S. Heer, M. Wermuth, K. Krämer, D. Ehrentraut and H. U. Güdel
Up-conversion excitation of sharp Cr^{3+ 2}E emission in YGG and YAG codoped with Cr³⁺ and Yb³⁺
J. Lumin. 94-95, 337-341 (2001)                 

Abstract: The ²E ⁴A₂ emission of Cr³⁺ in crystals and glasses is of great importance in science and technology. We found a new excitation mode for this emission in the garnets YGG and YAG codoped with Cr³⁺ and Yb³⁺. It is based on an up-conversion mechanism using ²F_5/2 of Yb³⁺ as an intermediate state. Excitation of the Yb³⁺ in the near infrared around 1000 nm leads to visible Cr^{3+ 2}E ⁴A₂ emission around 700 nm at low temperatures. The up-conversion luminescence intensity shows a strong temperature dependence due to an intrinsic loss mechanism. The up-conversion excitation spectra follow the Yb^{3+ 2}F_7/2 ²F_5/2 absorptions. From its time dependence the up-conversion mechanism is found to be dominated by an energy transfer step: Two excited Yb³⁺ ions simultaneously transfer their ²F_5/2 excitation to Cr³⁺.