Ultraviolet laser-induced sub-micron periodic domain formation in congruent undoped lithium niobate crystals

We report the formation of ordered sub-micron periodic surface domains on the –z face of congruent undoped lithium niobate single crystals induced by pulsed ultraviolet laser illumination of the sample faces under specific irradiation conditions. We demonstrate the utility of this simple light-induced technique for achieving periodic domain inversion and investigate the nature and spatial structure of these nano-domains by scanning force microscopy. We also demonstrate subsequent re-inversion of a small region of these light-induced nano-domains using scanning force microscopy.     

掺铁铌酸锂晶体的光电导衰减特性研究

利用电化学分析仪对掺铁同成分铌酸锂晶体进行瞬态光电导研究. 以不同掺铁浓度铌酸锂晶体为样品, 在不同强度的纳秒级脉冲光照射下对光电导的研究发现: 铌酸锂晶体的瞬态光电导是由较为复杂的电子迁移过程形成, 其衰减可以用一个指数函数叠加一个扩展指数来拟合. 拟合参数与光强、掺铁浓度存在以下依赖关系: 入射光强增强时, 幅值σ₁^max、σ₂^max、时间常数τ₂和扩展因子β 值增大, 在光强增大到一定时, τ₂和β出现饱和; 晶体的掺铁浓度升高时, σ₁^max、σ₂^max、τ₂ 值增大, 而β值减小. 根据实验结果, 从理论上提出了光电子导带迁移伴随光电子在小极化子上跳跃迁移的复合电荷传输模型. 该模型较好地解释了掺铁同成分铌酸锂晶体的光电导的衰减特点.     

Electro-optic and dielectric properties of Hafnium-doped congruent lithium niobate crystals

We report the optical and dielectric properties in hafnium (Hf)-doped lithium niobate (LN) crystals. We investigated samples of congruent composition with various doping concentration varying from 0 to 8 mol%. The clamped and unclamped values of the electro-optic coefficient r ₂₂ of Hf-doped LN and the corresponding dielectric permittivity as well, have been experimentally determined and compared with the results obtained in undoped congruent LN crystals. We show that the electro-optic and dielectric properties are only slightly affected by the introduction of hafnium ions, and therefore Hf-doped LN has the advantage of low photorefractive damage compared with the undoped congruent LN.     

Observation of ultra-long lifetime UV-induced infrared absorption in undoped and magnesium-doped congruent lithium niobate crystals

We report experimental observation of ultra-long lifetime UV-induced absorption in undoped and 5 mol.% Mg-doped congruent lithium niobate crystals between 400 nm and 2500 nm wavelength range. The saturated absorption coefficient change in undoped congruent lithium niobate was about 3 to 7 times that in Mg-doped congruent lithium niobate in the mid-infrared spectrum. The UV-illuminated LiNbO₃:Mg was fully recovered in the dark after 2 days, whereas the undoped congruent LiNbO₃ still exhibited nontrivial infrared absorption. We attribute the cause of the UV-induced absorption to the creation of hole-trapped intermediate states O⁻ near lithium vacancies.     

Light absorption in undoped congruent and magnesium-doped lithium niobate crystals in the visible wavelength range

Light absorption measurements of nominally undoped congruent lithium niobate crystals (CLN) as well as 5 mol% magnesium-oxide-doped lithium niobate (MgO:LN) crystals were performed in the light wavelength range of 350 to 800 nm. Absorption spectra reveal that—besides iron (Fe) impurities—chromium (Cr) impurities of less than 0.5 wt. ppm concentration contribute significantly to the total optical absorption in the CLN crystals with a maximum of 0.035 cm⁻¹ around 500 nm. The axial distribution of Cr within a CLN boule is examined, revealing that the bottom part of the boule contains less Cr and therefore light absorption is reduced as well. In the case of the MgO:LN crystals, Cr impurities also contribute significantly to the total optical absorption, which is on the order of 0.025 cm⁻¹ for ordinarily polarized light and 0.015 cm⁻¹ for extraordinarily polarized light around 500 nm.     

Low-temperature electrical conductivity of congruent lithium niobate crystals

The electrical conductivity of lithium niobate crystals was investigated at temperatures between 80 and 450 K as a function of the oxidation-reduction annealing conditions. The results are interpreted in terms of polaron electrical conductivity at room temperature and above. A reduction in the measurement temperature leads to “freezing out” of small-radius polarons, and hopping of Heitler-London bipolarons via unfilled Nb_Li sites becomes the main mechanism responsible for the electrical conductivity. Fiz. Tverd. Tela (St. Petersburg) 40, 1307–1309 (July 1998)     

Charge transport in highly iron-doped oxidized lithium niobate single crystals

The photorefractive properties of highly iron-doped lithium niobate crystals in as-grown and completely oxidized states are investigated. Iron concentrations range from 0.5 to 3 wt. %. Measurements of light-induced refractive-index changes, two-beam coupling gain, bulk-photovoltaic current densities, and conductivities are performed with visible light. Absorption spectra and the bulk photovoltaic currents are used to determine the Fe²⁺ concentration. The dark conductivity confirms the tunneling model for charge-carrier migration. Beam-coupling experiments as well as photocurrent measurements with highly oxidized samples indicate that the one-center model is valid in such highly doped crystals, but that hole conductivity dominates the charge transport. PACS 72.20.Jv; 72.40.+w; 81.40.Tv     

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.     

Direct ultraviolet writing of channel waveguides in congruent lithium niobate single crystals

We report the fabrication of optical channel waveguides in congruent lithium niobate single crystals by direct writing with continuous-wave ultraviolet frequency-doubled Ar+ laser radiation (244 nm). The properties and performance of such waveguides are investigated, and first results are presented.     

Temperature investigations of Raman spectra of stoichiometric and congruent lithium niobate crystals

In the temperature range 100–450 K, we have investigated Raman spectra of congruent and stoichiometric LiNbO₃ crystals. We have found that, in this temperature range, frequencies and widths of all the spectral lines depend linearly on temperature. However, the width of the line that corresponds to vibrations of the A₁(TO) symmetry of Li⁺ ions depends on temperature much more weakly than the width of the line that corresponds to vibrations of the A₁(TO) symmetry of Nb⁵⁺ ions. This fact indicates that the anharmonicity of vibrations of Nb⁵⁺ ions along the polar axis is much stronger compared to vibrations of Li⁺ ions. It is likely that this anharmonicity is noticeably contributed by O^2? ions, which are characterized by an anharmonic potential, vibrations of which, according to calculations from first principles, are mixed with vibrations of Nb⁵⁺ ions. The anharmonicity of vibrations of O^2? ions is evidenced by a strong temperature dependence of the width of the line that corresponds to vibrations of the A₁(TO) symmetry of O^2? ions perpendicularly to the polar axis. We have found that the temperature dependence of the intensity of lines that correspond to fundamental vibrations is nonmonotonic. At the same time, the temperature dependence of the intensity of “superfluous lines” is strictly linear. It is likely that this behavior of the intensities of lines of fundamental vibrations is related to the occurrence of clusters and microstructures in the crystal structure.     

Specific features of the dark conductivity in lithium niobate crystals of congruent composition

The temperature dependence of the dark conductivity in a series of doped and nominally undoped LiNbO₃ crystals of congruent composition is studied. The activation energies for the ionic and electronic contributions to the dark conductivity are determined, and the H⁺, ion diffusion coefficient is estimated. The correlation between the ionic and electronic contributions to the dark conductivity is established.     

纯铌酸锂晶体红外光谱的低温研究

测量了同成分纯铌酸锂的低温红外光谱,发现低温下铌酸锂晶体将会出现位于3200 cm^-1左右的新红外吸收峰.研究发现该峰与晶体中的氢离子无关,并且其峰强和峰形都随温度的升高发生复杂的变化.基于上述实验结果,认为该峰应该起源于电子在相邻的小极化子（Nb⁴⁺_Li）和自由极化子（Nb⁴⁺_Nb）之间的跃迁.另外,通过拟合发现新红外吸收峰可分解成三个高斯峰,这三峰应归因于能量有细微差别的三种跃迁. 关键词： 铌酸锂 红外吸收光谱 杂质缺陷     

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.     

Etch frustration in congruent lithium niobate single crystals induced by femtosecond ultraviolet laser irradiation

Modification of the chemical etching behaviour of undoped congruent z-cut lithium niobate single crystals is achieved by pre-illumination of the –z face of the crystal with sub-ps ultraviolet laser radiation at 248 nm, at energy fluences below the threshold for ablation. A systematic study of the effect of the energy fluence and total exposure on the etch frustration is presented. Received: 23 July 2001 / Accepted: 28 September 2001 / Published online: 20 December 2001     

Improved ultraviolet photorefractive properties of vanadium-doped lithium niobate crystals

A series of vanadium-doped lithium niobate (LN:V) crystals has been grown and their photorefractive properties were investigated. For 0.1 mol. % V-doped LiNbO(3), a fast photorefractive response of 160 ms was obtained with a 351 nm laser and a total light intensity of 583 mW/cm(2). The measurements of the x-ray photoelectron spectrum and electron paramagnetic resonance show that V(3+), V(4+), and V(5+) ions exist in these LN:V crystals. V(3+) and V(4+) ions correspond to the 420 and 475 nm absorption peaks, respectively. The fast photorefractive response and high sensitivity indicate that LN:V is a suitable candidate for UV photorefractive applications.     

Spectroscopic properties of trivalent chromium ion in lithium niobate crystals

The formulas of crystal field theory are derived for a particular case of the C _3v symmetry group. Convenient and numerically simple methods are proposed to take into account the polarization of the local environment of the Cr³⁺ impurity ion in LiNbO₃. The parameters of intraion interactions for Cr³⁺ in lithium and niobium positions in the lithium niobate lattice and the distortion of the niobium octahedron in lithium niobate, which is caused by the incorporation of trivalent chromium ion, are determined.     

Optical properties of lithium niobate and lithium tantalate crystals with impurities and defects

The photoinduced and Raman scattering in lithium niobate and lithium tantalate crystals with impurities and defects have been studied. An exciting laser beam propagated either along the ferroelectric Z axis or perpendicular to it. The conditions for exciting transverse and longitudinal polar optical modes in Raman spectra are established. The regularities of the excitation of Raman spectra in several polarization geometries (X(ZZ)Y, Z(XX, Y Y)Z, Z(XX, Y Y)Z, X(ZX)Y, X(ZX)X and X(ZX)X) have been investigated. Additional (extra) spectral lines are interpreted as a manifestation of a biphonon enhanced by the Fermi resonance and the result of violation of selection rules for pseudoscalar modes of the A ₂ type due to the reduction of the point symmetry group caused by the presence of impurities and defects in real crystals. The conditions for exciting coherent longitudinal and transverse modes in lithium niobate and lithium tantalate single crystals upon stimulated Raman scattering are analyzed. The temperature evolution of the spectra recorded in the X(ZZ)Y geometry near the ferroelectric phase transition point is explained based on the concept of effective soft mode and analysis of the isofrequency opalescence effect. Strong photoluminescence is found in copper-doped lithium niobate crystals.     

Photorefraction of molybdenum-doped lithium niobate crystals

Molybdenum-doped lithium niobate crystals were grown under different polarization conditions and their holographic properties were investigated. In contrast to current dopants, hexavalent molybdenum prefers niobium sites. Thereby, holographic storage becomes possible from the ultraviolet to the visible with considerably lower response time. The response time of 0.5 mol. % Mo-doped LiNbO(3) can be especially shortened to as small as 0.35 s with a still high saturation diffraction efficiency of about 60% at 351 nm. Molybdenum-doped lithium niobate thus is a promising candidate for all-color holographic storage applications.