Sharp 2 E upconversion luminescence of Cr 3 + in Y 3 Ga 5 O 12 codoped with Cr 3 + and Yb 3 +

We report on upconversion luminescence in ${\mathrm{Y}}_{3}{\mathrm{Ga}}_{5}{\mathrm{O}}_{12}$ codoped with ${\mathrm{Cr}}^{3+}$ and ${\mathrm{Yb}}^{3+}.$ This is, to our knowledge, the first upconversion process directly involving ${\mathrm{Cr}}^{3+}.$ At low temperature excitation around 10 314 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ leads to relatively strong red luminescence in the title compound. The sharp upconversion luminescence around 14 300 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ is identified as the ${}^{2}{E}^{4}{A}_{2}$ transition of ${\mathrm{Cr}}^{3+}.$ The upconversion excitation spectrum follows the ${\mathrm{Yb}}^{3+}$ ${}^{2}{\stackrel{\ensuremath{\rightarrow}}{{F}_{7/2}}}^{2}{F}_{5/2}$ absorptions in the range 11 200--10 315 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. From transient measurements the upconversion mechanism is found to be dominated by an energy transfer step: Two excited ${\mathrm{Yb}}^{3+}$ ions simultaneously transfer their excitation energy to the ${\mathrm{Cr}}^{3+}$ ion. In ${\mathrm{Y}}_{3}{\mathrm{Ga}}_{5}{\mathrm{O}}_{12}:2%$ ${\mathrm{Cr}}^{3+},$ 1% ${\mathrm{Yb}}^{3+}$ the efficiency of the process at 10 K and for 150 mW laser power is 6% and decreases with increasing temperature due to intrinsic loss mechanisms of the system.