We have studied the optical properties of copper-doped lithium niobate crystals using semiconductor light-emitting diodes and lasers emitting in the visible and ultraviolet spectral regions as...

A compositionally homogeneous nonlinear optical single crystal of double-doped LiNbO₃:Gd³⁺(0.003):Mg²⁺(0.65 wt.%) was obtained. Fine features of the LiNbO₃:Gd³⁺(0.003):Mg²⁺(0.65 wt.%) crystal structure were studied from the Raman spectra of the first and second orders upon excitation by laser lines in the visible (532 nm) and near-IR (785 nm) regions. When the Raman spectrum was excited by a 785 nm laser line in the frequency range of 1000–2000 cm⁻¹ for the first time, a number of low-intensity lines in the range of 900–2000 cm⁻¹, corresponding to the second-order Raman spectrum, were discovered. The same lines also appear in the spectrum upon excitation by a laser line with a wavelength of 532 nm, but their intensities are significantly (by an order of magnitude or more) lower. It is shown that in the structure of the double-doped LiNbO₃:Gd³⁺(0.003):Mg²⁺(0.65 wt.%), the crystal oxygen-octahedral clusters MeO₆ (Me–Li, Nb, Gd, Mg) are slightly distorted, and in addition, the value R = [Li]/[Nb] ≈ 1 is close to that for a nominally pure stoichiometric crystal.

A nonlinear optical double-doped single-crystal LiNbO₃:Gd:Mg (Gd concentration is 0.003, Mg—0.65 wt % in the crystal) has been researched by several optical methods: laser conoscopy, photoinduced light scattering (PILS), optical spectroscopy, and Raman scattering. The crystal has been shown to have no photorefraction effect and a high optical uniformity. Fine features of the crystal structure have been studied via Raman spectra. Spectra have been registered in the first and second orders, they have been excited by visible (532 nm) and near-IR (785 nm) lasers. Registered Raman spectra have the fundamental vibrations of the crystal lattice of the A₁(TO,LO)- and E(TO,LO)-type symmetry located in the range of 150–900 cm⁻¹. A number of low-intensity Raman bands in the 900–2000 cm⁻¹ region have been determined to correspond to the second-order Raman spectrum. These bands are polarized and appear only in certain polarization-scattering geometries. They appear in the spectrum excited by visible radiation, but their number and intensity are much lower than those excited by near-IR lasers. Oxygen-octahedral MeO₆ clusters in our case can contain Li, Nb, Gd, or Mg in the Me site. The clusters in the LiNbO₃:Gd:Mg crystal structure are slightly distorted compared with similar clusters in the nominally pure LiNbO₃ crystal. It has been established that the value R = [Li]/[Nb] in the studied crystal is ≈ 1. Such a ratio usually characterizes a nominally pure stoichiometric crystal.