Optical characterization of Cr3+ centers in LiNbO3

Low-temperature luminescence spectra of stoichiometric Cr:LiNbO3, congruent Cr:LiNbO3 and congruent Cr,Mg:LiNbO3 were studied. Dominant low-field and minor high-crystal-field optical centers are the Cr3+ impurity ions that preferentially occupy Li+ sites (CrLi) in the Cr:LiNbO3 crystals. Low-field centers related to Cr3+ substitution of Nb5+ (CrNb) occur in addition to CrLi in co-doped Cr,Mg:LiNbO3 samples. Application of high hydrostatic pressure leads to the transformation of dominant Cr3+ centers from low- to high-field type due to strong pressure-induced blue shift of the 4T2 state, resulting in its crossing with the 2E state of Cr3+. This level-crossing effect was observed for the dominant Cr3+Li and Cr3+Nb centers at pressures that correlate well with estimations based on the 4T2-2Eenergy gap (230 cm-1 and 1160 cm-1) and on the rate of their pressure-induced change (14.35 and 11.4 cm-1/kbar, respectively). We also studied inhomogeneous broadeningof the 2E?4A2transitions at ambient pressure for the minor high-field “defect” Cr3+Li centers in congruent LiNbO3. A fine structure in the spectral response of these centers was observed. The obtained results are discussed on the basis of a microscopic hierarchic model for perturbed Cr3+ ions in the LiNbO3 lattice.