S. Heer, M. Wermuth, K. Krämer and H. U. Güdel
Upconversion excitation of Cr^{3+ 2}E emission in Y₃Ga₅O₁₂ codoped with Cr³⁺ and Yb³⁺
Chem. Phys. Lett. 334, 293-297 (2001)                 

Abstract: The ²E ⁴A₂ emission of Cr³⁺ in crystals and glasses is of great importance in science and technology. We report a new excitation mode for this emission in Y₃Ga₅O₁₂ (YGG) codoped with Cr³⁺ and Yb³⁺. It is based on an upconversion mechanism using ²F_5/2 of Yb³⁺ as an intermediate state. Excitation in the near-IR at 969.6 nm leads to visible luminescence around 700 nm at low temperature. In YGG:2%Cr³⁺,1%Yb³⁺ at 15 K and for a laser power of 150 mW the efficiency of the process is 5.5%. From its time dependence the upconversion mechanism is found to be dominated by an energy transfer step: Two excited Yb³⁺ ions simultaneously transfer their ²F_5/2 excitation to Cr³⁺.