Optical Properties and Defects of Single-Crystal LiNbO3:Tb3+ Plates with a Post-Threshold Concentration of Terbium Ions

The macro- and microstructures of LiNbO3:Tb3+(2.25 wt%) and LiNbO3:Tb3+(2.9 wt%) crystals were studied. It was shown that a slight increase in terbium content dramatically alters the growth domain structure and enhances fractal self-organization. The defect structure features of LiNbO3:Tb3+(2.25 wt%) and LiNbO3:Tb3+(2.9 wt%) single-crystal plates with an after-threshold terbium concentration were studied using optical absorption spectra, Raman scattering spectra in the frequency range of 65–4000 cm−1, and IR absorption spectra in the frequency range of 1000–8000 cm−1. It was shown that the LiNbO3:Tb3+(2.25 wt%) plate has a lower concentration of NbLi point defects and a higher stoichiometry compared to the LiNbO3:Tb3+(2.9 wt%) plate. A good match was found between the Raman spectra of the LiNbO3:Tb3+(2.25 wt%) and LiNbO3:Tb3+(2.9 wt%) plates throughout the entire studied range. A significant difference was found in the frequencies of the lines corresponding to stretching vibrations of OH−-groups in the vibrational spectra of the studied plates. This observation is explained by the fact that OH−-groups in the LiNbO3 crystal structure are defects, and, therefore, the stretching vibrations of OH−-groups are not fundamental lattice vibrations. The existing selection rules for fundamental lattice vibrations, developed on the basis of group theory, do not apply to the defect sublattice.