Optical characterization of Cr3+ centers in LiNbO3

Low-temperature luminescence spectra of stoichiometric Cr:LiNbO 3 , congruent Cr:LiNbO 3 and congruent Cr,Mg:LiNbO 3 were studied. Dominant low-field and minor high-crystal-field optical centers are the Cr 3+ impurity ions that preferentially occupy Li + sites (Cr Li ) in the Cr:LiNbO 3 crystals. Low-field centers related to Cr 3+ substitution of Nb 5+ (Cr Nb ) occur in addition to Cr Li in co-doped Cr,Mg:LiNbO 3 samples. Application of high hydrostatic pressure leads to the transformation of dominant Cr 3+ centers from low- to high-field type due to strong pressure-induced blue shift of the 4 T 2 state, resulting in its crossing with the 2 E state of Cr 3+ . This level-crossing effect was observed for the dominant Cr 3+ Li and Cr 3+ Nb centers at pressures that correlate well with estimations based on the 4 T 2 - 2 E 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 inhomogeneous broadeningof the 2 E ? 4 A 2 transitions at ambient pressure for the minor high-field “defect” Cr 3+ Li centers in congruent LiNbO 3 . 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 Cr 3+ ions in the LiNbO 3 lattice.