Selective optical bleaching of Cr3+ luminescence at low temperature in MgO codoped near stoichiometric LiNbO3:Cr crystals

Selective R-line emission of the Cr31 ions in near stoichiometric LiNbO3 crystals codoped with MgO has been studied at low temperature under excitation into the 4A2!4T1 transition. The emission intensity of R lines associated with Cr31 ions located in Nb51 sites, @Cr#Nb centers, was observed to decrease with the illumination time, whereas those ascribed to other Cr31 ions, such as those located in the Li site, @Cr#Li centers, are optically stable. The selective bleaching is explained by considering the photoinization of Cr31 ions in Nb sites and the formation of Nb41 complexes ~small polaron!