Up-conversion excitation of sharp Cr3+ 2E emission in YGG and YAG codoped with Cr3+ and Yb3+

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 2F5/2 of Yb3+ as an intermediate state. Excitation of the Yb3+ in the near infrared around 1000 nm leads to visible Cr3+ 2E→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 Yb3+ 2F7/2→2F5/2 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 2F5/2 excitation to Cr3+.