Optical spectroscopy of Cr3+ ions in stoichiometric LiNbO3 crystals and co-doped with MgO

The optical spectroscopy of Cr3+ ions in near stoichiometric LiNbO3 crystals, ‘pure’ and co-doped with MgO, has been investigated. In the near stoichiometric LiNbO3:Cr (0.2%) optical absorption and emission spectra show the usual features of broad bands and narrow lines typical of congruent samples with the an additional of a narrow emission lines centred at 738 nm and has been ascribed to the zero-phonon line of the 4T2→4A2 transition. The concentration equilibrium between the two type of centres is strongly displaced towards the [Cr]Nb centre permitting thus to obtain with accuracy the parameters of the broad bands. The R-lines associated with the [Cr]Nb centre is only observable in low-temperature emission spectrum. The presence of Fano anti-resonance lines has been observed to be more pronounced in the near stoichiometric than in congruent samples.