Frequency mixing of photorefractive and ferroelectric gratings in lithium niobate crystals

Holographically recorded photorefractive gratings in periodically poled lithium niobate crystals (PPLNs) are investigated. The principal spatial frequency K of the grating is strongly suppressed. Sideband gratings with grating vectors K +/- G appear. From the measurements the domain grating vector G and the duty cycle of the domain structure can be obtained. These findings allow for fast nondestructive quality inspection of PPLN and are of importance for any optical application combining holography and PPLN.     

Dark conductivity in copper-doped lithium niobate crystals

The impact of protons and copper on the dark conductivity of copper-doped photorefractive lithium niobate crystals is investigated by observation of the dark decay of holograms. The dark conductivity is significantly smaller than that of iron-doped lithium niobate. A non-mono-exponential hologram decay is observed, which needs to be considered for multiplexing of holograms. Received: 25 March 2002 / Published online: 2 May 2002     

X-ray-induced photochromic effects in copper-doped lithium niobate crystals

X-Ray-induced absorption changes in Cu-doped lithium niobate crystals (LiNbO₃) are investigated. All induced photochromic effects are fully reversible, i.e., illumination with white light annihilates the absorption changes. With increasing doping level, a decrease in the saturation value of the absorption changes is found. Changes of the Li concentration of the crystal to a more stoichiometric composition via Li indiffusion further decreases the photochromic effect. For sufficient X-ray doses, the saturation value is independent of the radiation energy in the measured range from 50–150 kV. The spectral distribution of the X-ray-induced absorption changes and the slow dark decay indicate that the additional absorption results from an increase of the Cu⁺ concentration. Received: 20 December 2000 / Revised version: 24 January 2001 / Published online: 27 April 2001     

Influence of surroundings on photorefractive effect in lithium niobate crystals

In the paper results of the investigation of the influence of electric properties of the environment surrounding LiNbO₃ crystals on photorefractive effect induced in these crystals by Gaussian Ar⁺ laser beam with various intensities are presented. We show spatial and temporal dependences of changes of the refractive index obtained experimentally in LiNbO₃: Fe and LiNbO₃: Fe:Mn samples surrounded by media with different electric conductivities and different permittivities (water, air, water solution of CaCl₂). The space and time dependences of the refractive index changes induced by the Ar⁺ laser beam are observed by means of the Mach–Zehnder interferometer using light from HeNe laser. The experimentally obtained results are in a good agreement with those following from numerical calculations using the manifold mirroring method. The agreement between calculated and experimental results indicates that the polarization charge at the photorefractive crystal/surrounding medium boundary significantly influences the photorefractive process in the crystal. The experimentally observed slow spontaneous decrease of the refractive index change in a sample placed into a slightly conducting medium (air) after switching off the beam also indicates that the polarization charge in the sample's surroundings affects the photorefraction.     

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.     

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.     

铌酸锂晶体中的磁光折变效应研究

对于掺铁铌酸锂晶体中不同全息记录配置下的磁光折变效应做了比较系统的理论分析，给出了铌酸锂晶体所有的磁光生伏打非零张量元. 详细计算并给出了不同全息纪录配置下的所有体光生伏打、磁光生伏打电流的解析形式. 理论结果表明，由于磁光生伏打效应引起了光激发电流的变化，所以对于每种配置全息光栅的衍射效率都会受到外加磁场的影响；对于不同的全息记录配置，磁场对铌酸锂晶体光折变非线性性质的影响也不同.讨论了一种确定特定张量元的方法. 关键词： 磁光生伏打 磁光折变效应 光生伏打     

Bragg selectivity of space-charge gratings in multidomain lithium niobate crystals

It is shown experimentally and theoretically that a partial domain reversal in ferroelectric lithium niobate crystals containing a previously imprinted photorefractive space-charge grating results in a highly unusual change of the angular Bragg selectivity. This carries important in situ information about domain structures, including the quality of periodically poled samples.     

Study of the subsurface photorefractive effect in lithium niobate

A new phenomenon of a strong irreversible variation of the refractive index (n–0.2) in a skin layer (thickness 300 Å) has been discovered. The effect arises under the strong absorption of high-power uv radiation (249 nm, 5 ns, 20 MW/cm²). The characteristics of the subsurface photorefractive effect (SPRE) were identified by using the data on measurements of the reflection coefficient of a crystal and of the diffraction efficiency of a recorded phase grating.     

Flexible coherent diffraction lithography by tunable phase arrays in lithium niobate crystals

Flexible coherent diffraction lithography is proposed and preliminarily tested by means of an optical phase mask. The phase mask consisted of a two-dimensional hexagonal lattice of reversed ferroelectric domains engineered in a z-cut lithium niobate substrate and was electro-optically tunable. Appropriate phase shift values across adjacent reversed domains were induced by the application of an external electric field along the z-axis of the crystal via transparent electrodes. Photolithographic exposures of the self-imaging near-field diffraction intensity patterns, at various planes corresponding to the Talbot distances, were performed by using different values of the driving electric field signal.     

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-step holographic recording in photorefractive lithium niobate crystals using ultrashort laser pulses

We investigate non-volatile holographic data storage in photorefractive lithium niobate crystals. Infrared picosecond laser pulses are used to write holograms after sensitizing the crystal with blue light from a cw-laser. The dependence of the dynamic range and the photoconductivity on the pulse intensities and the recording wavelength is investigated in detail. The results can be explained by a two-center model if the mean intensity of the laser pulses is considered. We demonstrate that several fixed holograms can be multiplexed by employing the wavelength multiplexing technique.     

Orientational gratings in dye-doped polymer-dispersed liquid crystals induced by the photorefractive effect

We report the observation of orientational gratings induced by the photorefractive effect in dye-doped polymer-dispersed liquid crystals. The photorefractive origin of the grating-induction effect is supported by the results of two-beam coupling experiments and by the possibility of erasing the grating by uniform illumination. For all the samples a stable memory effect was observed.     

Near-field-optical-microscopy studies of micro-modifications caused by femtosecond laser irradiation in lithium niobate crystals

Near-field-optical-microscopy has been used to study the micro-modifications caused by femtosecond laser pulses focused at the surface and in the volume of lithium niobate crystals. We have found experimental evidence of the existence, close to femtosecond ablation craters, of periodic modifications in the surface reflectivity. In addition, the potential application of near-field-optical microscopy for the spatial location of permanent modifications caused by femtosecond pulses focused inside lithium niobate crystals has been also demonstrated.     

Domain switching in strontium-barium niobate crystals investigated by photorefractive self-focusing

The domain switching behaviour of undoped and Ce-doped Sr_0.61Ba_0.39Nb₂O₆ (SBN) crystals is investigated for different experimental conditions. For the investigation the photorefractive self-focusing technique is used. This method is demonstrated to be well suited for determining the polarization state of SBN crystals spatially resolved. We find that the repoling behaviour of nominally pure SBN is spatially inhomogeneous. The crystals are well repoled at the positive electrode, but are depolarized at the negative electrode. The larger the Ce doping concentration is, the smaller the depolarized crystal area. The domain switching depends on the switching velocity, while no influence of the electrode material is observed. This revised version was published online in March 2005. In the previous version, the published online date was missing     

Instability of moving gratings in photorefractive crystals

It is shown that a moving intensity grating in a photorefractive crystal with a sufficiently large lifetimemobility product can give rise to an instability against excitation of weakly damped space-charge waves. Increments and threshold of the instability are investigated for the threedimensional case. Subharmonics and other nonlinear scenarios of saturation are discussed.     

Amplification of holographic gratings in photorefractive crystals

In this work, we have theoretically and experimentally investigated the process of image amplification in photorefractive crystals as a result of the self-diffraction phenomenon. We have found that the amplification process is of the threshold type and is observed only for an angle of convergence of the light rays greater than a threshold value. The gain coefficient is rather high. For the LiNbO₃ crystal, at a wavelength of 0.63 µm the gain coefficient reached 15 dB; at a wavelength of 0.44 µm, the gain was greater than 50 dB.Tomsk State Academy for Control Systems and Radioelectronics. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 22–26, June, 1994.     

Discrete diffraction of light in optically induced bulk and planar photonic superlattices in photorefractive lithium niobate

Experimental multielement planar and channel waveguide structures induced by coherent radiation in bulk samples of photorefractive lithium niobate and planar waveguides on their basis including those formed in a projection scheme with scaling of image dimensions are described. The possibility of linear and nonlinear light localization in optically-induced bulk and planar one-dimensional photorefractive photonic superlattices is demonstrated. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 57–62, September, 2008.     

Photoelectric fields in lithium niobate crystals

Photovoltaic and diffusion fields in nominally pure single crystals of stoichiometric composition (R = Li/Nb = 1) grown in the presence of 6 wt% K₂O flux (LiNbO₃ stoich. K₂O) in nominally pure single crystals of complementary composition (LiNbO₃), complementary single crystals doped with Zn²⁺, Er³⁺ at wavelength of 476, 514.5, 530 nm are defined according to parameters of photo induced light scattering indicatrix. Photo induced changes of crystals’ refractive index are defined.