Before 2005

M. Ivker and A. J. Agranat, “Resistance degradation due to interstitial hydrogen in photorefractive potassium lithium tantalate niobate single crystals,” J. Applied Physics , vol. 96, no. 11. pp. 6405 - 6408, 2004. Publisher's VersionAbstract
Resistance degradation in potassium lithium tantalate niobate (KLTN) doped with iron and titanium was measured in a single sample containing various concentrations of interstitial hydrogen. In this crystal the degradation arose from the migration of interstitial hydrogen and not oxygen vacancies, as reported in previous research. Interstitial hydrogen and oxygen vacancy defects both arise to compensate the valence shortfall of the substitutional iron impurities and the thermodynamic balance between the two compensation mechanisms can be controlled using reaction chemistry techniques. Through appropriate annealing treatments a single crystal of KLTN was prepared in three states: hydrogen-rich oxidized, hydrogen-poor reduced, and hydrogen-rich reduced. The characteristic degradation times for the three cases were 29, 2710, and 26 min, respectively. The degradation rate is correlated with hydrogen concentration and not oxidation state of the crystal. Infrared absorption from near the two electrodes of the hydrogen-rich reduced crystal after degradation confirmed polarization of the hydrogen concentration. Electrocoloration was also found to correlate with hydrogen-it was observed in both hydrogen-rich states, but was absent from the hydrogen-poor crystal. (C) 2004 American Institute of Physics.
C. E. M. de Oliveira, G. Orr, N. Axelrold, and A. J. Agranat, “Controlled composition modulation in potassium lithium tantalate niobate crystals grown by off-centered TSSG method,” J. Cryst. Growth, vol. 273, no. 1-2. pp. 203 - 206, 2004. Publisher's VersionAbstract
Off-centered top-seeded solution growth (TSSG) method is demonstrated as an effective and simple way to generate controlled composition modulation in potassium lithium tantalate niobate (KLTN) single crystals. The changes in concentration were measured by differential interference contrast (DIC) microscopy. Large length with periodic modulations ranging from 1 to 5 mum in period was grown along a KLTN sample with period dispersion lower than 2%. (C) 2004 Elsevier B.V. All rights reserved.
E. DelRe, B. Crosignani, P. Di Porto, E. Palange, and A. J. Agranat, “Electro-optic beam manipulation through photorefractive needles,” Optics Letters, vol. 27, no. 24. pp. 2188 - 2190, 2002. Publisher's VersionAbstract

We demonstrate electro-optic spatial two-dimensional mode switching in a bulk sample of potassium lithium tantalate niobate. Spatial confinement, mode coupling, and electro-optic functionality are mediated by two photorefractive needle solitons of opposite electroholographic charges embedded together in their anisotropic lobular structure. (C) 2002 Optical Society of America.

A. J. Agranat, “Optical lambda-switching at telecom wavelengths based on electroholography ,” Topics in Applied Physics , vol. 89, pp. 129-156, 2002.Abstract
Electroholography is a wavelength-selective optical switching method based on governing of the reconstruction process of volume holograms by means of an electric field. Electroholography is based on the voltage-controlled photorefractive effect in the paraelectric phase. The basic switching device is an electrically controlled Bragg grating or a volume hologram stored in a volume of a paraelectric crystal by the photorefractive process. The basic electroholographic switching operation is the reconstruction of a volume grating (hologram), which requires that the Bragg condition be satisfied, and therefore is wavelength selective. In addition the applied field governs the efficiency of the reconstruction. Consequently, electroholographic switching includes grouping, multicasting, power management and non-intrusive data as an integral part of the switching operation. In preliminary measurements the performance envelope of the electroholography-based switch, is a cube of 1.8 mm3 was found to be as follows: The minimum net insertion loss is 0.5 dB per switching operation. The minimum loss when a beam propagates through a latent grating is 0.2%. The Polarization-Dependent Loss (PDL) in a device that includes diversity architecture is less than 0.4 dB and the Polarization Mode Dispersion (PMD) is less than 0.07 ps. Bit-error rate (BER) in a switch operating at 40 Gb/s was measured to be 10-13. These features make electroholography ideal for circuit switching applications. Finally, response times of approximately 10 ns were measured, opening the way to burst switching applications.
G. Bitton, Y. Feldman, and A. J. Agranat, “Relaxation processes of off-center impurities in KTN : Li crystals,” J. Non-Cryst. Solids, vol. 305, no. 1-3. pp. 362 - 367, 2002. Publisher's VersionAbstract
The dielectric and ferroelectric properties of K1-xLixTa1-yNbyO3 (KLTN) crystals as functions of temperature, frequency, and electric field are presented. Measurement of the polarization as a function of temperature indicates the existence of polarized microregions at temperatures well above the phase transition (PT) temperature. In the vicinity of the PT those microregions respond cooperatively and contribute to the observed macroscopic polarization. This cooperative behavior was also evident from dielectric relaxation measurements. Two relaxation processes were observed, one at high frequency and the other at low frequency. In both processes the relaxation time and the relaxation step (DeltaE) were found to increase as the PT temperature was approached. Those processes originate from movement of off-center ions in a multi-well potential. In the vicinity of the PT the correlation length of the host lattice is increased and the movements of the off-center ions become more and more correlated. The increased number of cooperatively relaxing ions increases the relaxation time and the relaxation amplitude. (C) 2002 Published by Elsevier Science B.V.
V. A. Trepakov, L. Jastrabik, S. Kapphan, E. Giulotto, and A. J. Agranat, “Phase transitions, related properties and possible applications of (K,Li)(Ta,Nb)O3 crystals,” Optical Materials, vol. 19, no. Photonic Materials for the 21st Century. Proceedings of the 2nd International Symposium on Laser, Scintillator and Nonlinear Opt, pp. 13 - 21, 2002. Publisher's VersionAbstract
It is shown that doping the highly polarizable KTaO3 perovskite simultaneously by Li and Nb (K1−xLixTa1−yNbyO3, KLTN) gives rise to new impressive dipole ordering effects, very unusual physical properties and strong responses with respect to doping parameters. Besides, KLTN has outstanding perspectives of application in electrically controlled holographic and compositionally graded pyroelectric devices.
A. J. Agranat, L. Secundo, N. Golshani, and M. Razvag, “Wavelength-selective photonic switching in paraelectric potassium lithium tantalate niobate,” Optical Materials, vol. 18, no. 1, pp. 195 - 197, 2001. Publisher's VersionAbstract
A generic wavelength-selective switching method is presented. The method is based on the voltage-controlled photorefractive effect in paraelectric crystals. The switch consists of a crystal at the paraelectric phase in which a space charge grating was stored by a photorefractive process. Diffraction occurs by the application of a uniform electric field that causes the space charge grating to induce an index grating. Results of the diffraction efficiency as a function of the applied field and the time dependence of the diffracted beam are presented. The performance envelop of a switch based on this effect is briefly discussed.
B. Pessach, G. Bartal, E. Refaeli, and A. J. Agranat, “Free Space Optical Cross-Connect Switch by Use of Electroholography,” Applied Optics , vol. 39, no. 5, pp. 746-758, 2000.Abstract
An electrically controlled holographic switch is proposed as a building block for a free-space optical interconnection network. The switch is based on the voltage-controlled photorefractive effect in KLTN crystals at the paraelectric phase. It is built of electrically controlled Bragg gratings stored in the volume of the crystal. A compact switch that connects four high-speed fiber-optic communication channels with high efficiency is demonstrated experimentally. The switch performance is investigated and optimized. This switch is extremely attractive for cascaded switching arrays such as those found in multistage interconnect networks.
O. Levi, et al., “A photo-oxidation mechanism for patterning and hologram formation in conjugated polymer/glass composites,” J. of Appl. Physics , vol. 88, no. 3. pp. 1236 - 1243, 2000. Publisher's VersionAbstract
Improved diffraction efficiency was observed in holograms stored in disordered conjugated polymer/glass composites. The conjugated polymers used were alkoxy substituted poly(phenylenevinylne) analogs and the glass matrices were zirconia-organosilica xerogels. Investigation of the mechanism of hologram formation revealed evidence of a photochromic process consisting of light induced photo-oxidation (bleaching) of the embedded conjugated polymer resulting in the formation of an absorption grating and a phase grating. Investigation of the hologram formation revealed that the process was oxygen dependent. Oxygen removal increases hologram formation time by more than an order of magnitude and halves the total hologram efficiency. The oxygen dependence was also highly correlated with photobleaching of the samples and beam interaction of the writing beams. The chemical transformations upon photobleaching were shown by infrared and Raman spectroscopy to involve chain scission and oxidation of the polymer at the vinylic position of the conjugated polymer. Film preparation of the composites was optimized showing a tenfold improvement in the holographic properties compared to our previous results. The optimized treatment method allows for a high, > 20%, diffraction efficiency, eta, to be obtained for the 2.5-mu m-thick polymer/glass films. Light sensitivity was compared for several polymer/glass composites and was correlated to the absorption curves and holographic diffraction efficiency showing that the new composites and film preparation techniques are promising for holographic materials sensitive in the blue and ultraviolet spectral regions. A method of information fixing by preventing oxygen entry to the composite film resulted in a fourfold increase of the erasure time. These findings suggest that holograms can be fixed for a long term by nonoxygen permeable coating, applied after hologram formation. (C) 2000 American Institute of Physics. [S0021- 8979(00)07114-0].
B. Crosignani, A. Degasperis, E. DelRe, P. Di Porto, and A. J. Agranat, “Nonlinear optical diffraction effects and solitons due to anisotropic charge-diffusion-based self-interaction,” Physics Review, vol. 82, no. 8. pp. 1664 - 1667, 1999. Publisher's VersionAbstract

We report the first observation of self-modified optical diffraction, beam ellipticity recovery and conservation, and intensity independent self-focusing in an anisotropic diffusion-type nonlinearity realized in ferroelectrics heated above the centrosymmetric transition. The interaction, a photorefractive diffusion-driven quadratic nonlinearity, constitutes the first known natural realization of a higher-order logarithmic nonlinearity and allows an analytical description of the observed phenomena and the prediction of a class of noncircular solitons with no characteristic length scale.

E. DelRe, M. Tamburrini, M. Segev, R. Della Pergola, and A. J. Agranat, “Spontaneous self-trapping of optical beams in metastable paraelectric crystals,” Physics Review Letters, vol. 83, no. 10. pp. 1954 - 1957, 1999. Publisher's VersionAbstract

We report on the observation of a new mechanism for self-trapping of optical beams: self-trapping that stems from spontaneous creation of ferroelectric crystalline clusters, seeded by a weak photorefractive diffusion field. This is an evident observation of the highly nonlinear aspects of propagation in a thermodynamically metastable system, including optically driven crystalline ordering in a medium undergoing a phase transition.

G. Bitton, A. J. Agranat, and V. M. Fridkin, “The Bulk Photovoltaic effect in Photorefractive KLTN Crystals,” Ferroelectric Letters , vol. 25, no. 1-2, pp. 1-9, 1999.Abstract
Experimental evidence of the bulk photovoltaic effect in photorefractive K1-xLixTa1-xNbyO3 (KLTN) crystals is shown. The dependence of the bulk photovoltaic effect on the pbotorefractive impurities type is presented. The current voltage characteristic is presented for different temperatures. It was found that, in Fe-doped KLTN crystals at low temperature, the bulk photovoltaic effect is the dominant transport mechanism contributing to the photorefractive process.
O. Levi, et al., “Conjugated polymeric composites for holographic storage,” Synthetic Metals , vol. 102, pp. 1178-1181, 1999.Abstract
We report here on the fabrication and optimization of composites of conjugated polymer in Sol-Gel and PVA matrices. FTIR spectroscopy as well as other observations show that efficient phase and absorption holograms can be created in the material upon illumination, via the photochromic effect. The main contribution to the absorption change is chain scission and photo-oxidation mechanism. Optimized holographic composites with diffraction efficiencies as high as 26% are shown. Beam interaction between the writing beams is observed for these composites, while the diffraction efficiencies was improved by an order of magnitude.