Electrical fixing in near-stoichiometric lithium niobate crystals

Photorefractive gratings are recorded in lithium niobate crystals in the presence of externally applied electric fields close to the coercive field. After optical erasure a voltage pulse above the poling threshold is applied, and by these means a photorefractive grating reappears that can be erased optically. The results point to electrical fixing, i.e., ferroelectric domain reversal induced by a space-charge field.     

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.     

Localized holographic recording in doubly doped lithium niobate

Persistent holograms are recorded locally with red light in a LiNbO>(3) crystal doped with Mg and Fe. Selective erasure is realized by use of a focused UV sensitizing light. We demonstrate the recording of 50 localized images as well as selective erasure in a 4 mm x 4 mm x 4 mm crystal. A comparison of the total recording time for M holograms obtained with the conventional distributed-volume recording and the localized methods is presented.     

Effect of broadband incoherent light on the photoresponse of lithium niobate crystals

The effect of broadband incoherent light on the local photoresponse of lithium niobate crystals has been experimentally investigated. It is shown that a previously induced electric field affects the dependence of the photovoltage on time and impurity type and concentration. The pyroelectric effect significantly influences the photoprocesses in lithium niobate crystals, especially in the initial stage of illumination.     

Image recording in doped crystals of lithium niobate

In lithium niobate crystals doped with iron, copper, rhodium, and ruthenium ions, the optical image recording is feasible when the illumination gradient is directed parallel to the polar axis of the crystal. Depending on the dopant type, the doping level, and external conditions (for instance, the recording time), the storage time of the image can reach 60 days. The effect of polarization of recording radiation on the contrast of the recorded image is investigated.     

Incremental holographic recording in lithium niobate with active phase locking

Angular-multiplexed hologram recording in iron-doped lithium niobate crystals was carried out with near-infrared light. An incremental recording schedule with active phase locking of the light pattern onto the hologram was used. Continuous and reproducible recording of holograms of equal efficiency was achieved, and a hologram multiplexing number, M/#=2 , for a 5-mm-thick crystal was obtained at a 760-nm wavelength of light.     

Ferroelectric domain inversion in near-stoichiometric lithium niobate for high efficiency blue light generation

LiNbO₃ single crystals with a composition close to stoichiometry ([Li]/[Li+Nb]=0.496), 16 mm in diameter and 40 mm in length were grown by the Czochralski method using K₂O flux. The domain reversal characteristics of near-stoichiometric LiNbO₃ single crystals were investigated. The switching field required for 180° ferroelectric domain reversal in the near-stoichiometric crystal at room temperature was 7.5 KV/mm. This is about one third of the switching field required for conventional LiNbO₃ crystals. Domain reversal (180°) in near-stoichiometric LiNbO₃ samples of 1.0 mm thickness has been achieved. Samples have been evaluated by second harmonic generation and conversion efficiencies of up to 32% have been obtained. Received: 8 November 2000 / Accepted: 29 January 2001 / Published online: 20 June 2001     

Nonvolatile holographic storage in iron-doped lithium tantalate with continuous-wave laser light

Holograms have been recorded in congruent LiTaO(3):Fe with continuous-wave laser light by use of a two-step process. Blue gating light (lambda=488 nm) sensitizes the crystals for holographic recording with red light (lambda=660 nm) of a diode laser. Refractive-index changes of as much as 1.0x10(-5) are achieved for intensities of the red light of 1 W/cm(2) . The saturation values are proportional to the intensity of the writing light. Nondestructive readout with red light is possible, and the holograms remain erasable for blue light.     

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.     

Enhancement of ultraviolet photorefraction in highly magnesium-doped lithium niobate crystals

We investigate UV photorefraction in Mg-doped LiNbO(3) crystals. Strong UV photorefraction is achieved in highly Mg-doped LiNbO(3) crystals with high two-wave mixing gain, fast response, and low noise. It is also demonstrated experimentally that so-called damage-resistant dopants such as Mg are damage resistant only in the visible and that they will enhance photorefraction in the UV.     

Infrared holographic recording in lithium tantalate crystals by means of the pyroelectric effect

Infrared holographic recording in a two-step process is demonstrated in stoichiometric iron-doped lithium tantalate crystals. Through absorption of two intersecting infrared pulses (A = 1064 nm) a temperature grating and thus a modulated pyroelectric field build up. Free electrons, excited by homogeneous light of a shorter wavelength (lambda = 532 nm) drift in this field, and a phase hologram is stored that can be read nondestructively. The change in refractive index depends mainly on the absorption coefficient at the wavelength of the recording light and on the intensity of the infrared light. The proposed method may be extended to telecommunication wavelengths by choice of suitable dopants.     

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.     

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     

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.     

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.     

Femtosecond holography in lithium niobate crystals

Spatial gratings are recorded holographically by two femtosecond pump pulses at 388 nm in lithium niobate (LiNbO3) crystals and read out by a Bragg-matched, temporally delayed probe pulse at 776 nm. We claim, to our knowledge, the first holographic pump-probe experiments with subpicosecond temporal resolution for LiNbO3. An instantaneous grating that is due mostly to the Kerr effect as well as a long-lasting grating that results mainly from the absorption caused by photoexcited carriers was observed. The Kerr coefficient of LiNbO3 for our experimental conditions, i.e., pumped and probed at different wavelengths, was approximately 1.0 x 10(-5) cm2/GW.     

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     

Observation of superluminal and slowdown light propagation in doped lithium niobate crystals

We investigate the group velocity of light in a one-dimensional volume grating inside lithium niobate crystals doped with different impurities. The superluminal and slowdown light propagations are both observed in the crystals. The relationships between the group refractive index and the grating amplitude and phase shift are presented and discussed.     

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.     

The dielectric behaviour of doped near-stoichiometric lithium niobate in the terahertz range

The dielectric properties of near-stoichiometric LiNbO₃:Fe and LiNbO₃:Ce single crystals have been investigated using terahertz time domain spectroscopy in a frequency range of 0.7-1.6 THz at room temperature. When coupled with an applied external optical field, obvious photorefractive effects were observed, resulting in a modulation of the complex dielectric constant for the crystals. The variation in refractive index, |Δn|, had a linear relationship with the applied light intensity, accompanied by a step-like decrease at high intensity. The findings were attributed to the internal space charge field of the photorefraction and the light-induced domain reversal in the crystals.