Raman spectra of photorefractive lithium niobate single crystals

We have conducted comparative studies of the Raman spectra of lithium niobate (LiNbO₃) crystals of different compositions for excitation in the visible and near IR regions. We have observed that the photorefractive effect is one of the factors leading to line broadening. For this reason, the linewidths may be greater upon Raman excitation in the visible region than for excitation in the near IR region. This may be explained by formation in the crystal, when illuminated by laser radiation in the visible region, of a three-dimensional sublattice of nanostructures and microstructures (with refractive index and other physical parameters different from the parameters of the host crystal) from which photorefractive light scattering occurs. Formation of nanostructures and microstructures makes an additional contribution (besides the contribution due to random and dynamic disorder in the arrangement of the structural units) to the broadening of the Raman lines in the visible region of the spectrum. Illumination of the crystal by radiation in the near IR region does not induce a sublattice of nanostructures and microstructures, due to a significantly smaller photorefractive effect.     

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)     

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     

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.     

Charge transport properties of undoped congruent lithium niobate crystals

Time-resolved light-induced birefringence measurements based on a phase compensation technique are used to determine the charge transport properties of congruent, nominally-undoped lithium niobate crystals (LiNbO₃). Some of the crystals are conventionally oxidized. The steady-state space-charge field E _pv, the bulk photovoltaic coefficient β, and the photoconductivity σ _ph are determined in the intensity range (30–30 000) W/cm². The photovoltaic coefficient β increases by one order of magnitude over this intensity range, the space-charge field E _pv even by two orders of magnitude. We discuss the results in the context of the known one- and two-center charge transport models for LiNbO₃. The experimental findings presented here are of relevance for the long-standing problem of optical damage in such crystals, which inhibits their use in high-intensity applications like nonlinear optics.     

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.     

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.     

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.     

A Raman study of single-crystal congruent lithium niobate following electric-field repoling

We describe a study of the time dynamics of the spectral position of six major peaks in the Raman spectrum of single-crystal congruent lithium niobate, following the process of electric-field repoling. All of the peaks observed show a small (<1 cm^-1) frequency shift after repoling. Two peaks are shifted to higher frequencies while the other four are shifted to lower frequencies. The shift generally recovers over time towards the original value. The 153 cm^-1 and 432 cm^-1 peaks are seen to recover to their original positions before poling, with time constants of approximately 8 h and 4 h, respectively, whereas the higher frequency peak at 872 cm^-1 does not appear to recover at all. The other peaks exhibit incomplete recovery. We compare the measured values of temporal recovery with published relaxation times for internal electric fields, and make an additional comparison with our measured etch-rate data for the -z face of lithium niobate as a function of delay time following repoling. PACS 77.84.Dy; 78.30; 77.80.Dj     

Synchronously pumped femtosecond optical parametric oscillator of congruent and stoichiometric MgO-doped periodically poled lithium niobate

A synchronously pumped femtosecond optical parametric oscillator based on congruent MgO-doped periodically poled lithium niobate (c-MgO:PPLN) is reported. The system, operating at room temperature, was pumped by a mode-locked Ti:sapphire laser. The wavelengths of the signal and idler waves were tuned from 870 nm to 1.54 μm and 1.58 to 5.67 μm, respectively, by changing the pump wavelength, the grating period or the cavity length. Pumped by 1.1 W of 755 nm laser radiation, the OPO generated 310 mW of 1080 nm radiation. This signal output corresponds to a total conversion efficiency of 50%. Without dispersion compensation the OPO generated phase-modulated signal pulses of 200 fs duration. Besides the OPO of c-MgO:PPLN, an OPO of stoichiometric (s) MgO:PPLN was investigated. Because of the reduced sensitivity to photorefractive damage, both crystals allowed efficient OPO operation at room temperature. Received: 19 August 2002 / Revised version: 11 December 2002 / Published online: 19 March 2003 RID="*" ID="*"Corresponding author. Fax: +49-631/205-3906, E-mail: andres@physik.uni-kl.de     

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.     

Interrelation of intrinsic and extrinsic defects – congruent, stoichiometric, and regularly ordered lithium niobate

A new approach to the intrinsic and extrinsic defect subsystems, considering them as one integrated functional system, is developed for complex oxides. The strong interrelation of these subsystems becomes especially apparent when concentrations of both defect classes are comparable. A new parameter, reflecting the total intrinsic defect concentration, is introduced to determine the degree of crystal imperfection. The necessity of at least two calibrated samples – congruent and regularly ordered crystals – is substantiated. The accuracy by which the parameter can be determined can serve as a criterion in comparison of different methods of crystal characterisation. A procedure for crystal calibration by several physical characteristics is illustrated. Such a heuristic generalisation of the concept of mutual correlation of the whole defect system and the material only became possible on the basis of experimental and theoretical study of large numbers of LiNbO₃ crystals. The spectra of EPR, NMR, ENDOR, and other properties of crystals with different compositions, diverse modifiers (K, Mg,...), and various probe impurities have been analysed. It is found that crystals with vanishingly small concentration of intrinsic defects offer extraordinary informative opportunities. Their micro- and macroscopic properties are discussed. The developed ideas have a general character; therefore they should also be valid for other non-stoichiometric complex oxides. Received: 3 December 1998 / Revised version: 15 February 1999 / Published online: 12 April 1999     

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.     

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.     

Raman spectra of lithium hydroxide single crystals

First order Raman frequencies, linewidths, intensities and assignments to the symmetry species of all modes predicted by group theory at 20°C and at 78 K are reported for the first time. Assignments of the fundamental frequencies to stretching vibrations or librations of the OH^? ions, internal vibrations of Li⁺ ions of the same layer or translational modes of the Li⁺ and OH^? layers are derived from factor group analysis and correlation methods. They correspond to the constructions of unit cell modes by Wickersheim[2].     

The two photorefractive centres in iron doped nearly stoichiometric lithium niobate crystals

This paper investigates the photorefractive properties of iron doped lithium niobate with different [Li]/[Nb] ratios The experimental results show two photorefractive centres for iron doped near-stoichiometric lithium niobate crystal Besides Fe^2＋ and Fe^3＋ ions, small polarons and bipolarons are considered as another photoactive centre.     

Two-mode nature of the Raman spectrum of lithium niobate crystals

The ordering of the host and impurity cations in nominally pure (with different values of Li/Nbratio) and doped lithium niobate crystals is studied using Raman spectroscopy. It is shown that depending on the composition-controlled ordering of structural units of the cationic sublattice the crystal may exhibit, in the region of bridge stretching vibrations of the oxygen ions, either single-mode or two-mode behavior. The nominally pure lithium niobate crystals show, within the homogeneity region, single-mode behavior, while the crystals doped with divalent or trivalent cations show single-mode behavior at low concentrations of the dopant and two-mode behavior at higher concentrations.     

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.     

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.