Control of sensitivity to hologram storage in LiNbO3 using an accessory photovoltaic crystal

Enhancement of sensitivity to hologram storage in a LiNbO₃ crystal is reported by using an accessory photovoltaic LiNbO₃ crystal, highly iron doped and reduced. With an illumination of 600 mW/cm² at λ=0.48 μm of the photovoltaic crystal the photogenerated electric field is about 10 kV/cm. This field is applied to a storage crystal having a higher resistivity and improves substantially the photorefractive sensitivity and saturation of the diffraction efficiency.     

Theory of 2×2 switch based on electro-optic and piezoelectric effects in photorefractive LiNbO3 crystal

Based on the electro-optic and piezoelectric effects, the principle of a 2×2 bypass-exchange switch in photorefractive LiNbO₃ crystal is discussed. In photorefractive volume grating, the Bragg condition can be changed by applying a field during readout. That is, by applying a specific field E_s or zero, the diffraction efficiency will be 0 or 1, respectively and this property can be used to realize the bypass or exchange operation of the switch. In this paper, considering both the piezoelectric and electro-optic effects caused by the applied electric field, we analyzed relations of both the specific field and the incident intensity ratio with respect to the writing angles and polarizations of writing beams.     

Feedback controlled periodic states for different kinds of photorefractive nonlinear response

We investigate analytically and numerically the influence of the type of the photorefractive nonlinear response on the periodic states (attractors) which occur during feedback controlled 2W-coupling and correspond to almost 100% diffraction efficiency of the dynamic index grating. In addition to the case of the local response typical, for example, for LiNbO₃ crystals we consider the cases of nonlocal (diffusive) response (BaTiO₃, SBN) and resonant response (DC-biased BSO, BTO, and BGO crystals). It is shown that the conditions for the transition to the periodic states and their apparent characteristics are strongly different for the two limiting cases above. Received 16 July 2002 / Received in final form 29 October 2002 Published online 4 March 2003     

Photorefractive centers in LiNbO3, studied by optical-, Mössbauer- and EPR-methods

The photorefractive properties of LiNbO₃∶Fe and LiNbO₃∶Cu have been studied in combination with optical absorption-, Mössbauer- and EPR-measurements. The charge states of Fe in successively reduced LiNbO₃∶Fe have been investigated with respect to the influence on the photorefractive sensitivity and saturation value of the refractive index change. The results of this experiment demonstrate clearly the close correlation between the concentration of Fe²⁺ impurities and the optical absorption band around 2.6 eV in LiNbO₃∶Fe, which is known to give rise to an anisotropic charge transport upon optical excitation. The resulting photocurrents determine the photorefractive sensitivity mainly in the initial state of halographic exposure. With increasing conversion from Fe³⁺ to Fe²⁺ the photorefractive sensitivity saturates and the saturation value of the refractive index change decreases remarkably. In the case of LiNbO₃∶Cu a similar behaviour of the photorefractive storage parameters after successive reduction treatments has been observed qualitatively. However, in contradiction to LiNbO₃∶Fe the Cu²⁺ centers cannot be related to the photorefractive sensitivity of LiNbO₃∶Cu. These results are discussed with respect to the predictions of two models concerning the microscopic nature of the photorefractive process in doped LiNbO₃.     

High-sensitivity read-write volume holographic storage in reduced KNbO3 crystals

Reduced KNbO₃ is a photoconductive ferroelectric in which holograms can be recorded by the photorefractive effect. Read-write volume hologram storage and erase sensitivities ofS ⁻¹=100 J/cm² andS ⁻¹=84 J/cm² (S=d(Δn)/d(I₀t)‖_t=0) have been measured at zero applied electric field, where the charge transport is shown to be due to diffusion of photoexcited electrons. By applying an electric field along thec-axis, the migration length of the photoexcited electrons becomes comparable to the holographic grating spacing. This leads to storage sensitivities comparable to high-resolution photographic plates. Experimental data on storage and erase sensitivity as a function of the grating spacing, applied electric field, writing light intensity and temperature are reported and interpreted on the basis of the theoretical results of Young et al. and Amodei. Changes of the intensity ratio of the writing beams by self diffraction (beam coupling), reflections from surfaces and the residual dark conductivity are assumed to cause experimental results which deviate from the theoretical models. It is shown, that in reduced KNbO₃ and other ferroelectric photoconductors having photocarrier transport lengths much larger than the unit cell dimension, photovoltaic currents do not contribute significantly to the build-up of space-charges leading to the photorefractive effect.     

Application of interference microscopy to the study of hologram build-up in LiNbO3 crystals

Interference microscopy was applied to direct microscopic observation of temporal evolution of phase holograms in LiNbO₃:Fe photorefractive crystals. First a hologram was recorded in the sample, and diffraction efficiency was monitored during hologram build-up using inactinic laser light. Thus kinetics of hologram build-up could be determined. The initial hologram was erased using white light. Then a series of write-erase cycles were performed with increasing exposure times. Holograms were observed by interference microscope after each exposure. The time elapsed between the exposure and the microscopic observation was negligible compared to the relaxation time of the hologram. The obtained temporal evolution of the grating profile gives a deeper insight into the physical mechanism of hologram formation in photorefractive materials than simple diffraction efficiency measurements. A congruently grown sample of LiNbO₃ doped with 10⁻³ mol/mol Fe in melting was studied by this method. Sample thickness was set to 300 μm to allow correct microscopic observation. Plane-wave holograms were recorded in the samples using an Ar-ion laser at λ = 488.0 nm of grating constants of 3, 6.5 and 8.8 μm.     

Photorefractive effects in LiNbO3:Fe under external electric fields

The photorefractive properties of LiNbO₃:Fe crystals are studied in the near uv-region under the influence of externally applied electric fields. Improvements of sensitivity have been obtained in oxidized samples showing large dark storage times.     

Electric-field enhancement of beam coupling in Sn2P2S6

The influence of an external field on photorefractive recording in Sn₂P₂S₆ (SPS) crystals is studied. A large gain factor of more then 15 cm^-1 is achieved for a grating spacing of 12 μm at λ=0.9 μm. For an applied field exceeding ±200 V/cm a switching of the beam coupling direction is detected, exhibiting a pronounced hysteresis. Received: 25 October 2000 / Revised version: 18 January 2001 / Published online: 21 March 2001     

High photorefractive sensitivity at 860 nm in reduced rhodium-doped KNbO(3)

We show that high-temperature reduction in a CO-CO(2) atmosphere increases the photorefractive sensitivity of KNbO(3):Rh at 860nm by 4 orders of magnitude compared with that of the as-grown crystal. The effective trap density is increased by a factor of 3, and the photoconductivity by a factor of 30, and the photorefractive response at a grating spacing of 0.15 mu;m is accelerated by a factor of 400. The grating buildup time at a grating spacing of 0.7 microm and an intensity of 1Wcm(-2) is 0.5 s, a value comparable with that of as-grown KNbO(3):Fe at visible wavelengths. The optical and photorefractive parameters of Rh-doped KNbO(3) subjected to reduction treatment are characterized for wavelengths of 0.48-1.064 microm .     

Grating formation in Fe:LiNbO<Subscript>3</Subscript> photorefractive crystal by chirped 800-nm femtosecond laser pulses operated at 1 kHz

We demonstrate an extension of forming a photorefractive volume grating in an Fe:LiNbO₃ crystal by chirped intense femtosecond laser pulses generated from a Ti:Al₂O₃ regenerative amplifier at 1 kHz. We confirm that one-photon absorption is still dominant in the Fe:LiNbO₃ crystal up to 70 GW/cm². To generate a photorefractive grating at such a low laser repetition rate within a practical writing time, the laser pulse intensity is increased to >30 GW/cm². Furthermore, we demonstrate the amplification of femtosecond laser pulses by the chirped volume grating that is written in the Fe:LiNbO₃ by two-wave mixing.     

Dynamics of photorefractive response in sillenite crystals with double-ionized donor centers and shallow traps

The dynamics of the photorefractive response in the Bi₁₂TiO₂₀ crystal with double-ionized donor centers and shallow traps is analyzed. Consideration is given to the time dependences of the recording of a photorefractive grating in the absence of an external electric field, its storage in the dark, and its development upon the application of an external field and turning on of a reading light beam.     

运动折射率相位栅和背向光折变散射

本文按照Kukhtarev的光折变理论讨论了在LiNbO₃:Fe晶体内自发形成的运动折射率相位栅和由它所引起的强烈的背向光折变散射。 关键词：     

非相干背景光辐照对LiNbO3∶Fe与LiNbO3∶Fe∶In晶体中二波耦合的控制

理论分析和实验观测了非相干背景光辐照下掺杂LiNbO₃晶体中的二波耦合,发现非相干背景光能够在很大范围内灵活有效地控制信号光增益,抑制扇形效应,提高信噪比,缩短光栅的建立时间.入射光光强比一定时,适当地增大背景光光强可以使光栅响应时间减小一个数量级.尤其对于抽运光光强远大于信号光光强的情况下,上述作用更为明显. 关键词： 光折变 二波耦合 非相干背景光     

Diffraction efficiency and energy transfer during hologram formation in reduced KNbO3

Volume phase-hologram formation by the photorefractive effect in KNbO₃ is accompanied by a stationary energy transfer between writing beams. The change in energy transfer by applying an electric field on the reduced crystals is shown to be due to an efficient increase in migration length which can reach values comparable or larger than the fringe spacing. It is demonstrated that photovoltaic contribution to the diffraction efficiency and energy transfer is rather small in reduced KNbO₃ and that diffusion of photogenerated free holes is the dominant charge transport for the photorefractive effect in unbiased crystals. The experimental results for diffraction efficiency and energy transfer as a function of grating spacing, electric field, light intensity and temperature is well described by a recent theory of Kukhtarev and Vinetskii.     

Photorefractive effects in long, narrow BSO crystals with applied electric field

In a photorefractive Bi₁₂SiO₂₀ crystal with high applied electric ac field of square-wave shape a fast two-wave coupling response (less than 1 s) and a slow hologram readout decay (minutes) was found for a wavelength of 633 nm. This can be explained by electron–hole transport with two trap levels. An intensity dependence of the slower complementary grating was found. Illuminating with the readout wave without applied electric field leads to a very slow grating decay (many hours). Received: 18 November 1998 / Revised version: 3 February 1999 / Published online: 7 April 1999     

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.     

Enhancement of the diffraction efficiency of the hologram recorded in photorefractive media under the action of external electric field

Diffraction efficiency as a function of the applied electric field for the non-linear regime has been calculated by solving numerically the beam coupling equations. The refractive index variation used in the beam coupling equations was calculated directly from the material rate equations via the total space charge field. The diffraction efficiency of the holograms recorded in photorefractive media is not only a function of the applied external electric field but also a function of crystal thickness, diffusion field, reduced fringe contrast modulation ratio and absorption coefficient of the materials. The effects of these parameters on the efficiency of the holograms have been studied in details. In the absence of the external applied field, it is found that the diffraction efficiency of the holograms could be maximized for a thinner photorefractive crystal having lower absorption coefficient and higher value of diffusion field, which could exist at a much lower value of reduced fringe contrast modulation ratio of the index grating. More efficient holograms can be recorded in the presence of the externally applied electric field as compared to the case of no external field.     

Ultraviolet photorefractive effect in Mg-doped near-stoichiometric LiNbO3

The ultraviolet photorefractive effect of Mg-doped near-stoichiometric LiNbO₃ crystals prepared by vapor transport equilibration (VTE) technique was studied at 351 nm. It was found in the near-stoichiometric LiNbO₃ crystals that the ultraviolet photorefractive effect could be enhanced greatly with the increase of Mg concentration. Based on the activation energy of dark decay of the photorefractive grating, possible centers responsible for the ultraviolet photorefractive effect were also discussed.     

Photorefractive effect in sillenite crystals with shallow traps in a sign-alternating electric field

The results of a theoretical analysis of the photorefractive response in crystals with shallow traps to in a sign-alternating, square-wave electric field are presented. The numerical analysis method developed imposes no restrictions on the frequency of the external field and the period of the photorefractive grating. The parameters characterizing deep donor and shallow trap centers are estimated on the basis of investigations of two-beam interaction in a Bi₁₂SiO₂₀:Cd crystal with the application of a sign-alternating, square-wave electric field. Fiz. Tverd. Tela (St. Petersburg) 40, 2037–2043 (November 1998)     

LiTaO3 as holographic storage material

The optical storage properties of LiTaO₃:Fe are investigated and compared with those of the isomorphous compound LiNbO₃:Fe. Absorption, photocurrent, photoconductivity and holographic measurements are reported. In the case of photovoltaic writing similar results for LiTaO₃- and LiNbO₃-crystals are obtained. However, in the case of photoconductive writing using external electric fields LiTaO₃:Fe-crystals yield much better results due to large photoconductivity values. Considering the recording sensitivity and the extremely large storage time LiTaO₃:Fe turns out to be one of the most promising materials for photorefractive storage of volume phase holograms.