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Whispering-gallery resonators made of undoped and MgO-doped congruently grown lithium niobate are used to study electro-optic refractive index changes. Hereby, we focus on the volume photovoltaic and the pyroelectric effect, both providing an electric field driving the electro-optic effect. Our findings indicate that the light-induced photorefractive effect, combining the photovoltaic and electro-optic effect, is present only in the non-MgO-doped lithium niobate for exposure with light having wavelengths of up to 850 nm. This leads to strong resonance frequency shifts of the whispering-gallery modes. No photorefractive effect was observed in the MgO-doped material. One has to be aware that surface charges induced by the pyroelectric effect result in a similar phenomenon and are present in both materials.

The influence of different oxidizing and reducing treatments on the photorefractive properties of tetragonal KTa1-xNbxO3:Fe crystals is investigated. Elementary holograms are recorded and photoconductivity, saturation value of refractive index change and gain factor are determined. The experimental results are interpreted by a model which assumes one kind of photorefractive centers with both, electrons and holes as free charge carriers, taking into account space charge limitations. We find that oxidizing treatments increase the hole contribution to photoconductivity and the effective density of photorefractive centers.

A nonlinear optical phenomenon, namely, the surface waves in photorefractive crystals with a diffusion nonlinearity (of the gradient type), is suggested and analyzed. These waves can be guided along the boundary of the crystal with a metal, a dielectric with a lower average refractive index, or a similar photorefractive crystal with the opposite sign of the diffusion nonlinearity. Our estimates predict that for a typical photorefractive crystal such as ${\mathrm{BaTiO}}_{3}$ this mechanism can ensure efficient concentration of light power in a crystal layer of \ensuremath{\sim}10 \ensuremath{\mu}m thickness.

A photorefractive system is one which is simultaneously photoconductive and electrooptic. Diffraction gratings or holograms can be produced in a photorefractive material by the photogeneration, drift or diffusion and subsequent trapping of mobile charges. The grating is produced by the internal space charge field set-up by these charges which, via the electrooptic effect, produces an index of refraction grating. Until 1990 all photorefractive systems were inorganic crystals such as LiNbO,, BaTiO,, B,,SiO,, InP:Fe, GaAs, or multiple quantum well materials. In 1990, the first observation of the photorefractive effect in an organic material, a carefully grown, doped molecular crystal was described. This was followed in short time by the discovery of photorefractive polymers. The early photorefractive systems, polymeric and crystalline, were inefficient compared to inorganic systems but in recent years the efficiency and sensitivity of photorefractive polymers have become equal to inorganic crystals. This paper describes the origins of the photorefractive effect and the design, synthesis and characterization of photorefractive polymers. In addition an orientational enhancement of the photorefractive diffraction efficiency is discussed. This orientational enhancement does not occur in crystalline systems, since it relies on the ability of the optically nonlinear chromophores to be aligned not only by an externally applied field, but also by the sinusoidally varying space charge field produced during photorefractive grating formation.

A photorefractive polymer composite containing a trapping layer which exhibits a significant enhancement in its photorefractivity is reported. The photorefractive polymer composite containing a trapping layer showed a grating life time as high as 600 s, while the grating life time was only 7 s in the photorefractive polymer composite without a trapping layer. The grating growth rate was found to be unaffected by the introduction of a trapping layer. The diffraction efficiency and gain coefficient also increased by the introduction of a trapping layer. The enhancement in the photorefractive properties is due to the increase in the space-charge field developed in the photorefractive polymer composite containing a trapping layer.

Synchrotron light changes the refractive index of poly(methyl methacrylate) (PMMA). Refractive-index enhancements as well as reductions depending on dose and energy of the radiation used can be observed. This effect allows the manufacturing of, for example integrated-optical components, as is demonstrated by realization of waveguides in PMMA.

We have observed an extremely large electro-optically induced photorefractive effect in nematic liquid crystal doped with single-walled carbon nanotubes and fullerene C60. The effective refractive index change coefficient can be as large as 7 cm2/W, which is >1000 times larger than previous observations. We describe the basic mechanisms and conditions necessary for occurrence of such nonlinearities.

Single component photorefractive materials based on small molecules have been synthesized. The sidechains located in the electron-withdrawing group of these molecules were systematically changed in order to investigate their influence. It was demonstrated that the photorefractive performance of these materials could be fine-tuned by changing the length of the sidechain. Excellent optical quality and photorefractive properties were obtained. A large net optical gain of 280 cm−1 at a low external field (38.3 V/μm) and a diffraction efficiency of 82% were observed at 780 nm. The results indicate that an optimum sidechain length exists for this type of molecules.

We present a photorefractive hybrid liquid crystal system that allows strong photorefractive effects on surface plasmon polaritons. We demonstrate its capability to couple energy between two 1.03 eV surface plasmon polariton modes with an efficiency of 25.3±2.3%. We present the energy and grating pitch dependence of the diffraction and a model that can qualitatively explain them.

Special features of light-induced scattering of radiation (λ = 0.44 μm) with extraordinary polarisation in the SBN-75 photorefractive crystal are studied in detail and an efficient technique is proposed for exciting surface waves in this crystal. In the experiment carried out the efficiency of the surface wave excitation was ~ 30%, which is 50 times greater than that achieved earlier in exciting nonlinear surface waves in the optical region. The patterns of the near and far fields of the surface wave are presented. It is found that at small excitation angles (0-1.5°) the presence of a metal changes the character of arising surface waves. Using method of images for calculation of electrostatic fields a model is proposed explaining the peculiarities of light propagation close to the crystal-metal boundary.

In prior work, we exploited the nonlinearity inherent in four-wave mixing in organic photorefractive materials for adaptive filtering. In this paper, we extend our work further and demonstrate new applications which involve: dislocation, scratches and defect enhancement. With the availability of the organic photorefractive materials with large space-bandwidth product, it should open the possibility of using the adaptive filtering techniques in quality control systems.

In prior work, we demonstrate optical correlation via dynamic range compression in two-beam coupling using thin-film organic materials. In this paper, we continue the effort; characterize the performance of this correlator for variety of input. We successfully demonstrated correlation results almost free of cross- correlation and noise for extremely complicated noisy image were the signal image consist of several targets and reference image superposed of many templates.

Made available in DSpace on 2013-08-28T14:08:40Z (GMT). No. of bitstreams: 1 WOS000302956100001.pdf: 702896 bytes, checksum: fe030cb74b90e5d2582e37784a8acf88 (MD5) Made available in DSpace on 2013-09-30T18:51:01Z (GMT). No. of bitstreams: 2 WOS000302956100001.pdf: 702896 bytes, checksum: fe030cb74b90e5d2582e37784a8acf88 (MD5) WOS000302956100001.pdf.txt: 20260 bytes, checksum: 54491e0a4cc918c26b1e2ec227c9971f (MD5) Previous issue date: 2012-03-01 Submitted by Vitor Silverio Rodrigues (vitorsrodrigues@reitoria.unesp.br) on 2014-05-20T14:16:43Z No. of bitstreams: 2 WOS000302956100001.pdf: 702896 bytes, checksum: fe030cb74b90e5d2582e37784a8acf88 (MD5) WOS000302956100001.pdf.txt: 20260 bytes, checksum: 54491e0a4cc918c26b1e2ec227c9971f (MD5) Made available in DSpace on 2014-05-20T14:16:43Z (GMT). No. of bitstreams: 2 WOS000302956100001.pdf: 702896 bytes, checksum: fe030cb74b90e5d2582e37784a8acf88 (MD5) WOS000302956100001.pdf.txt: 20260 bytes, checksum: 54491e0a4cc918c26b1e2ec227c9971f (MD5) Previous issue date: 2012-03-01 Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) FUNCAMP-Fundo de Apoio ao Ensino, Pesquisa e Extensao (FAEPEX-PAPIDC) Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) We report on the recording of stabilized running holograms in photorefractive materials with arbitrarily selected phase shift phi between the transmitted and difracted beams propagating along the same direction behind the photorefractive crystal. The dependence of the diffraction efficiency and of the hologram speed on phi, in such stabilized holograms, can be easily measured and used for material characterization. In this communication we applied for the first time this technique for studying and characterizing hole-electron competition in a nominally undoped titanosillenite crystal sample. (C) 2012 Optical Society of America Univ Estadual Campinas, Fac Tecnol, Limeira, SP, Brazil Universidade Federal do ABC (UFABC), Santo Andre, SP, Brazil UNESP, Dept Fis, Inst Geociencias & Ciencias Exatas, Rio Claro, SP, Brazil Univ Estadual Campinas, Inst Fis, Campinas, SP, Brazil UNESP, Dept Fis, Inst Geociencias & Ciencias Exatas, Rio Claro, SP, Brazil

In our proposal a light intensity distribution generated by an incoherently illuminated planar amplitude grating is projected into a photorefractive crystal. This 3D distribution is mapped as an index refractive perturbation via the photorefractive effect thereby generating a volume phase grating. The self-imaging phenomenon in the Fresnel field of this volume phase grating coherently illuminated is theoretically and experimentally analyzed. A model to simulate this volume grating that considers the 3D light intensity distribution formed in the crystal combined with the photorefractive grating formation theory is proposed. A path-integral approach to calculate the self-image patterns which account for the inhomogeneous propagation through the photorefractive grating is employed. The experimental and theoretical results show that the self-images location coincides with that of the self-images generated by planar phase grating of the same period. Moreover, the self-images visibility depends on three parameters: the exit pupil diameter of the incoherent recording optical system, the external electric field applied on the crystal, and the crystal thickness. To study the visibility behavior, a phase parameter which includes the three mentioned parameters is proposed. The self-images visibility shows the typical sinusoidal dependence found in planar phase grating. A good agreement between theoretical and experimental results is observed.

High quality inverse opal structures are synthesized from the photorefractive SnO2 medium by a simple two step infiltration and calcination process. Optical characterization reveals the high index contrast and applications of them in the field of sensors and integrated optics.

Enhanced and fast photorefractive index grating is developed in compound semiconductor, when the two pico-second light pulses couple inside compound semiconductor crystal at 77 K under Voigt configuration, near the resonance condition in the presence of magnetic field.

We report on the experimental evidence of the light intensity dependence of the Debye screening length l(s) in undoped photorefractive titanosillenite crystals (Bi12TiO20) by measuring the holographic gain and diffraction efficiency in a two-wave mixing experiment under 532 nm wavelength laser light. Debye length shows saturation at high values of the light intensity. Results are in agreement with the theoretical development Aberto

We demonstrate image restoration and recognition via photorefractive two-beam coupling using organic photorefractive materials. Our results outperforms the simulations based on the phase-only and some optimal correlation filters.

We characterize the photorefractive behavior of a photorefractive multicomponent polymer composite of PVK‐TCP:C60:DEANST. Efficient plasticization of the host polymeric matrix and utilization of a nonlinear chromophore with a large dipole moment provide a large poling‐induced electro‐optic coefficient. Diffraction efficiencies as high as 40% and asymmetric net two‐beam coupling gain coefficients in excess of 130 cm−1, surpassing those of known inorganic single‐crystalline photorefractive media, are reported.

PHOTOREFRACTIVE materials are of considerable interest for the development of all-optical devices1. The photoref ractive effect appears in materials that exhibit an electric-field-dependent refractive index and that are photosensitive, such that the spatial distribution of photogenerated charge carriers is modified on irradiation with light. The diffraction pattern formed by the interference of two coherent light beams within such a material generates a non-uniform internal electric field that in turn modulates the refractive index. The resulting refractive-index pattern forms a grating that can diffract light and thereby give rise to two-beam coupling, whereby one of the writing beams gains energy at the expense of the other—a property that can be exploited in photonic devices. Although the best photorefractive materials currently available are inorganic crystals such as LiNbO3, there is considerable interest in the development of photorefractive polymers2–8, owing to their structural flexibility, ease of processing and lower cost. We describe here a polymer composite with excellent photorefractive properties. We have achieved a diffraction efficiency approaching 100% and a net two-beam coupling gain of more than 200 cm–1, making these polymeric materials suitable for immediate application in areas such as dynamic holographic storage and optical information processing1.

The photorefractive effect is observed and described in a polymeric composite consisting of poly[N-vinylcarbazole] charge transporting matrix, fullerene C{sub 60} photosensitizing molecules, and second-order active molecules of (diethylamino)nitrostyrene. The results of photoconductivity, electrooptic modulation, degenerate four-wave mixing, and two-beam coupling experiments are reported. The composite shows relatively high field-dependent electrooptic coefficient and charge carrier photogeneration quantum efficiency. The observed photorefractive diffraction efficiencies compare well with those of well-known inorganic materials. Time-dependent measurements reveal very fast kinetics of light induced phase grating formation and erasure. A possibility of efficient switching of the photorefractive effect with an external electric field is shown. 25 refs., 10 figs.

Organic photorefractive materials have only recently been considered as alternatives to inorganic crystals such as lithium niobate, despite the fact that they offer possible advantages, such as mechanical flexibility, low fabrication costs, and the ability to chemically tune the properties of the material. The influence of the nonlinear optical chromophore on various phenomena observed in a photorefractive polymer is reported on the basis of a study of samples derived from poly(N-vinylcarbazole) sensitized with 2,4,7-trinitro-9-fluorenone.

Near-ultraviolet light induces band-to-band electron transitions and is used to record interband photorefractive gratings in undoped ${\mathrm{KNbO}}_{3}$. The high absorption constant in the range from 500 to 6000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ dictates the use of thin crystals or of the crystal volume near the surface. The probing of the grating is usually performed in the visible. We measured a fast grating response of 10 \ensuremath{\mu}s at 1 W/${\mathrm{cm}}^{2}$ for refractive index changes of 2\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}5}$. The hologram diffraction efficiency reaches \ensuremath{\eta}=1.4% in a longitudinal recording geometry with a grating approximately 200 \ensuremath{\mu}m thick. In a transverse geometry we measured \ensuremath{\eta}=60% in 7.5-mm-long crystals. The photorefractive sensitivity of ${\mathrm{KNbO}}_{3}$ is similar to that of multiple-quantum-well structures while the resolution is better due to smaller carrier mobilities and because of shorter optical wavelengths used. Interband photorefractive gratings are highly robust with respect to intensive illumination in the visible. A theoretical model that describes interband electron transitions predicts the important role of free carriers in both bands. It explains the transition from a linear to a sublinear dependence of photoconductivity on light intensity which is observed already at intensities of a few mW/${\mathrm{cm}}^{2}$. The predictions are consistent with dynamic and steady-state holographic investigations performed under variation of grating spacing, applied electric field, ultraviolet and visible light intensities, and grating readout depth.

Photorefractive properties of an iron-doped tetragonal KTa0.52Nb0.48O3 single crystal are investigated with Q-switched light pulses (wavelength 532 nm). Pyro-electric fields significantly enhance light-induced refractive index changes.

We report the simultaneous diffraction cancellation for beams of different wavelengths in out-of-equilibrium dipolar glass. The effect is supported by the photorefractive diffusive nonlinearity and scale-free optics, and can find application in imaging and microscopy. © 2011 Optical Society of America.

Surface Plasmon Polaritons (SPP) are of fundamental interest in a wide variety of applications including optical signal processing and sensing. The ability to manipulate these SPP is essential for continued development in this field. The high sensitivity of SPP to variations in the refractive index near the metal opens up the possibility of manipulating them using a photorefractive liquid crystal cell [1] adjacent to a thin gold layer, see Figure 1 (A).

Imaging in atmospheric turbulence and target recognition in cluttered environments have been research topics for many years. Currently, there are some well-established techniques for image restoration and recognition; however, if the atmospheric turbulence becomes a severe scattering medium and the surrounding environment is very cluttered, most conventional methods, such as inverse filtering and Wiener filtering, will be inadequate for correcting and recognizing the captured images. In this paper, we experimentally demonstrate nonlinear dynamic range compression techniques for image restoration and correlation via two-beam coupling and four wave mixing in organic photorefractive films.

In this paper, we demonstrate optical correlation via dyna mic range compression in two-beam coupling using thin-film organic materials. In contrast to the first demonstration, in which it was not possible to demonstrate correlation with complicated input, here we demonstrate correlation with extremely challenging cases involving finger prints, images in clutter, and SAR images. Our correlation results outperform many correlation results, including ones based on optimal filters. 1. INTRODUCTION In this paper, we demonstrate dynamic range compression co rrelation via a photorefractive two-beam coupling correlator (TBC) which proposed more than a decade ago . 1, 2 The correlator operates by pump ing a clean reference beam by the joint spectra of reference and signal images. The TBC has been studied extensively through computer simulations. In these studies, it was found that, for some cluttered signals, the TBC outperforms, in terms of peak-to noise ratio 2-4 , other correlators based on phase-only filters

The lens-like effects, both photorefractive effect and thermo-optical effect, were studied in the LiNbO3: Fe sheet. The threshold light intensity above which the photorefractive effect was overwhelmed by the thermal effect was determined and the thermo-optical coefficient dn/dT is evaluated by the Z-scanning method. The limiting characteristics of LiNbO3: Fe sheet is examined.

A nonlinear optical chromophore for photorefractive applications containing a 2-dicyanomethylen- 3-cyano-2,5-dihydrofuran acceptor group is presented. When doped into a plasticized composite of poly(n-vinylcarbazole), large gain coefficients (Γ) are observed with photorefractive speed similar to the best composites reported in the literature while maintaining low sample absorption (∼15 cm−1).

The photorefractive effect of a ferroelectric liquid crystal doped with a photoconductive compound was investigated. The photorefractive effect appeared only at the temperature at which the sample exhibits ferroelectricity. The refractive index grating formation time was measured to be ∼30 ms. In the ferroelectric phase, reorientation of the spontaneous polarization vector was found to be induced by the internal space-charge field.

We introduce holographic scattering as a technique to determine the activation energy for thermal fixing of refractive index patterns in photorefractive crystals. After recording a parasitic hologram at ambient temperature, we measured the time dependence of the transmitted intensity at the fixing temperature, to determine the time constant. The temperature dependence of the latter allowed us to evaluate the thermal activation energy. For comparison, we performed an equivalent experiment employing the standard two-wave mixing method. The values obtained using the two techniques agree very well.

This letter reports a series of molecular materials that shows efficient photorefractive effect. The materials which consist of monolithic methine dye molecules can form amorphous films. A net photorefractive gain of 215 cm−1 and a diffraction efficiency of 87.6% at a semiconductor laser wavelength of 780 nm are observed. These materials are the simplest molecular materials and possess the best photorefractive performances. Due to the large photorefractive effect, optical self-defocusing and instability of the laser beam are observed in these materials when the applied electric field is above a threshold.

We present results of the photorefractive properties of a chiral smectic A liquid-crystalline phase. In our approach, the mesophase is sandwiched between two photoconducting layers. In contrast to the effect due to the dielectric anisotropy typical of nematic phases, the refractive-index modulation is due to the electroclinic effect. We measured the grating spacing and the field dependence of the gain coefficient and obtained a net two-beam coupling gain of 600 cm−1 at 35 V/μm.

In this paper, we exploit the nonlinearity inherent in four-wave mixing in organic photorefractive materials and demonstrate edge enhancement, contrast conversion, and defect enhancement in a periodic structure. With the availability of these materials, which have large space-bandwidth products, edge enhancement, contrast conversion and defect enhancement are possible.

We present the evolution equation of surface waves due to the two-photon photorefractive effect. It is shown theoretically that the surface waves can be supported by two-photon photorefractive media. The spatial frequency of photorefractive surface waves can be changed by an applied external electric field, which indicates a method for transforming the surface waves between high-frequency modes and low-frequency modes.

The general equations applicable to the description of different photorefractive effects in electro-optic crystals, taking into account their piezoelectric properties, have been considered in this paper. The photorefractive gratings formed by plane light waves slowly changing in time in boundless piezoelectric media have been analyzed in detail. The influence of piezoelectric properties of the crytals on the effective static dielectric constant at different orientations of the photorefractive grating vector has also been considered.

Photorefractive compound semiconductors are attractive for optical processing because of fast material response, compatibility with semiconductor lasers, and availability of cross polarization diffraction for enhancing signal-to-noise ratio. This paper presents a collection of recent experimental results on optical processing using photorefractive GaAs and InP. The results demonstrate the feasibility of using photorefractive compound semiconductors as dynamic holographic interaction media for optical processing applications.

The photorefractive effects of the as-grown and proton-implanted KNbO3 crystals were studied by two-wave mixing at 62.2µW/cm2 illumination. At this pump power level, the as-grown crystal showed little photorefractive effect, while the proton-implanted crystal showed an obvious effect. Furthermore, there seemed to exist two photorefractive gratings with response times of 22.54 and 1596.30s, at 62.2µW/cm2 pump power level, in the proton-implanted sample. This phenomenon has been attributed to the influence of proton-implantation.

The optical and photoconductive fatigue of fast photorefractive polymers have been studied in a family of C60-sensitized polymer composites containing styrene-based chromophores with varying ionization potential. Changes in response time and in photoconductivity were studied for exposures up to 104 J/cm2. Increasing the chromophore ionization potential beyond that of the polyvinylcarbazole host was found to stabilize the response time. Studies of the electric-field dependence of the steady-state diffraction efficiency in various samples confirm the role of C60 anions as possible traps.

Photoinduced variations in the refractive index of nitrogen-doped silica are directly measured in the wavelength range from 350 to 2500 nm. It is found that the change in the refractive index under the action of 193 nm laser radiation significantly increases in the long-wavelength region and amounts to ~10-3 at λ=2.5 μm (for the energy fluence ~104 J cm-2). This means that the nature of the photorefractive effect in nitrogen-doped silica is related to photostructural transformations affecting the phonon part of the optical absorption spectrum.

The drift length of charge carriers has a significant influence on the dynamics of the space-charge field in organic photorefractive materials. This letter introduces a relatively simple method for the determination of the drift length, which takes into account that the charge carrier mobility depends on the sample thickness. By combining results of time-of-flight and holographic time-of-flight experiments using the stochastic transport model of Scher and Montroll, the effective drift length can be determined as 2.4 μm in the investigated photorefractive glass.

Permanent photorefractive gratings with considerable refractive index modulations (~3×10–4) were written in silica optical fibers doped with very low levels of SnO2(~0.15 mol %) using ultraviolet (UV) laser radiation at 248 nm. The photoinduced refractive index change does not show any sign of saturation even for long exposure times (i.e., total fluence >20 kJ/cm2). A comparison with GeO2:SiO2 fibers shows that under similar UV intensities saturation takes place for shorter exposure time and comparable refractive index changes are obtainable for GeO2 concentrations ~10 mol %.

The linear and the circular bulk photovoltaic and photorefractive effects in a piezoelectric La3Ga5SiO14: Fe crystal have been studied. It is shown that the photorefractive effect is induced by generation of the photovoltaic voltage.

Summary form only given. In this paper, we present the results of theoretical analysis and experimental investigation of contradirectional two-wave mixing with partially coherent waves in photorefractive crystals using coupled quasi-monochromatic pump and signal waves. Because the complex amplitudes are stationary random processes of time we can define the corresponding self coherence functions and mutual coherence function. Because the coherence time of a wideband laser source is substantially less than the relaxation time of a typical photorefractive material, the position and profile of the photorefractive grating are nearly temporally constant. As an approximation, the amplitude of the grating is proportional to the mutual coherence of the two waves.

We demonstrate the extension of the photorefractive effect in BaTiO(3) crystals to the 1.5-microm wavelength regime, which is obtained by two-photon absorption induced by intense femtosecond light pulses. The strong dependence of the effect on the light's intensity permits nondestructive readout of the photorefractively induced pattern by low-intensity light. One can thus use the photorefractive effect in the 1.5-microm regime to form optical components for communication applications.

A laser beam propagating in a photorefractive crystal generates a refractive-index profile that deflects the beam to one side. This nonlinear deflection can be balanced by total internal reflection at the crystal surface to produce self-induced photorefractive surface waves. Theoretical and experimental evidence for this effect is given.