Complex study of the structural and optical homogeneity of lithium niobate crystals

Abstract

Methods of Raman spectroscopy, laser conoscopy, optical microscopy, and electron spin reso nance have been used to study the photorefractive properties and structural and optical homogeneity of the following lithium niobate (LiNbO 3 ) crystals: nominally pure crystals of congruent composition (LiNbO 3con ); LiNbO 3 :Cu[0.015 wt %] crystals grown from a melt of congruent composition and nominally pure crystals of stoichiometric composition grown from a melt with 58.6 mol % Li 2 O (LiNbO 3st ). A small deformation of optical indicatrix and regular microdomain structures of fractal type are revealed for the LiNbO 3 :Cu[0.015 wt %]; the microdomain structures may be due to the nonuniform impurity incorporation into the structure. It is shown that oxygen octahedra in the LiNbO 3 :Cu[0.015 wt %] crystal are deformed in comparison with the octahedra in LiNbO 3st and LiNbO 3con crystals and that the main and impurity cations are clusterized along the polar axis. It is established that the LiNbO 3 :Cu[0.015 wt %] crystal exhibits photo refractive properties not only due to the presence of intrinsic defects with localized electrons, as in the case of LiNbO 3st , but also due to the charge exchange in copper cations (Cu 2+ Cu + ) under illumination. Fig. 3. PRLS in (a) LiNbO 3st , (b) LiNbO 3con , and (c) LiNbO 3 :Cu[0.015 wt %] single crystals; λ 0 = 514.5 nm; Р = 30 mW; the numbers 1-3 indicate the central, second, and third layers, respectively. CRYSTALLOGRAPHY REPORTS Vol. 59 No. 5 2014 Translated by Yu. Sin'kov