Optical properties of LiNbO3:Cr crystals co-doped with germanium oxide

Lithium niobate (LiNbO3) crystals doped with chromium ions show a clear green colouring reflecting the absorption profile of the dominating [Cr]Li defect centres. A significant change in its colouration takes place when it is co-doped with other valency impurities such as Mg2+, Sc3+ and W6+, above a certain threshold concentration. This concentration singularity has been attributed to the formation of [Cr]Nb centres coexisting with the [Cr]Li centres. In this work, we extended the investigation on the effect of co-dopant ions in Cr:LiNbO3 to tetravalent cation such as GeO2. A singularity in the relative intensity of the 4A2→4T1 and 4A2→4T2 absorption band was observed for a concentration of ∼1.5 mol%, compared with 4.5 mol% for Mg2+. The photoluminescence emission spectra also reveal a new emission band, at a lower energy than the [Cr]Li centre, corresponding to this threshold concentration. A charge compensation model is proposed to explain the role of cation impurities and results are compared with those of other valence impurities.