Concentration Dependence of Optical Properties of Double-Doped LiTaO:Cr:Nd Crystals.

LiTaO crystals doped with Cr and Nd ions are promising for developing active nonlinear laser media. In this work, the defect structure of LiTaO crystals, including those doped with Cr and Nd, is examined. X-ray patterns of all six investigated LiTaO:Cr:Nd crystals are identical and correspond to a highly perfect structure. Using optical microscopy, the presence of defects of various shapes, microinhomogeneities, and lacunae was revealed. The optical absorption and Raman scattering spectra of a series of nonlinear, optical, double-doped LiTaO:Cr:Nd (0.06 ≤ [Cr] ≤ 0.2; 0.2 ≤ [Nd] ≤ 0.45 wt%) crystals showed that at concentrations of doping Cr ions less than 0.09 wt% and Nd ions less than 0.25 wt%, the crystal structure is characterized by a low level of defects, and the optical transmission spectra characterized by narrow lines corresponding to electron transitions in Nd ions. In this case, for the radiative transition in the cation sublattice, the existence of three nonequivalent neodymium centers is observed, and for the radiative transition, two nonequivalent centers are observed. IR absorption spectroscopy in the OH-stretching vibration range revealed two main spectral regions: 3463-3465 cm, associated with stoichiometry changes, and 3486-3490 cm, linked to complex defects such as (V)-OH and (Ta)-OH. A distinct low-intensity line at ~3504 cm was observed only in doped crystals, attributed to (Nd)-OH defects that significantly distort the oxygen-octahedral clusters due to the larger ionic radius of Nd compared to Ta. In contrast, Cr-related defects cause only minor distortions. The Klauer method indicated that the highest concentration of OH-groups occurs in the LiTaO:Cr (0.09 wt%):Nd (0.25 wt%) crystal, where multiple complex defects are present.