Origin of athermal refractive index changes observed in Yb3+ doped YAG and KGW

Time-resolved excited-state absorption and fluorescence measurements were performed in the Yb3+ doped YAG and KGW laser crystals. The results indicate that the athermal refractive index changes observed in previous transient interferometric and diffraction grating experiments can be related to polarisability changes caused by the existence of strong UV charge transfer absorption bands.