Photoionization energies of Cr3+-doped LiNbO3

We report on a study of intra-ion and electron transfer transitions in Cr3+-doped LiNbO3. The energies of the meta-stable emitting levels of Cr3+ on both Li and the Nb sites are determined by standard optical spectroscopy techniques. Photoconductive measurements are used to determine the ionization energies of both species of Cr3+ ions. From these results, energy level diagrams are created which give the energies of the optically active levels with respect to the host valence and conduction bands. The emitting 4T2 level of Cr3+ on a Li site and on a Nb site is found to lie 1.5 and 1.2 eV below the conduction band bottom, respectively. These results lead to the suggestion that the lack of laser action found in LiNbO3:Cr3+ may be due to strong excited state absorption transitions from the meta-stable 4T2 level to the host conduction band.