High diffraction efficiency in SBN with applied fields near the coercive field

We present the experiments and results of our investigation of electrical fixing in SBN:60. We propose an optical method for determining the value of the coercive field in ferroelectric crystals. An interferometric method is used to map the change in the index of refraction with negative applied fields, where the minimum of the index change is an indication of the coercive field. From this experiment, values of 1.55 kV±20 V for the coercive voltage and 147±6 pm/V for the linear electro-optic coefficient are found. Two electrical-fixing techniques that result in very high diffraction efficiencies are presented, discussed and compared to previous publications on electrical fixing in SBN. High diffraction efficiencies of about 95% were achieved with the application of negative fields near the coercive region, during and after holographic recording in the crystal. Received: 6 December 2000 / Revised version: 13 February 2001 / Published online: 27 April 2001     

Dynamic light beam deflection caused by space charge waves in photorefractive crystals

During holographic recording in photorefractive crystals (BSO, BGO, and BTO) by an oscillating interference pattern we observe a strong dynamic deflection of the laser beams reflected from the crystal’s surface. The theoretical treatment shows that this new effect is associated with a nonlinear interaction of space charge gratings resulting in a quasi-homogeneous oscillating space charge field which provides deformations of the crystal via the piezoelectric effect. Received: 3 August 1999 / Accepted 5 August 1999 / Published online: 8 September 1999     

Electromechanical and optical effects in photorefractive materials

From thermodynamic considerations, we derive a consistent set of equations coupling the steady-state photoinduced photorefractive space-charge field to mechanical deformations, dc permittivity, and refractive index changes. For the first time to our knowledge, these equations include piezoelectricity, electrostriction, the elasto-optic effect, the linear and quadratic electro-optic effects, as well as the influence of the volume forces and electric torques. We determine the conditions on the material parameters, for which volume forces and torques are of some importance on the optical properties. These equations resulting from a macroscopic approach are valid whatever the physical microscopic origins of the various effects. Non centrosymmetrical and centrosymmetrical materials are considered. The order of approximation is the lowest possible able to describe the mentioned effects in both types of materials. Received: 18 November 1998 / Revised version: 18 February 1999 / Published online: 12 April 1999     

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     

Estimation of trap concentration in photorefractive crystals by a technique of adaptive holographic interferometry

12 GeO₂₀, Bi₁₂SiO₂₀, Bi₁₂TiO₂₀). Received: 17 June 1998/Revised version: 3 August 1998     

Compact and portable holographic camera using photorefractive crystals. Application in various metrological problems

We present a compact holographic interferometer that uses a photorefractive crystal of the sillenite family as a holographic recording medium. Its development is based on a previous prototype that showed lack of flexibility and portability. We briefly discuss the main improvements leading to a compact device. Applications of this instrument in various metrological problems are shown, among which are two that were not already considered using holography, namely measurement of a thermal expansion coefficient and detection of fingerprints. Received: 20 December 2000 / Revised version: 8 February 2001 / Published online: 20 April 2001     

Holographic study of chain length in photopolymerizable compositions

We describe a study of the free-radical homopolymerization kinetics for a system based on acrylamide, triethanolamine and methylene blue by analyzing temporal variations of the diffraction efficiency. The modulation index is related to the component’s parameters, giving as a result a method that can be used to determine the chain length of the polymer and the kinetic rate constants. By using this method we have studied the effect of intensity on the kinetic parameters of the photopolymerization process. Received: 20 August 2001 / Published online: 7 February 2002     

Temperature distribution and thermal-induced defects in photorefractive barium titanate crystals illuminated with intense cw or pulsed laser irradiations

The temperature distribution in a parallelepipedic crystal irradiated with a Gaussian repartition of light is calculated by solving the three-dimensional heat equation and taking into account the losses through the sides of the samples. The theoretical and experimental maps of temperature are satisfactorily compared. For barium titanate crystals, the resistance to intense cw laser (25.7 kW/mm² at 514.5 nm) and to Nd:YAG pulsed laser (peak power of 9.2 MW at 532 nm) is investigated. The defects induced by the irradiation are analysed. Under pulsed illumination, the damage threshold is found to be 0.54 GW/cm² in a nominally undoped BaTiO₃ and 0.44 GW/cm² in a rhodium-cobalt doped BaTiO₃ crystal. Received: 7 March 2000 / Revised version: 14 April 2000 / Published online: 16 August 2000     

Nonlinearity of a response in photorefractive crystals with a square-wave applied field

The space-charge-field formation in a photorefractive crystal illuminated by a one-dimensional light pattern is studied when an alternating square-wave electric field is applied to the crystal. We derive the general nonlinear equation for the time-averaged distribution of the space-charge field and employ the simplified versions of this equation to analyze the space-charge-field formation in the case of a Gaussian beam and interference light pattern. The borders of the applicability of simplified equations and analytic expressions to describe a self-action of light beams and the large modulation effects under two-beam coupling are estimated from the numerical calculations. Received: 18 November 1998 / Revised version: 26 January 1999 / Published online: 12 April 1999     

Real-time holographic interferometry using sillenite photorefractive crystals. Study and optimization of a transportable set-up for quantified phase measurements on large objects

We present the development of a holographic interferometer that uses a photorefractive crystal of the sillenite family as holographic recording medium. The aim of this work is to achieve a transportable and breadboard instrument, with a flexible design and which is user friendly for the quantified measurements of displacements on large scattering objects. The state of the art of the use of photorefractive crystals in holographic interferometry is presented. Based on the latter, a method that is the best suited to our mentioned objectives is chosen: the real-time interferometric technique associated with the crystal configuration exhibiting diffraction anisotropy. On this basis, we have studied and compared two imaging systems, we have experimentally determined the holographic figures of merit of three sillenite crystals samples, and, finally, we have shown the use of two measurement quantification methods. The result of these investigations is a certified and transportable holographic camera prototype containing all the necessary equipment for its operation, which can be easily adapted to various applications. Among these, we show its use in the detection of defects in composite structures, in the measurement of time-evolving deformations, and in the measurement of vibration modes. Received: 4 December 1998 / Revised version: 6 March 1999 / Published online: 12 April 1999     

New photopolymerizable holographic recording material based on polyvinylalcohol and 2-hydroxiethylmethacrylate (HEMA)

We have developed a new polyvinylalcohol-based photopolymeric holographic recording material. The recording is obtained by the copolymerization of acrylamide and 2-hydroxiethylmethacrylate (HEMA). Diffraction efficiencies near 70% are obtained with exposures of ≈65 mJ/cm² in materials of 110-μm thickness. The addition of HEMA increases the thickness of the film, which results in improved storage capacity for holographic optical storage. Received: 7 March 2002 / Published online: 2 May 2002     

Optical holographic memory using angular-rotationally phase-coded multiplexing in a LiNbO3:Fe crystal

We propose a new holographic memory scheme based on random-phase-coded multiplexing in a photorefractive LiNbO₃:Fe crystal. Experimental results show that rotating a diffuser placed as a random-phase modulator in the path of the reference beam provides a simple yet effective method of increasing the holographic storage capabilities of the crystal. Combining this rotational multiplexing with angular multiplexing offers further data-storage possibilities. The advantage of using post-image versus pre-image phase modulation of the object beam is demonstrated. Received: 15 August 2000 / Published online: 10 January 2001     

Nonlinear interactions and scattering of space charge waves in sillenites

For the first time the effects of overall doubling, spatial doubling, and overall rectification of space charge waves in a photorefractive crystal (Bi₁₂GeO₂₀, BGO) have been detected and theoretically studied. The effects of overall doubling and overall rectification resemble second-harmonic generation and rectification of light in nonlinear optics. A good agreement between theory and experiment is found for the effect of spatial doubling, while for overall doubling and overall rectification effects only a qualitative agreement between theory and experiment has been established until now. Received: 13 July 2001 / Published online: 10 October 2001     

Photophysical properties of a crown ether-bearing [60]fulleropyrrolidine

The photophysical properties of the first crown ether-bearing [60]fulleropyrrolidine (CFP) have been studied. The absorption and fluorescence spectra are hypsochromically shifted by metal ions complexation via intramolecular electron transfer which is confirmed by AMI computation. The third-order optical nonlinearity of CFP is measured by time-resolved degenerate four-wave-mixing experiment. The third-order optical susceptibility χ⁽³⁾ of CFP is 8.97×10⁻¹⁴ esu. The nonlinear optical temporal response is second-order exponential, the fast process is determined as 120 ps. Received: 24 March 1999 / Revised version: 11 May 1999 / Published online: 25 August 1999     

A method for single-pass distortion correction using holography in rhodium-doped barium titanate

We present a holographic method of one-way image transmission with ‘real-time’ correction of distortion through an optical system. The technique is demonstrated using a crystal of photorefractive rhodium-doped barium titanate as the holographic medium and corrects for phase distortions imposed upon a US Air Force test chart. This technique is suitable for applications in both cw and pulsed regimes, and for either fixed or changing distortions. Received: 26 October 1998 / Revised version: 19 December 1998 / Published online: 12 April 1999     

Holographic angle multiplexing combined with peristrophic multiplexing in a photorefractive LiNbO3 crystal

We have investigated angle multiplexing combined with peristrophic (rotational) multiplexing in a photorefractive LiNbO₃ crystal of cylindrical shape. The peristrophic multiplexing was achieved by rotating the recording medium while the angle multiplexing, by varying the incident angle of the reference beam. Angle multiplexing in synchronization with rotation of the sample made it possible to continuously record and retrieve holograms. In the combined multiplexing, holograms were stored at different peristrophic positions and thus their angular selectivity could be much improved in comparison to that of only angle-multiplexed holograms. The theoretical angular selectivity was derived, along with its dependence on the sample rotation. The experimentally measured selectivity was in agreement with the theoretical one. The angular selectivity of angle-multiplexed holograms was measured to be 0.03° in the absence of sample rotation and it became less than 0.0075° with a sample rotation by 5.4°.     

Structural and electrical properties of polycrystalline PbZr0.5Ti0.5O3 films deposited on La0.5Sr0.5CoO3 coated silicon by sol-gel process

La_0.5Sr_0.5CoO₃ (LSCO) films have been grown on Si (100) by a metalorganic chemical liquid deposition (MOCLD) technique using lanthanum acetate, strontium acetate and cobalt acetate as the starting materials. Subsequent PbZr_0.5Ti_0.5O₃ (PZT) films were deposited onto LSCO films by a modified sol-gel method. Field-emission scanning electron microscopy and X-ray diffraction analysis show that PZT and LSCO films are polycrystalline and entirely perovskite phase. At an applied electric field of 250 kV/cm, the Pt/PZT/LSCO capacitor shows no polarization fatigue after 3×10⁹ switching cycles and an internal electric field; the remnant polarization P_r and the coercive field E_c are about 22 μC/cm² and 73 kV/cm, respectively. The dielectric constant of PZT films is 650 at a frequency of 1 kHz. Received: 20 February 2001 / Accepted: 6 June 2001 / Published online: 30 August 2001     

Fabrication of surface relief gratings on transparent dielectric materials by two-beam holographic method using infrared femtosecond laser pulses

Fabrication of surface relief-type gratings in transparent dielectrics, which are hard to machine, has been achieved by a holographic technique using two infrared femtosecond (fs) pulses from a mode-locked Ti:sapphire laser. The present method can be applied for a variety of transparent dielectrics, Al₂O₃ (sapphire), TiO₂, ZrO₂, LiNbO₃, SiC, ZnO, CdF₂, MgO, CaF₂ crystals, and SiO₂ glass. It is found that the grating formation is due primarily to laser ablation processes. Planar surface relief gratings can be fabricated by colliding two fs laser pulses on the surface of substrates which move at a constant speed, synchronized with the laser repetition rate. Received: 1 March 2000 / Published online: 7 June 2000     

Photorefractive properties of Cr-doped Sr0.61Ba0.39Nb2O6 related to crystal purity and doping concentration

The purity and the concentrations of the constituents Sr, Ba, Nb and Cr were determined in single crystals of chromium-doped Sr_0.61Ba_0.39Nb₂O₆ (SBN) by instrumental neutron activation analysis (INAA) and X-ray fluorescence analysis (XRF). Experiments with different Cr concentrations C_Cr reveal constant Sr and Ba concentrations as well as a decreasing Nb concentration with an increasing C_Cr. Therefore, Cr is incorporated at Nb lattice sites. The distribution coefficient of Cr is 1.2 between 100 and about 7000 ppm Cr and 1 above. From measurements of the holographic two-beam coupling gain Γ for different C_Cr, we deduced response times for the buildup of holographic gratings. The saturation value of Γ depends in a nonlinear manner on C_Cr and reaches its maximum at about 6000 ppm Cr. The inverse photorefractive response time of the grating erasure depends linearly on the erasure intensity for all doping concentrations. Thus a one-center model can be assumed for the charge transport in SBN:Cr. Received: 8 December 2000 / Revised version: 23 January 2001 / Published online: 21 March 2001