Infrared Excitation Processes for the Visible Luminescence of ${\mathrm{Er}}^{3+}$, ${\mathrm{Ho}}^{3+}$, and ${\mathrm{Tm}}^{3+}$ in ${\mathrm{Yb}}^{3+}$-Sensitized Rare-Earth Trifluorides

Infrared reflectance and excitation measurements, together with studies of the dependence of the luminescence on excitation intensity and sensitizer concentration, in the fluorides of La, Gd, and Y doped with ${\mathrm{Yb}}^{3+}$ and ${\mathrm{Er}}^{3+}$, ${\mathrm{Ho}}^{3+}$, or ${\mathrm{Tm}}^{3+}$ indicate that infrared quantum counteraction is accomplished by absorption of energy in the ${\mathrm{Yb}}^{3+}$ ions followed by two (for ${\mathrm{Ho}}^{3+}$ or ${\mathrm{Er}}^{3+}$) or three (for ${\mathrm{Tm}}^{3+}$) transfers to the activator, resulting in production of strong visible luminescence from absorption in the ${\mathrm{Yb}}^{3+}\left({}_{}{}^2{}_{}{}^{}F_{\frac{7}{2^{-}}}^{}{}_{}{}^{}{}_{}{}^2{}_{}{}^{}F_{\frac{5}{2}}^{}{}_{}{}^{}\right)$ band. Saturation of the population of intermediate levels, in the case of ${\mathrm{Tm}}^{3+}$-activated materials, was observed and is believed to account for previous results which were interpreted as evidence of a so-called "cooperative sensitization," in which only two (simultaneous) transfers were required.