ₄→³F₄ laser transition occurring around 1.5 μm