Optical Properties of ${\mathrm{Yb}}^{3+}$ and ${\mathrm{Nd}}^{3+}$-${\mathrm{Yb}}^{3+}$ Energy Transfer in YAl${\mathrm{O}}_3$

The optical absorption and emission spectra, $4f$ energy levels, fluorescence lifetime, and quantum efficiency of ${\mathrm{Yb}}^{3+}$ ions in YAl${\mathrm{O}}_3$ are reported. Fluorescence kinetics and ${\mathrm{Nd}}^{3+}\to {\mathrm{Yb}}^{3+}$ energy transfer were studied as a function of temperature in crystals codoped with ${\mathrm{Yb}}^{3+}$ and ${\mathrm{Nd}}^{3+}$. Back transfer from ${\mathrm{Yb}}^{3+}$ to ${\mathrm{Nd}}^{3+}$ becomes important for ${\mathrm{Yb}}^{3+}$ relaxation at temperatures $\gtrsim {200}^{\circ }\mathrm{K}$. The rate and efficiency of ${\mathrm{Nd}}^{3+}\to {\mathrm{Yb}}^{3+}$ energy transfer in YAl${\mathrm{O}}_3$ are determined and compared with results for other materials. Stimulated-emission cross sections, ${\mathrm{Cr}}^{3+}\to {\mathrm{Nd}}^{3+}\to {\mathrm{Yb}}^{3+}$ sensitization schemes, and possible ${\mathrm{Yb}}^{3+}$ laser action are also discussed.