S. Heer, M. Wermuth, K. Krämer and H. U. Güdel
Sharp ²E upconversion luminescence of Cr³⁺ in Y₃Ga₅O₁₂ codoped with Cr³⁺ and Yb³⁺
Phys. Rev. B 65, 125112/1-10 (2002)                 

Abstract: We report on upconversion luminescence in Y₃Ga₅O₁₂ codoped with Cr³⁺ and Yb³⁺. This is, to our knowledge, the first upconversion process directly involving Cr³⁺. At low temperature excitation around 10 314 cm^–1 leads to relatively strong red luminescence in the title compound. The sharp upconversion luminescence around 14 300 cm^–1 is identified as the ²E ⁴A₂ transition of Cr³⁺. The upconversion excitation spectrum follows he Yb^{3+ 2}F_7/2 ²F_5/2 absorptions in the range 11200 - 10315 cm^–1. From transient measurements the upconversion mechanism is found to be dominated by an energy transfer step: Two excited Yb³⁺ ions simultaneously transfer their excitation energy to the Cr³⁺ ion. In Y₃Ga₅O₁₂:2% Cr³⁺, 1% Yb³⁺ 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.