Cooperative Energy Transfer from ${\mathrm{Yb}}^{3+}$ to ${\mathrm{Tb}}^{3+}$ in Y${\mathrm{F}}_3$

The transfer of energy from $\mathrm{Yb}{}_{}{}^2{}_{}{}^{}F_{\frac{5}{2}}^{}{}_{}{}^{}$ to $\mathrm{Tb}{}_{}{}^5{}_{}{}^{}D_4^{}{}_{}{}^{}$ by the cooperative action of two ${\mathrm{Yb}}^{3+}$ ions has been observed in Y${\mathrm{F}}_3$. The correlation between the $\mathrm{Tb}{}_{}{}^5{}_{}{}^{}D_4^{}{}_{}{}^{}$ intensity and the $\mathrm{Yb}{}_{}{}^2{}_{}{}^{}F_{\frac{5}{2}}^{}{}_{}{}^{}$ and $\mathrm{Tb}{}_{}{}^5{}_{}{}^{}D_4^{}{}_{}{}^{}$ lifetimes as a function of Tb concentration supports the conclusion that cooperative transfer has taken place. At an exciting (9300 Å) intensity of 1 W ${\mathrm{cm}}^{-2\mathrm{}}$ the emitted $\mathrm{Tb}{}_{}{}^5{}_{}{}^{}D_4^{}{}_{}{}^{}$ (4900, 5460, 5850, and 6200 Å) power from a thin layer of powdered material was 0.8×${10}^{-6\mathrm{}}$ W ${\mathrm{cm}}^{-2\mathrm{}}$.