The effect of stoichiometry and Mg doping on the Raman spectra of LiNbO3:Mg crystals

LiNbO₃:Mg crystals doped with 0–8 mol. % Mg with stoichiometric, intermediate and congruent compositions were systematically investigated by Raman spectroscopy in backscattering y(zx)y, y(zz)y and z(xx)z geometries. The damping was found to be a very sensitive parameter for the characterization of the crystal composition. The half-widths of E(TO₃)–E(TO₉) and A ₁(TO₁)–A ₁(TO₄) bands having significant composition dependence for the undoped LiNbO₃ crystals show only a weak Mg concentration dependence below the photorefractive threshold, which is a consequence of the counteracting effect of the decreasing Nb_Li and increasing Mg_Li contents. The half-widths of the bands, however, increase linearly with growing Mg content for samples above the threshold, irrespective of the Li/Nb ratio. The change in the Mg concentration dependence at a given Li/Nb ratio determines the same threshold value as that concluded from IR and UV spectroscopic measurements. The half-width of the main A ₁(LO₄) band at 873 cm^-1 increases linearly with growing Mg concentration, but no threshold effect is observed. However, the ratio of the area of the main band and the high-frequency sideband shows a threshold effect that can be interpreted by the existing defect incorporation models. The small Raman band at about 740 cm^-1 attributed earlier to Nb_Li vibration is also detected in above-threshold LiNbO₃:Mg crystals, which can be explained by the vibration of Nb ions in Mg₄Nb₂O₉ defect clusters appearing at high Mg concentrations. PACS 77.84.Dy; 63.20.Mt; 42.70.Mp; 78.30.-j     

Photorefractive light scattering in lithium niobate crystals doped with Mg2+, B3+, Y3+, and Ta5+

We have investigated the photorefractive (photoinduced) light scattering in lithium niobate single crystals: LiNbO₃, LiNbO₃:B, LiNbO₃:Y(0.46 mas %), LiNbO₃:Y(0.24):Mg(0.63 mas %), and LiNbO₃:Ta(1.13):Mg(0.0109 mas %) that were grown from congruent melts. We have found that the shape of the speckle structure of this scattering and the kinetics of the development of its indicatrix depend substantially on the type of the impurity dopant in the lithium niobate crystal. We have observed that, upon laser irradiation of crystals doped with Y³⁺, Ta⁵⁺:Mg²⁺, and Y³⁺:Mg²⁺, the shape of their scattering indicatrix changes with time. At the same time, the LiNbO₃:B crystal is characterized by a complete absence of time changes in its speckle structure, which indicates that the photorefractive effect in this crystal is substantially lowered.     

Dispersion of the dielectric function real part for Mg:LiNbO3 crystals at terahertz frequencies

We study dispersion of the dielectric function real part ε′ in the terahertz-frequency range for bulk and periodically poled congruent LiNbO₃ and Mg:LiNbO₃ crystals. The concentration of Mg in Mg:LiNbO₃ samples was close to 5 mol%, which is the photorefractive threshold. Approximate expressions for extraordinary polariton dispersion dependence were obtained in the range 0.5–6.5 THz. The influence of Mg-dopant on the optical properties of crystals in the terahertz range is revealed. Changes of the defect structure of lithium niobate crystals are discussed.     

Studies of absorption spectra and the photovoltaic effect in LiNbO3:Mg:Fe crystals

The absorption spectra, photoconductivities and photovoltaic currents of LiNbO₃:Fe crystals with different Mg doping levels and Li/Nb ratios in the oxidized state have been investigated at room temperature. The Fe²⁺ ions in LiNbO₃:Mg:Fe with Mg content above a critical value are more easily oxidized than in crystals with Mg content below a critical value. The photoconductivity of LiNbO₃:Mg:Fe crystals with Mg content above a critical value is one order of magnitude higher than those with Mg content below a critical value, however, the photovoltaic current of the former is one order of magnitude lower than the latter. The differences are postulated to be due to different sites of Fe in these two classes of crystals.     

Stoichiometric Mg:LiNbO(3) as an effective material for nonlinear optics

Photorefractive damage, optical absorption, photoconductivities, and photogalvanic currents of stoichiometric LiNbO(3) single crystals with different Mg doping levels have been investigated. Nominally pure stoichiometric LiNbO(3) shows lower photorefractive damage resistance than congruent crystal; however, stoichiometric crystals doped with MgO of more than 1.8 mol. % exhibit no measurable photorefractive damage at 532 nm to intensities of as much as 8 MW/cm(2) . This remarkable damage resistance can be attributed not only to increased photoconductivity but also to decreased photogalvanic current. Stoichiometric Mg:LiNbO(3) also demonstrates the shortest absorption edge, 302 nm, and a single-domain nature with low scattering losses.     

Investigation of optical homogeneity and photorefractive properties of lithium-niobate single crystals by the Raman-spectroscopy and laser-conoscopy methods

Photorefractive properties and structural and optical homogeneity of (1) LiNbO₃:Cu crystals ([Cu] = 0.015 mas %) grown from a congruent melt, (2) nominally pure stoichiometric crystals grown from a melt with 58.6 mol % of Li₂O (LiNbO_3stoich), and (3) nominally pure congruent crystals (LiNbO_3congr) have been studied using the Raman-spectroscopy method with excitation in the UV, visible, and near-IR ranges; the laser-conoscopy method; and the electron paramagnetic resonance-spectroscopy method. In optically uniaxial LiNbO₃ crystals, a weak optical biaxiality has been revealed, which is attributed to an insignificant deformation of the optical indicatrix. This deformation can be caused both by the initial structural inhomogeneity of crystals and by the photorefractive effect. It has been shown that, under the action of light, charge exchange of copper cations Cu²⁺ → Cu⁺ takes place in the crystal LiNbO₃:Cu ([Cu] = 0.015 mas %). The LiNbO₃:Cu crystal exhibits photorefractive properties not only because of the occurrence of intrinsic defects with electrons localized at them, as is the case with the LiNbO_3stoich and LiNbO_3congr crystals, but also due to the charge exchange of copper cations under the action of the laser radiation.     

Investigation of optical photorefractive properties of Zr:Fe:LiNbO3 crystals

A series of Zr:Fe:LiNbO₃ crystals with various levels of ZrO₂ doping were grown by Czochraski technique. The optical damage resistance and photorefractive properties were deeply explored. The results showed that the ability optical damage resistance increased remarkably when the concentration of ZrO₂ is over threshold concentration, but which is lower than that of traditional damage resistant additive MgO. While, the holographic storage properties can be greatly enhanced by proper level of ZrO₂ doping in Fe:LiNbO₃. In terms of ions' site occupation model, the photo-damage resistant ability enhancement and the change of the photorefractive properties were discussed.     

Optical homogeneity,defects, and photorefractive properties of stoichiometric,congruent, and zinc-doped lithium niobate crystals

Using the laser-conoscopy method, the photorefractive light-scattering method, and the Raman light-scattering method, we have studied the structural and optical homogeneities and photorefractive properties of (i) stoichiometric lithium niobate crystals (LiNbO₃(stoich)), which were grown from a melt with 58.6 mol % of Li₂O; (ii) congruent crystals (LiNbO₃(congr)); and (iii) congruent crystals that were doped with Zn²⁺ cations (LiNbO₃:Zn; [Zn] = 0.03–1.59 mol %). We have shown that the speckle-structure of the photorefractive light scattering in all the crystals is three-layer. The shapes of the second and third layers repeat in general the shape of the first layer. We have shown that the differences that are observed between the Raman spectra, the photorefractive light scattering, and the conoscopic patterns of the examined crystals are caused by the fact that defects are distributed inhomogeneously over the volume of these crystals and that Zn²⁺ cations are incorporated inhomogeneously into the lattice. This leads to the appearance of local changes in the elastic characteristics of the crystal and to the appearance of mechanical stresses, which locally change the optical indicatrix and, correspondingly, the conoscopic pattern and the Raman spectrum.     

Specific features of growth and structure of LiNbO<Subscript>3</Subscript> : Zn crystals near the ZnO concentration threshold of 6.76 mol %

The crystallization conditions and Raman spectra of LiNbO₃ : Zn crystals (0.02–8.91 mol % ZnO in the melt) have been investigated. It has been established that the most favorable conditions for growing optically and compositionally homogeneous heavily doped LiNbO₃ : Zn crystals, which are characterized by a low photorefractive effect, are implemented in the ZnO concentration range of ~4.0–6.76 mol % in the melt. Since the distribution coefficient K _eff decreases significantly with an increase in the ZnO concentration in the melt, one can obtain LiNbO₃ : Zn crystals with significantly different defect structures but identical zinc concentrations. A change in the zinc concentration in crystals has been shown to induce a stepwise change in the sequence order of the main (Li and Nb) and doping (Zn) cations and vacancies and stepwise anisotropic expansion of the oxygen octahedra along the polar axis. The number of kinks in the concentration behavior of the spectral-line widths (five kinks for the lines with frequencies of 630 (A ₁(TO)) and 876 cm^–1 (A ₁(LO))) significantly exceeds the number of thresholds (two) known from the literature.     

Photorefractive light scattering in LiNbO3: Cu crystals

The time dependence of the photorefractive light scattering in lithium niobate single crystals of the congruent composition (LiNbO₃) doped by “photorefractive” Cu cations (0.015 wt %) is studied for a radiation power of 160 mW. The data for the photorefractive scattering are confirmed by changes in Raman scattering spectra.     

Enhanced nonvolatile holographic properties in Zn,Ru and Fe co-doped LiNbO3 crystals

A series of LiNbO₃ crystals doped with various concentrations of ZnO and fixed concentrations of RuO₂ and Fe₂O₃ have been grown by the Czochralski method from the congruent melts. The type of charge carriers was determined by Kr⁺ laser (476 nm) and He–Ne laser (633 nm). The results revealed that the holes were the dominant charge carriers at blue light irradiation. Dual-wavelength and two-color techniques were employed to investigate the nonvolatile holographic storage properties of Ru:Fe:LiNbO₃ and Zn doped Ru:Fe:LiNbO₃ crystals. The essential parameters of blue nonvolatile holographic storage in Zn:Ru:Fe:LiNbO₃ crystals were enhanced greatly with the increase of Zn concentration. This indicates that the damage resistant dopants Zn²⁺ ions enhance the photorefractive properties at 476 nm wavelength instead of suppressing the photorefraction. The different mechanisms of blue photorefractive and nonvolatile holographic storage properties by dual wavelength recording in Zn:Ru:Fe:LiNbO₃ crystals were discussed.     

Preparation and holographic storage properties of tri-doped Mg:Mn:Fe:LiNbO3 crystals

Doping MgO, MnO and Fe₂O₃ in LiNbO₃ crystals, tri-doped Mg:Mn:Fe:LiNbO₃ single crystals were prepared by the conventional Czochralski method. The UV-vis absorption spectra were measured and the shift mechanism of absorption edge was also investigated in this paper. In Mg:Mn:Fe:LiNbO₃ crystal, Mn and Fe locate at the deep level and the shallow level, respectively. The two-photon holographic storage is realized in Mg:Mn:Fe:LiNbO₃ crystals by using He-Ne laser as the light source and ultraviolet as the gating light. The results indicated that the recording time can be significantly reduced for introducing Mg²⁺ in the Mg:Mn:Fe:LiNbO₃ crystal.     

Effects of the ordering of structural units of the cationic sublattice of LiNbO3:Zn crystals and their manifestation in Raman spectra

We have studied the Raman spectra of congruent crystals of lithium niobate (LiNbO₃) that were doped with Zn²⁺ ions in the range of concentration of 0–1.59 mol %. We have revealed a region of a more ordered structure such that the order of sequence of basic ions, impurity ions, and vacancies along the polar axis of the cationic sublattice is more regular, while the oxygen octahedra are close to ideal. In this case, crystals have a higher optical quality and are more stable with respect to optical damage. An increased ordering of the structure is realized because small amounts of Zn²⁺ cations displace Nb_Li defects and order the alternation of cations and vacancies along the polar axis and make the crystal less defect with respect to Li⁺ vacancies. Our results are important for industrial production of optically perfect lithium-niobate crystals by doping a congruent crystal with small concentrations of Zn²⁺ ions, since, in this case, technological regimes of crystal growth almost do not differ from regimes of growing of nominally pure congruent crystals.     

Structural homogeneity of photorefractive LiNbO<Subscript>3</Subscript> 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 Nb_Li and Li□ structural defects by Zn²⁺ cations, the crystal structure is ordered and, simultaneously, the number of defects with localized electrons decreases.     

Raman spectra of lithium niobate crystals heavily doped with zinc and magnesium

We have examined the Raman spectra of heavily doped lithium niobate single crystals (at close-to-threshold concentrations of doping cations): LiNbO₃:Zn (4.5 mol % ZnO), LiNbO₃:Mg (5.01 mol %):Fe (0.005 mol %), LiNbO₃:Mg (5.1 mol %), and LiNbO₃:Mg (5.3 mol % MgO). Low-intensity lines with frequencies at 209, 230, 298, 694, and 880 cm^–1 have been revealed for the first time. Analysis of the data from the literature on lattice dynamics calculations from first principles (ab initio) does not make it possible to unambiguously state that these lines correspond to fundamental vibrations of the А ₂ symmetry species, which are forbidden for the С _3V ⁶ (R3c) space group. At the same time, ab initio calculations unambiguously indicate that the experimentally observed low-intensity “superfluous” lines with the frequencies at 104 and 119 cm^–1 cannot correspond to vibrations of the А ₂ symmetry species. It is most likely that they correspond to two-particle states of acoustic phonons with a total wave vector equal to zero.     

The correlation of doping ions to holographic storage properties of Mg:Mn:Fe:LiNbO3 crystals

The congruent Mn(0.1 wt%):Fe(0.03 wt%):LiNbO₃ crystals doped with different concentration of MgO(0,1,3,6 mol%) have been grown by Czochralski method in air atmosphere. Some crystal samples were reduced in Li₂CO₃ powder. The defects and doping ions location in crystals were investigated by UV-Vis. absorption spectrum as well as infrared transmittance spectrum analysis. In two wave coupling experiments we determined the writing time, maximum diffraction efficiency and the erasure time of four crystal samples with He-Ne laser at 633 nm. The results indicated that Mg(3 mol%):Fe:Mn:LiNbO₃ was the most proper holographic recording media material among four crystals in the paper.     

LiNbO3 with the damage-resistant impurity indium

We report on a detailed investigation (EPR, SHG, optical absorption, luminescence and Raman scattering) on the new damage-resistant impurity indium in LiNbO₃, where the increased photoconductivity strongly reduces the photorefractive effect. EPR and optical absorption measurements point to a complete disappearance of the Nb antisite in LiNbO₃: In for all In concentrations. We believe that the very effective driving out of Nb antisites by In is due to the trivalent charge state of In and the possibility of charge self-compensation. Similarities in the properties of Mg-, Zn- or Indoped samples are discussed. Simultaneous doping with In and Zn leads to an addition of both contributions, in particular for optical frequency doubling and luminescence. Raman studies prove that In does not improve the stoichiometry of the crystals. Indium doping provides the possibility to control simultaneously phase-matching conditions and to reduce drastically photorefraction. Therefore, In co-doped LiNbO₃ compositions are promising materials for applications after solving contemporary growth problems.     

Photorefractive centers in LiNbO3, studied by optical-, Mössbauer- and EPR-methods

The photorefractive properties of LiNbO₃∶Fe and LiNbO₃∶Cu have been studied in combination with optical absorption-, Mössbauer- and EPR-measurements. The charge states of Fe in successively reduced LiNbO₃∶Fe have been investigated with respect to the influence on the photorefractive sensitivity and saturation value of the refractive index change. The results of this experiment demonstrate clearly the close correlation between the concentration of Fe²⁺ impurities and the optical absorption band around 2.6 eV in LiNbO₃∶Fe, which is known to give rise to an anisotropic charge transport upon optical excitation. The resulting photocurrents determine the photorefractive sensitivity mainly in the initial state of halographic exposure. With increasing conversion from Fe³⁺ to Fe²⁺ the photorefractive sensitivity saturates and the saturation value of the refractive index change decreases remarkably. In the case of LiNbO₃∶Cu a similar behaviour of the photorefractive storage parameters after successive reduction treatments has been observed qualitatively. However, in contradiction to LiNbO₃∶Fe the Cu²⁺ centers cannot be related to the photorefractive sensitivity of LiNbO₃∶Cu. These results are discussed with respect to the predictions of two models concerning the microscopic nature of the photorefractive process in doped LiNbO₃.     

Growth and optical properties of Mg, Fe Co-doped LiTaO3 crystal

Mg, Fe double-doped LiTaO₃ and LiNbO₃ crystals have been grown by Czochralski method. The optical properties were measured by two-beam coupling experiments and transmitted facula distortion method. The results showed that the photorefractive response speed of Mg:Fe:LiTaO₃ was about three times faster than that of Fe:LiTaO₃, whereas the photo-damage resistance was two orders of magnitude higher than that of Fe:LiTaO₃. In this paper, site occupation mechanism of impurities was also discussed to explain the high photo-damage resistance and fast response speed in Mg:Fe:LiTaO₃ crystal.     

Photorefractive properties for holographic application of Ce:Fe:LiNbO3 crystals with various [Li]/[Nb] ratios

Ce:Fe:LiNbO₃ crystals with various [Li]/[Nb] ratios were grown by the Czochralski method from melts having compositions varying between 48.6 and 58 mol% Li₂O. The Ce, Li and Nb concentrations in the grown Ce:Fe:LiNbO₃ crystals were analyzed by the inductively coupled plasma atomic emission spectrometer (ICP-AES). It was found that as the [Li]/[Nb] ratio increases in the melt, the [Li]/[Nb] ratio in the crystal and the distribution coefficients of Ce ions increase also. The photorefractive properties of the Ce:Fe:LiNbO₃ crystals were experimentally studied by the two-wave coupling method. The results show that as the [Li]/[Nb] ratio increases, the dynamic range decreases, but the photorefractive sensitivity and the signal-to-noise ratio improve. In a coherent volume 0.192 cm³ of a Ce:Fe:LiNbO₃ crystal with [Li]/[Nb] ratio of 1.2, 3800 holograms with 800×600 pixels have been successfully multiplexed in a compact volume holographic data storage system.