Fluorescence mechanisms in Tm3+ singly doped and Tm3+, Ho3+ doubly doped indium-based fluoride glasses.

The ${\mathrm{Tm}}^{3+}$\ensuremath{\rightarrow}${\mathrm{Tm}}^{3+}$ and ${\mathrm{Tm}}^{3+}$\ensuremath{\rightleftarrows}${\mathrm{Ho}}^{3+}$ energy transfers have been studied in both Tm-doped and (Tm,Ho)-doped indium-based fluoride glasses over a wide temperature range for several dopant concentrations. The regime of diffusion among the ${\mathrm{Tm}}^{3+}$ ions is clearly established and the cross-relaxation $^{3}$${\mathit{H}}_{4}$, $^{3}$${\mathit{H}}_{6}$${\ensuremath{\rightarrow}}^{3}$${\mathrm{F}}_{4}$${,}^{3}$${\mathit{F}}_{4}$ has been proved and its efficiency calculated. The temperature dependence of the thermal equilibrium observed between the lowest excited states $^{3}$${\mathit{F}}_{4}$ of ${\mathrm{Tm}}^{3+}$ and $^{5}$${\mathit{I}}_{7}$ of ${\mathrm{Ho}}^{3+}$ ions in codoped glasses is explained in terms of the Boltzmann distribution of the state populations governed by efficient forward and backward energy transfers. When ${\mathrm{Tm}}^{3+}$ ions are excited in the $^{3}$${\mathit{H}}_{4}$ level near 0.8 \ensuremath{\mu}m, it is shown that two cross-relaxation processes compete: one between ${\mathrm{Tm}}^{3+}$ ions and the other between ${\mathrm{Tm}}^{3+}$ and ${\mathrm{Ho}}^{3+}$ ions. Their efficiencies are shown to strongly depend on the relative Tm and Ho concentrations.