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 pr eferentially occupy Li+ sites (Cr-Li) in the Cr:LiNbO3 crystals. Low-field centers related to Cr3+ substitution of Nb5+ (Cr-Nb) occur in addition to C r-Li in co-doped Cr,Mg:LiNbO3 samples. Application of high hydrostatic pres sure leads to the transformation of dominant Cr3+ centers from low- to high -field type due to strong pressure-induced blue shift of the T-4(2) state, resulting in its crossing with the E-2 state of Cr3+. This level-crossing e ffect was observed for the dominant Cr-Li(3+) and Cr-Nb(3+) centers at pres sures that correlate well with estimations based on the T-4(2) - E-2 energy 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 inhomog eneous broadening of the E-2 --> (4)A(2) transitions at ambient pressure fo r the minor high-field "defect" Cr-Li(3+) centers in congruent LiNbO3. A fi ne structure in the spectral response of these centers was observed. The ob tained results are discussed on the basis of a microscopic hierarchic model for perturbed Cr3+ ions in the LiNbO3 lattice.