New long-lived memory effects in LiNbO3 crystals

This paper describes a new way of recording information by changing the potential contour at the surface of z-cut crystals of LiNbO₃. The effect of various kinds of external perturbations on the rate of decay of optical images written using this method is investigated, along with the possibility of writing information on crystals with transition-metal impurities. Fiz. Tverd. Tela (St. Petersburg) 40, 337–339 (February 1998)     

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.     

化学计量比LiNbO3晶体的激光显微拉曼光谱研究

对用助熔剂提拉法生长的近化学计量比LiNbO3晶体进行了激光显微拉曼光谱测试分析,与同成分LiNbO3晶体相比较,A1(TO)与E(TO)模的谱线数目、频率基本不变,验证了LiNbO3晶体属置换式固溶体的结论.738cm-1处拉曼振动峰（A1(LO)模）的相对强度随晶体中Li2O含量的增加发生了明显的变化,在两种掺杂晶体中,此振动峰已经消失.根据Li空位模型,从占位和结构上进行了分析讨论.在152和872cm-1处拉曼振动峰半高宽随晶体中Li/Nb的增大而变窄,对二者关系进行了线性拟合,利用拟合公式可根据拉曼振动峰半高宽计算晶体中的Li/Nb值. 关键词： 化学计量比 拉曼光谱 线性拟合 半高宽     

Electro-optic properties in undoped and Cr-doped LiNbO3 crystals

3 along the ferroelectric c axis in pure and chromium-doped crystals. We have studied four series of samples with various crystal compositions and doping concentrations to separate the influence of the intrinsic defects related to the non-stoichiometry and the effect of the introduction of chromium. In pure and slightly doped crystals, the electro-optic coefficient r_c shows a non-monotonous dependence on the controlled crystal composition, which reflects a similar behaviour of the corresponding dielectric permittivity. In the highly Cr-doped crystals r_c displays a large decrease with increasing Cr concentration whatever the crystal composition. The results have been discussed by means of the LiNbO₃ structure and the substitution site dynamics. Received: 14 November 1997/Accepted: 2 January 1998     

Two-center holographic recording in highly doped LiNbO3 crystals

In this paper, we report the observation of electron tunneling during two-center recording in LiNbO₃:Fe:Mn doped with high Fe concentration. Unlike tunneling in the singly doped lithium niobate that erases the hologram, the effect in two-center recording can improve the diffraction efficiency of the hologram after the recording while it is kept in the dark. A detailed study of this effect in LiNbO₃:Fe:Mn is presented both theoretically and experimentally. We show that the hologram strength may increase or decrease due to tunneling, depending on the experimental condition of recording. PACS 42.40.-i; 42.70.Ln; 42.40.Lx     

Site spectroscopy of Hf doping in Hf-doped LiNbO3 crystals

By means of Raman scattering spectroscopy we determine the site location occupied by Hf ions in photorefractive damage resistant Hf-doped LiNbO₃. At room temperature the two lowest frequency phonons are related to sites B (Nb) and A (Li), and are used to directly probe the dopant ion. For the low concentration range, the Hf ions go into the sites A whereas, for high concentration, Hf ions occupy both sites A and B. Results are compared with data obtained for pure congruent and stoichiometric crystals, and the proposed mechanism of incorporation of Hf ions in the LiNbO₃ lattice explains the threshold in the behaviour of photorefractive properties.     

Ultraslow shock waves of electron density in LiNbO3 crystals

We show, experimentally and theoretically, that the application of modest voltages, U0=(0.1-1) kV, to LiNbO3ratioFe crystals at sufficiently high temperatures, T approximately (550-700) degrees C, leads to the formation of ultraslow shock waves (moving discontinuities) of the electron density owing to the removal of electrons from Fe2+ centers. Behind the sharp wave front, almost all iron centers are in the Fe3+ state, the sample is optically transparent, and its transport properties are strongly modified. The front velocity decreases during the propagation; it is controlled by the electron mobility.     

Spectral line intensities of Yb3+ ion in LiNbO3 crystals

Optical spectra of absorption and luminescence of LiNbO₃:Yb³⁺ crystals are investigated experimentally and theoretically. Within the framework of the point charge approximation of crystal field, the Stark problem is solved: crystal field parameters and wave functions of Stark states are determined. Line strengths due to indirect electric-dipole and magnetic-dipole inter-Stark transitions are calculated and values of the Judd-Ofelt parameters are determined. Values of the branching ratios of luminescence. Einstein coefficients and, lifetime of the excited sublevel are calculated. A satisfactory agreement of calculated results with experimental data is obtained.     

Domain inversion in LiNbO3 and Zn-doped LiNbO3 crystals by the electron-beam irradiation of the nonpolar Y-surface

Individual domains and domain gratings were fabricated on nonpolar Y-cuts of LiNbO₃ and LiNbO₃-Zn crystals by electron beam irradiation. The domains which nucleated in the irradiation points are frontally growing along the direction +Z within a thin (of about several microns) surface layer. The regularities of this motion are discussed in the framework of the approach to formation of space-charge fields under e-beam charging of insulators. The obtained dependency of the domain length on the exposure time permits us to propose the viscous-friction mechanism for the observed frontal domain growth. The velocity of the frontal growth in LiNbO₃-Zn is higher than in LiNbO₃ obviously due to a decreased number of pinning centers at the Nb-antisites. In LiNbO₃-4 %Zn crystals planar domain gratings were fabricated by means of point-to-point irradiations along the X- and Z-directions with specified distances between the irradiation points. It is shown that the domain gratings are generated by a total field of point charges $ \vec{E} = \sum\nolimits_{i = 1}^{n} {\vec{E}_{i} } $ , where E _i is the space-charge field induced in any irradiation point, and n is the number of points. Some preliminary estimates indicate that the frontal growth of domains under e-beam irradiation occurs at fields E < E _c.     

Domain-inversion characteristics of congruent and stoichiometric LiNbO3 crystals

The characteristics of the ferroelectxic domain inversion structures fabricated by applying voltage at room temperatture in Z-cut congruent LiNbO3 and stoichiometric LiNbO3 crystals were examined. The voltage required for domain reversal in congrueat crystals is about 2.8 times larger than the voltage for stoichiometric crystals. The periodicity of the sstoichiometric crystals are more uniforn than that of the congruent crystals.     

Photorefraction and anisotropic light scattering in LiNbO3-Fe crystals

By holographic recording in LiNbO₃-Fe and LiTaO₃-Fe crystals a new light scattering effect has been observed with an optical indicatrix along the optical axis. The kinetics of the light scattering depends on the intensity, wavelength as well as the polarization of the incident light The holographic volume grating is created by the interference of an incident light and light scattered by crystal inhomogeneities.     

Multiplexing holograms in LiNbO3:Fe:Mn crystals

Persistent holograms are recorded with red light in lithium niobate crystals doped with manganese and iron. Different erasure mechanisms are investigated, and a recording schedule for multiplexing holograms with equal diffraction efficiencies is proposed. To test the recording schedule experimentally, we multiplex 50 plane-wave holograms with the proposed recording schedule.     

Periodic domain structure in Czochralski-grown LiNbO3: Y crystals

A study is reported of the correlation between the impurity distribution and the ferroelectric domain structure in a periodically polarized LiNbO₃: Y crystal grown by the Czochralski method. The domain walls forming near the impurity concentration modulation extrema are shown to be shifted by the temperature gradient below the Curie point. A new positive domain was observed to form near the trace of an electron beam scanning the $\{ 01\bar 10\} $ crystal surface in the course of x-ray microprobe analysis.     

Ultraviolet band edge photorefractivity in LiNbO3:Sn crystals

The ultraviolet (UV) band edge photorefractivity of Sn-doped LiNbO(3) (LN:Sn) at 325 nm has been investigated. A sharp decrease of beam distortion, which is accompanied by a significant increase in the photoconductivity, is observed in LN:Sn crystals with Sn-doping concentrations at or above 2.0 mol%. The diffraction efficiency, the holographic recording sensitivity and response rate, and the two-wave coupling gain coefficient are greatly enhanced when the Sn-doping concentration reaches 2.0 mol% or more. Unlike LiNbO(3) doped with Hf in which the UV gratings can be erased easily by a red beam, the UV gratings in LN:Sn can withstand long-term red beam illumination. Electrons are determined to be the dominant light-induced charge carriers responsible for the UV band edge photorefraction. The observed enhancement on the UV band edge photorefractivity is found to be associated with the showup of an absorption band around 325 nm in LN:Sn crystals with Sn-doping concentrations at or above 2.0 mol%.     

Scattering reduction for highly diffractive holograms in LiNbO3 crystals

We show that self-stabilized recording in photo-voltaic LiNbO₃ crystals produces highly diffractive holograms with a 3- to 4-fold light scattering reduction. Low scattering is probably due to the nonstationary mechanism involved in self-stabilized recording that may not allow parasitic scattering patterns in the crystal to be efficiently recorded.     

Anisotropy of interchain interaction in polymeric (SN)x crystals

The technique of polarized reflection microspectroscopy was used to study anisotropy of polymeric sulfur nitride (SN)_x in plasma edge region. The spectra taken from different faces of single crystals indicate the presence of strong anisotropy of interchain interaction due to orientation of the π-electron orbitals and suggest two-dimensional metallic behavior of the π-electron system in (SN)_x.     

Features of infralow-frequency polarization relaxation processes in LiNbO3 single crystals

The dielectric response of lithium niobate single crystal in ac fields with amplitudes E of 3.74 to 13.1 kV cm^?1 is investigated at frequencies of 1 and 10 Hz in the temperature range of 70 to 200°C. The increase in effective dielectric permittivity ?_eff′ and the effective dielectric losses ?_eff″ is found to be associated with the formation of short-radius Nb _Li ⁴⁺ polarons at temperatures T > 130°C and their contribution to polarization relaxation in a LiNbO₃ crystal.