N. A. Teplyakova

Complex investigations of structural and optical homogeneities of low-photorefractivity lithium niobate crystals by the conoscopy and photoinduced and Raman light scattering methods

Using photoinduced light scattering, conoscopy, and Raman spectroscopy methods, we have studied stoichiometric lithium niobate crystals and congruent crystals that were doped with Mg(0.078, 0.89 mas %), Zn(0.03, 0.52, 0.62), Cu(0.015), B(0.12), Gd(0.51), Y(0.46), Gd(0.23):Mg(0.75), Mg(0.86):Fe(0.0036), Ta(1.13):Mg(0.011), and Y(0.24):Mg(0.63) cations. It has been found that, depending on the kind of the pattern of photoinduced light scattering, investigated specimens can be divided into three groups. We have shown that the asymmetry of the indicatrix of photoinduced light scattering of LiNbO3 crystals is caused by birefringence of exciting laser radiation as it propagates perpendicularly to the polar axis of the crystal, whereas the asymmetry of the Raman spectrum arises due to the occurrence of spontaneous polarization, the vector of which is directed along the polar axis, and by birefringence. The pattern of the photoinduced light scattering depends on the difference of the refractive indices Δn = no − ne of the ordinary (no) and extraordinary (ne) rays and their energies E. If Eno {ie259-1} Ene, the proportion of the photoinduced light scattering has the shape of a three-layer round spot. For equal energies, the pattern has the shape of a symmetric figure-eight. At Eno < Ene, the figure-eight is asymmetric. In this case, its large “lobe” is directed in the positive direction of the polar axis of the crystal.

Structural homogeneity of photorefractive LiNbO3 crystals doped with 0.03–4.5 mol % of ZnO

Using the electronic spectroscopy method, the laser-conoscopy method, and the Raman light-scattering method, we have studied the structural homogeneity of LiNbO3 crystals doped with 0.03–4.5 mol % of ZnO. We have found that, as the laser radiation power is increased to 90 mW, the conoscopic patterns of crystals show additional distortions, which are attributed to the manifestation of the photorefractive effect. For the LiNbO3 crystal doped with 4.5 mol % of ZnO, in which the photorefractive effect is low, we have revealed a considerable shift (compared to the remaining crystals) of the optical absorption edge toward the shortwavelength range, which indicates a high structural homogeneity of this crystal. We have shown that, in the LiNbO3 crystal doped by 0.05 mol % ZnO, due to the displacement of NbLi and Li□ structural defects by Zn2+ cations, the crystal structure is ordered and, simultaneously, the number of defects with localized electrons decreases.

Structural disorder and optical properties of congruent lithium niobate crystals doped with zink and boron

From the concentration dependence of the widths and intensities of Raman lines and from the patterns of photoinduced light scattering, it is found that the mechanism of incorporation of Zn2+ cations into the LiNbO3 crystal structure changes with increasing concentration of zinc in the melt, which leads to an abrupt anisotropic expansion of oxygen octahedra along the polar axis. In this case, the number of kinks in the concentration dependence of linewidths (5) considerably exceeds the number of concentration threshold (2) known from the literature. It is shown that B3+ ions almost do not enter the cationic sublattice of the LiNbO3 crystal but changes the melt structure, so that the LiNbO3:B crystal is characterized by a high structural and optical homogeneity and a low photorefractive effect.

Structure and optical properties of LiNbO3:ZnO (3.43–5.84 mol %) crystals

LiNbO3:ZnO (3.43–5.84 mol %) crystals have been studied using Raman spectroscopy and fullprofile analysis of X-ray diffraction data. The results demonstrate that, at ZnO concentrations above 3.95 mol %, their structure is free of NbLi basic defects, which are characteristic of congruent lithium niobate crystals. Increasing the Zn concentration leads to changes in the arrangement of structural units in the cation sublattice along the polar axis and distorts the BO6 (B = Nb, Li, Zn, or a vacancy) oxygen octahedra. In the Raman spectra of the crystals, the width of the line at a frequency of 876 cm–1, which corresponds to stretching modes of the oxygens in the BO6 octahedra, has been shown to increase considerably, which may be due to changes in the character of bonding in the B–O–B bridges in response to changes in the Zn concentration in the crystals. The gradual increase in the electro-optical coefficients of the LiNbO3:ZnO crystals with increasing zinc concentration can be accounted for by changes in the ionic contribution to these bonds.

Photorefractive properties of congruent lithium niobate crystals doped with zinc

Optical and structural homogeneity and photorefractive properties of LiNbO3:Zn crystals (0.03 to 4.5 mol % Zn) were investigated by the methods of laser conoscopy, photoinduced light scattering, and Raman spectroscopy. It is established that the photorefractive effect nonmonotonically depends on the concentration of Zn2+ and is almost absent in LiNbO3:Zn crystals containing 0.05 and 4.5 mol % Zn. The suppression of photorefraction is explained by a decrease in the number of defects with the electrons localized on them due to the replacement of the NbLi and VLi defects by Zn2+ cations, by the appearance of shallow traps near the bottom of the conduction zone, and by the radiative recombination of laser-induced photo-electrons. The conoscopic pattern of a single-axis crystal of high optical quality was observed in a LiNbO3:Zn crystal containing 4.5 mol % Zn upon irradiation by the laser with a power of 90 mW. This pattern is of much better quality than the one observed with a laser radiation power of 1 mW, which is explained by curing of the defects by the laser radiation. An area of highly ordered structure with high alternation ordering of main and doped cations and vacancies along the polar axis in the cation sublattice is detected.

Optical properties of LiNbO3:Mg(5.21 mol %) and LiNbO3:Fe(0.009 mol %):Mg(5.04 mol %) crystals

Using methods of electronic spectroscopy, laser conoscopy, photoinduced (photoreactive) light scattering, and Raman light-scattering spectroscopy, we have studied the optical homogeneity, optical transmission, and photorefractive properties of single crystals LiNbO3:Mg(5.21 mol %) and LiNbO3:Fe(0.009 mol %):Mg(5.04 mol %) that were grown from congruent melts. We have ascertained that doping with “nonphotorefractive” Mg2+ cations causes suppression of the photorefractive effect in a lithium-niobate crystal. Upon double doping (Fe:Mg), if the concentration of Mg2+ cations exceeds the threshold concentration, the photorefractive effect is almost not observed and the presence of “photorefractive” Fe cations does not affect the photorefractive effect as strongly as in congruent crystals doped with Fe.

Structure and Properties of Boron-Doped LiNbO 3 Single Crystals

We have studied the macro-and microstructure and properties of LiNbO3〈B〉 crystals in comparison with LiNbO3〈Zn〉 and stoichiometric and congruent lithium niobate crystals. The optical characteristics of the LiNbO3〈B〉 crystals have been shown to approach those of the stoichiometric LiNbO3 and LiNbO3〈Zn〉 crystals.

Growth of LiNbO3:Er Crystals and concentration dependences of their properties

A series of lithium niobate (LiNbO3) crystals of congruent and stoichiometric compositions, doped with erbium, have been grown under non-steady-state thermal conditions. A series of LiNbO3:Zn crystals, nominally pure LiNbO3 crystals of congruent and stoichiometric compositions, and a LiNbO3:B crystal have also been grown. Both growth conditions and concentration dependences of physicochemical, ferroelectric, and structural characteristics of LiNbO3:Er crystals are investigated. The growth regular domain microstructures and periodic nanostructures in LiNbO3:Er crystals are analyzed by optical microscopy and atomic force microscopy (AFM). A comparative study of the optical homogeneity and photorefractive properties of LiNbO3:Er crystals of congruent and stoichiometric compositions and the Raman spectra of LiNbO3 crystals of different compositions is performed.

Lithium ion conductivity and mobility in the Li0.12Na0.88Ta0.4Nb0.6O3 solid solution

Lithium ion mobility and the superionic phase transition in the Li0.12Na0.88Ta0.4Nb0.6O3 solid solution have been studied using temperature-dependent ionic conductivity measurements and Raman spectroscopy. From the temperature dependences of the conductivity and the width of a Raman line corresponding to Li+ and Na+ vibrations in the AOx (A = Na+, Li+) polyhedra, the average lifetime of the Li+ ion in its equilibrium position and the height of the barrier to hopping have been estimated at ≃3.9 × 10−13 s and ≃16 kJ/mol, respectively.

Optical homogeneity and photorefractive properties of stoichiometric and congruent lithium niobate crystals grown using charges of different origins

A stoichiometric lithium niobate crystal (LiNbO3 st) and congruent lithium niobate crystals grown from a charge prepared using cyclohexanone as an extractant (LiNbO3 cong(CHN)) and a charge prepared using cyclohexanone and carboxylic acid dimethylamides as extractants (LiNbO3 cong(CHN + A)) have been characterized by photoinduced light scattering and laser conoscopy. The results demonstrate that the LiNbO3 cong(CHN + A) crystals are rather homogeneous along their growth direction and possess good optical properties, similar to those of the LiNbO3 cong(CHN) crystal. At the same time, the LiNbO3 cong(CHN + A) crystal offers significantly better electro-optical properties (re = 29.3 pm/V).