The role of the Mg²⁺ ions in Cr³⁺ spectroscopy for near-stoichiometric LiNbO₃ crystals

The optical spectroscopy of Cr³⁺ ions doped into near-stoichiometric LiNbO₃ crystals, pure and co-doped with MgO, has been investigated. In the near-stoichiometric LiNbO₃:Cr(0.2 mol%):Mg(2 mol%) crystal, the optical spectra resemble those previously observed for congruent LiNbO₃:Cr:MgO samples when the total MgO content exceeds the 4.6 mol% threshold. The coexistence of two types of Cr³⁺ centre ([Cr]_Li and [Cr]_Nb) characterized the optical and luminescence spectra of this sample. The concentration equilibrium between the two types of centre is strongly displaced towards the [Cr³⁺]_Nb centre, permitting us to obtain with accuracy the parameters of the broad bands. The R-line associated with the [Cr]_Nb centre is only observable in the low-temperature emission spectrum. The Fano anti-resonance lines present have been observed to be more pronounced for the near-stoichiometric samples than for congruent ones.