Optical bleaching of Cr3+ luminescence in near stoichiometric LiNbO3 crystals codoped with MgO

The emission intensity of the R-line of Cr3+ ions in near stoichiometric LiNbO3crystals codoped with MgO has been studied at a low temperature under excitation of the 4A2→4T1 transition. The R-line emission intensity associated with Cr3+ ions in the Nb5+ site, [Cr]Nb centre, was observed to decrease with the illumination time whereas the Cr3+ ions located in the Li site, [Cr]Li centres, are optically stable. This selective bleaching is explained by considering the photoionization of Cr3+ ions in Nb5+ sites and the formation of Nb4+.