Abstract

Strong refractive index changes of purely electronic origin have been reported and analysed recently in the case of the well-known laser material Nd:YAG when it is intensively pumped with diodes or flash lamps [1,2], It was shown rather unambiguously that the 4f→5d interconfigurational transitions of the Nd³⁺ ions were indeed responsible for polarisability changes which could account for most of the observed athermal refractive index variations in this material. It was noticed however that these bands may not be the only cause and that other types of absorption bands such as ligand to metal charge transfer (LMCT) band may also contribute. Such a possibility was recently examined in the case of Yb:YAG in which large thermal refractive index changes were also evidenced [3] while the 4f→5d interconfigurational transitions of Yb³⁺ in this material are lying at much higher energies than in the case of Nd³⁺ and thus should less contribute to the polarizability changes. As a matter of fact, we confirmed this hypothesis both by registering excited-state absorption spectra in the near UV spectral domain and by performing new transient interferometric measurements, as in Ref. 3, thus by using an He-Ne at 632 nm as a probe, but using the same kind of laser pump source for both experiments, i.e. a Q-switched Nd:YAG pumped OPO laser delivering 10 ns pulses of about 3 mJ around 970 nm. We also extended this investigation to the other well-known laser material Yb:KGW.

© 2007 IEEE