掺杂对铌酸锂晶体非挥发全息存储性能的影响

通过研究掺镁、掺锌和掺铟同成分铌酸锂晶体的紫外-红光双色全息存储性能,发现双色记录响应时间均比单色记录时明显缩短,最多的可减小3个数量级;双色记录灵敏度大幅度提高,在掺镁5 mol.%的晶体中可达到11 cm/J.在掺杂浓度超过抗光损伤阈值的铌酸锂晶体中,均可实现非挥发全息存储.但是,在掺镁、锌样品中,深、浅能级中心上的光栅反相,而在掺铟样品中则表现为同相.这是由于掺杂离子的种类不同,在铌酸锂晶体中形成的缺陷中心也不同所引起的. 关键词： 掺杂 铌酸锂晶体 非挥发 全息存储     

Photorefractive properties of iron-doped lithium tantalate crystals

Iron-doped lithium tantalate crystals are grown by the Czochralski method and their photorefractive properties are examined with holographic methods. Dynamic range, holographic sensitivity, photoconductivity, and dark storage time are measured in dependence on the iron concentration and light intensity. The largest refractive-index change for ordinarily polarized light is 3.5×10^-4, in comparison with 6.2×10^-4 for iron-doped lithium niobate. Due to a small mobility of protons the dark storage time of holograms in lithium tantalate is larger than that in lithium niobate. PACS 42.40.Pa; 42.70.Ln     

High-efficiency nonvolatile holographic storage with two-step recording in praseodymium-doped lithium niobate by use of continuous-wave lasers

We have observed efficient two-photon, two-step recording in a praseodymium-doped lithium niobate crystal by use of cw lasers. Single-photon erasure during the readout at near-infrared wavelengths was found to be negligible. Nonvolatile holographic image storage was demonstrated. This progress is an important step in the realization of an economically feasible nonvolatile read-write holographic recording system based on low-cost semiconductor diode lasers.     

Superior real-time holographic storage properties in doped potassium sodium strontium barium niobate crystal

We demonstrate superior holographic storage performance in a cobalt doped potassium sodium strontium barium niobate (Co:KNSBN) crystal that possesses a fast response time of 1.4 ms, a large photorefractive sensitivity of 13 x 10(-3) cm(3) J(-1) under a total writing intensity of 1 W/cm(2) , and high spatial resolution of 45 line pairs/mm. Reconstructed images with high fidelity have been obtained in real-time holographic storage. The dynamic properties of the index grating, the dependence of response time on writing intensity, and the dark decay of diffraction signal with increased writing intensity indicate that two species and shallow traps exist in Co:KNSBN crystal.     

Multilayer volume holographic optical memory

We demonstrate a scheme for volume holographic storage based on the features of shift selectivity of a speckle reference-wave hologram. The proposed recording method permits more-efficient use of the recording medium and yields greater storage density than spherical or plane-wave reference beams. Experimental results of multiple hologram storage and replay in a photorefractive crystal of iron-doped lithium niobate are presented. The mechanisms of lateral and longitudinal shift selectivity are described theoretically and shown to agree with experimental measurements.     

Two-photon apodization in lithium niobate

We demonstrate a novel apodization technique for holographic data storage using two-photon recording in stoichiometric lithium niobate. The gating light-intensity profile is used to achieve grating apodization inside the bulk of the crystal during recording in the transmission geometry. Experimental Bragg-selectivity curves and theoretical fits indicate a >20-dB drop in multiplexing cross talk.     

Fast electrically switchable holographic optical elements in LiNbO3

We present here the realization of fast electrically switchable holographic optical elements based on electric field multiplexing of volume holograms in lithium niobate crystals. We demonstrate the electrical control of holographic lenses and holographic mirrors for fast switching of the focal length and the direction of the reconstructed light beam, respectively. The switching time in the range of few hundred microseconds has been demonstrated using this technique.     

Laser-induced preferential domain nucleation in hafnium-doped congruent lithium niobate crystal

The focused visible laser-induced preferential domain nucleation effect is investigated in 3 mol% hafnium-doped congruent lithium niobate crystal. The local phase variation is in-situ monitored during laser-induced preferential domain nucleation. The variations of phase distributions are reconstructed by digital holographic interferometry. The nucleation field decreases exponentially with increasing irradiation intensity. The space charge field along the z direction is thought to be an important mechanism for the laser-induced preferential domain nucleation. Laser-induced hafnium-doped congruent lithium niobate crystals appear to be a promising candidate for further development of ferroelectric domain engineering.     

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.     

Simultaneous occurrence of beam deflection, holographic recording, and self-interference in one lithium niobate crystal

Coupling of the acousto-optic effect and the photorefractive effect in a magnesium-doped lithium niobate crystal was studied by a holographic recording technique. The process of self-interference that is due to the coupling of these two effects was observed. Our results demonstrate that beam splitting and deflection, holographic recording, and self-interference can occur simultaneously in one crystal.     

Nonvolatile two-color holographic recording in near-stoichiometric lithium niobate crystals gated by incoherent ultraviolet light

Nonvolatile two-color holographic recording gated by incoherent ultraviolet （UV） light centered at 365 nm is investigated in near-stoichiometric lithium niobate crystals. The influence of thermal treatment on the two-color recording is studied. The results show that thermal reduction tends to improve the two-color recording performance, whereas thermal oxidation degrades the two-color recording. With an incoherent 0.2-W/cm2 UV gating light and a 0.25-W/cm2 semiconductor recording laser at 780 nm, a two-color recording sensitivity of 4 × 10^-3 cm/J and a recording dynamic range characterized by M/# of 0.12 are achieved in a 2.2-mm thermally reduced near-stoichiometric lithium niobate crystal. We attribute the improvement to the prolonged lifetime of small polarons and the increased absorption at the gating wavelength due to thermal reduction.     

Activation Energy of Proton Migration in Mn- and Fe-Doped Lithium Niobate Obtained by Holographic Methods

The activation energy of thermal fixing of photorefractive gratings is determined in congruent and nearly stoichiometric lithium niobate crystals, both doped with iron or manganese. The novel technique called holographic scattering method is compared with the standard two-wave mixing method. A measurement of the angular distribution of the self scattered intensity and its possible analytical function is presented. The mathematical problems of the holographic scattering method are discussed applying the angular distribution functions.     

Experimental and simulated performance of lithium niobate 1–3 piezocomposites for 2 MHz non-destructive testing applications

Lithium niobate piezocomposites have been investigated as the active element in high temperature resistant ultrasonic transducers for non-destructive testing applications up to 400 °C. Compared to a single piece of lithium niobate crystal they demonstrate shorter pulse length by 3×, elimination of lateral modes, and resistance to cracking. In a 1–3 connectivity piezocomposite for high temperature use (200–400 °C), lithium niobate pillars are embedded in a matrix of flexible high temperature sealant or high temperature cement.In order to better understand the design principles and constraints for use of lithium niobate in piezocomposites experiments and modelling have been carried out. For this work the lithium niobate piezocomposites were investigated at room temperature so epoxy filler was used. 1–3 connectivity piezocomposite samples were prepared with z-cut lithium niobate, pillar width 0.3–0.6 mm, sample thickness 1–4 mm, pillar aspect ratio (pillar height/width) 3–6, volume fraction 30 and 45%. Operating frequency was 1–2 MHz.Experimental measurements of impedance magnitude and resonance frequency were compared with 3-D finite element modelling using PZFlex. Resonance frequencies were predicted within 0.05 MHz and impedance magnitude within 2–5% for samples with pillar aspect ratio ⩾3 for 45% volume fraction and pillar aspect ratio ⩾6 for 30% volume fraction. Laser vibrometry of pulse excitation of piezocomposite samples in air showed that the lithium niobate pillars and the epoxy filler moved in phase. Experiment and simulation showed that the thickness mode coupling coefficient k_t of the piezocomposite was maintained at the lithium niobate bulk value of approximately 0.2 down to a volume fraction of 30%, consistent with calculations using the (Smith and Auld, 1991 [1]) model for piezocomposites.     

Lifetimes of thermally fixed holograms in LiNbO(3):Fe crystals

The thermal erasure decay at different temperatures of holographic gratings thermally fixed in iron-doped lithium niobate has been exhaustively studied. An activation energy of 0.94 eV was found to be independent of the grating fringe spacing. At a given temperature the decay time is strongly dependent on the grating spacing. This dependence is well fitted to a square fringe spacing law, as predicted theoretically. From the experimental data the room-temperature hologram lifetime under illumination is extrapolated, giving as a result t(years)=(5.2+/-0.2)?(2) , with the grating spacing ? in micrometers.     

Grating spacing dependence of nonvolatile holographic recording with arbitrary charge transport lengths

Grating spacing dependence of nonvolatile holographic recording in doubly doped lithium niobate crystals is theoretically investigated allowing arbitrary charge transport lengths. It is shown that the nonvolatile refractive index modulation initially increases with increasing grating spacing, then a saturation behavior arises because of the dominant bulk photovoltaic effect. Although different charge transport length results in different nonvolatile refractive index modulation, the grating spacing dependence of nonvolatile holographic recording obeys almost the same rules for arbitrary charge transport lengths. The experimental results obtained by recording nonvolatile holograms in LiNbO₃:Cu:Ce crystals with different grating spacing are consistent with the theoretical analyses.     

Cu-diffused layers in LiNbO3 for reversible holographic storage

Some holographic recording characteristics of Cu-diffused layers fabricated in Rh-doped LiNbO₃ have been examined in comparison with those fabricated in undoped lithium niobate.The diffusion layers, whose thickness depends on the diffusion temperature, are found to be enormously susceptible to optically-induced refractive index changes and to be able to attain a high diffraction efficiency. A distinguished difference is found between the persistence of holograms stored in Rh-doped and undoped LiNbO₃.     

Oscillation of nonvolatile holographic recording in LiNbO3:Fe:Cu crystals

The oscillation of diffraction efficiency is observed in the nonvolatile holographic recording of lithium niobate crystals doped with iron and copper. The physics of oscillation in doubly doped lithium niobate crystals is studied by using Runge–Kutta methods, and the oscillation can be attributed to the redistribution of electrons in the deeper and shallower traps of the crystals in the initial phase of holographic recording. The effects of Fe concentration and intensity ratio of red beams to UV beam (I_R/I_UV) on the oscillation are investigated theoretically. The results show that with lower Fe concentration, the amplitude of oscillation is larger and with lower intensity ratio I_R/I_UV, the duration of the oscillation is longer.     

Fast photorefractive response of vanadium-doped lithium niobate in the visible region

A series of vanadium-doped lithium niobate crystals was grown and their photorefractive properties were investigated with a 532 nm laser. At a total light intensity of 471 mW/cm(2), a short response time of only 0.57 s was achieved for 0.1 mol.% vanadium in LiNbO(3). The photorefractive process is dominated by the diffusion field instead of the photovoltaic field. The dominant charge carriers are electrons. The possible mechanism for the fast photorefractive response is discussed.     

双掺杂近化学计量比铌酸锂晶体的全息光学特性研究

在LiNbO3中掺入0.2 mol％ MnO和0.1 mol％ Fe2O3,采用顶部籽晶法生长了双掺杂近化学计量比铌酸锂晶体.紫外吸收测试结果表明,晶体成分趋近于化学计量比.采用二波耦合光路测试了晶体的光折变性能.晶体的指数增益系数达到28 cm－1,衍射效率为68.3％,响应时间为亚秒级.利用晶体进行了体全息存储实验,实验结果显示,双掺杂近化学计量比晶体的图像存储质量明显好于相同掺杂的同成分晶体,记录速度较同成分晶体提高了二个数量级.     

OH~- absorption and nonvolatile holographic storage properties in Mg:Ru:Fe:LiNbO_3 crystal as a function of Mg concentration

Mg:Ru:Fe:LiNbO3 crystals with various concentrations of MgO (in mole) and fixed content of RuO2 and Fe2O3 (in mass) are grown with the Czochralski method from the congruent melt. Their infrared transmission spectra are measured and discussed to investigate the defect structure. With the increase of Mg2+ concentration the blue nonvolatile holographic storage capability is enhanced. The nonvolatile holographic storage properties of dual-wavelength recording of Mg(7 mol%):Ru:Fe:LiNbO3 nonvolatile diffraction efficiency, response time, and nonvolatile sensitivity reach 59.8%, 70 s, and 1.04 cm/J, respectively. Comparing Mg(7 mol%):Ru:Fe:LiNbO3 with Ru:Fe:LiNbO3 crystal, the response time is shortened apparently. The nonvolatile diffraction efficiency and sensitivity are raised largely. The mechanism in blue photorefractive nonvolatile holographic storage is discussed.