S. Heer, K. Petermann and H. U. Güdel
Upconversion excitation of Cr³⁺ emission in YAlO₃ codoped with Cr³⁺ and Yb³⁺
J. Lumin. 102-103, 144-150 (2003)                 

Abstract: We report on upconversion (UC) luminescence in YAlO₃ codoped with 0.3% Cr³⁺ and 6% Yb³⁺. At 10 K excitation around 10,400 cm^–1 leads to red Cr³⁺ pair luminescence at 14,000 cm^–1. The UC excitation spectrum of the red Cr³⁺ luminescence is very similar to the squared Yb³⁺ absorption spectrum. This demonstrates that two Yb^{3+ 2}F_7/2²F_5/2 excitations are involved in the UC process. From transient measurements, the UC mechanism is found to be dominated by an energy transfer step: Two excited Yb³⁺ ions simultaneously transfer their excitation energy to a nearby Cr³⁺ pair.