Spectroscopic characterization of potential tunable solid state laser materials - Cr(3+)-doped germanium garnets

A systematic characterization of the compounds Ca3M2(1-x)Cr2x(GeO4)3 where M = Ga, Al, Sc has been performed using emission, absorption and EPR spectroscopies. Unusual oxidation states were detected in the Ga and Al germanates. EPR spectra unmistakably point to the presence of some Ga(2+) or Ga(4+) and Al(2+) or Al(4+) ions, respectively, even in undoped samples. Optical absorption spectra further suggest that some tetrahedral Cr(4+) impurity is present in Cr-doped samples. Both observations correlate well with the pronounced inhomogeneities revealed by fluorescence lifetime measurements and low-temperature emission spectra. On the other hand, all the methods of characterization indicate that the Sc germanate is free of contaminants and maintains chromium solely in the (3+) oxidation state. Furthermore, it is a true low-field material. This makes the Sc germanate the best candidate of the germanium garnets for a tunable solid state laser. The spectroscopic properties of the Sc germanate which are directly relevant to laser action are compared to those of GSGG:Cr(3+).