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

Fluorescence dynamics of the H-3(4) and F-3(4) 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 H-4(4) and to shorten the lifetime of the F-3(4) energy levels of the Tm3+ ion in order to alleviate the problem of self-termination of the H-3(4) --> F-3(4) laser transition occuring around 1.5 mu m.