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We have developed high-performance photorefractive (PR) materials containing ternary composites of 2-ethylhexyl plasticized poly(triarylamine) (PTAA), a chromophore, and a photosensitizer. PTAAs containing carbazole or fluorene in a main chain and 2-ethylhexyl in a side chain are synthesized via palladium-complex catalyzed polyamination. The spectroscopic, thermal, and hole-transport properties of these polymers are analysed. The carbazole-containing polymer shows good compatibility with chromophores, affording the high chromophore loading of polymer/chromophore = 70/30 wt%. This polymer can be utlilised in PR devices as a host material without any additional plasticizer. The PR performance of a polymer/chromophre/sensitizer (69/30/1 wt%) composite was evaluated using an optical four-wave mixing technique (FWM) to determine the change in refractive index Δn and the average-response time constant τ. This composite exhibits results comparable to the inorganic crystals BSO or SPS when the optical sensitivity is maximized at 15.8 cm3 kJ−1, with Δn = 5.3 × 10−4 and τ = 2 milliseconds under a pump-laser intensity of 149 mW cm−2. This is one of the highest values obtained for an organic PR composite with a pump laser operating at 633 nm.

Photorefractive materials are capable of reversibly changing their index of refraction upon illumination. That property allows them to dynamically record holograms, which is a key function for developing an updateable holographic 3D display. The transition from inorganic photorefractive crystals to organic polymers meant that large display screens could be made. However, one essential figure of merit that needed to be worked out first was the sensitivity of the material that enables to record bright images in a short amount of time. In this review article, we describe how polymer engineering was able to overcome the problem of the material sensitivity. We highlight the importance of understanding the energy levels of the different species in order to optimize the efficiency and recording speed. We then discuss different photorefractive compounds and the reason for their particular figures of merit. Finally, we consider the technical choices taken to obtain an updateable 3D display using photorefractive polymer. By leveraging the unique properties of this holographic recording material, full color holograms were demonstrated, as well as refreshing rate of 100 hogels/second.

Publisher Summary The electrooptic effect is the change in the refractive index of a material induced by the presence of a dc (or low-frequency) electric field. In some materials, the change in refractive index depends linearly on the strength of the applied electric field. This change is known as the linear electrooptic effect or Pockels effect. Linear electrooptic effect can occur only for materials that are noncentrosymmetric. Although the linear electrooptic effect can be described in terms of a second-order nonlinear susceptibility, a very different mathematical formalism has historically been used to describe the electrooptic effect. The photorefractive effect is the change in refractive index of an optical material that results from the optically induced redistribution of electrons and holes. The photorefractive effect is quite different from most of the other nonlinear-optical effects. The photorefractive effect tends to give rise to a strong optical nonlinearity. Moreover, under certain circumstances, two beams of light can interact in a photorefractive crystal in such a manner that energy is transferred from one beam to the other. This process, which is often known as two-beam coupling, can be used to amplify a weak, image-bearing signal beam by means of an intense pump beam.

Unidirectional ( plane) 1, 3, 5-triphenylbenzene (3PB) single crystal 150 mm long and 25 mm diameter was grown by SR technique. The grown crystal was subjected to UV–vis-NIR spectral analysis determine linear optical properties over the range of 200–1100 nm. The relevant optical parameters such as transmittance, absorption coefficient, extinction coefficient, band gap energy, reflectance, and refractive index were determined which reveal the suitability of crystal to couple with a photomultiplier tube or other light sensor. The wide band gap energy 3.65 eV of the 3PB crystal shows significant transmittance in the Vis and NIR region. The refractive index of the crystal is determined to be constant above 350 nm, and it confirms the required parameter to set high light yield. The temperature dependence refractive index was measured in the range of 30–70 °C, it illustrates the faster propagation light in the crystal. Third order nonlinear optical parameters of the grown crystal were estimated by Z-scan technique. It showed that the low absorption coefficient and negative refractive index. The second order hyperpolarizability (γ) of 3PB molecule was found to be 8.6411 × 10−34 esu. The obtained all results in the present work which makes the 3PB crystal promising candidate for optical power limits, optical switching and other nonlinear optical applications. The Laser Damage Threshold (LDT) was determined to be 2.1612 GW/cm2 and it confirms the suitability of crystal for device fabrication.

Photorefractive holograms have received great attention for decades due to their transmissive geometries and dynamic nature. The PR mechanism involves charge generation, charge transport, charge trapping, and modulation of the refractive index. So far, photorefractive hologram is regarded as one of the most important holographic materials with dynamic features. Since the first PR mechanism was described about 50 years ago, extensive investigations have been undertaken by many researchers and it is anticipated that PR materials will be the center of optoelectronic material research in the near future. This article reviews the current understanding of the mechanism responsible for the PR effect and current research trends, and briefly introduces recent progress in applications and material design.

The refractive index is crucial in determining the detailed nature of the propagation of the electromagnetic waves in a medium. There is a growing demand for high-resolution measurement of the refractive index because newly synthesized materials are usually small and optical devices and the elements of metamateirals are being miniaturized. In addition, the inhomogeneous broadening effects of organic materials cannot be described by the mathematical dielectric models, which are essential to extract the refractive index from conventional spectroscopic measurements. Here, we present a novel method called surface plasmonometry, which extracts the complex refractive index of a material using surface plasmons without any dielectric models. By changing the length of the nanoslot antennas, various plasmon modes are excited and coupled to the target material, which becomes a unique characteristic of the refractive index. Since the optical parameters can be extracted at each wavelength through a machine learning al...

We report on the photoconductivity and the photoelectric conversion measured on a thin photorefractive sillenite crystal plate, between transparent electrodes, in the longitudinal configuration where the current is measured along the same direction of the light beam through the sample. Its behavior is based on the already reported light-induced Schottky effect. The wavelength for optimal photoconductivity is determined. A specific parameter is formulated here for quantitatively determining the photoelectric conversion efficiency of the sandwiched material.

The photorefractive effect presents a periodical change of the refractive index of an optical environment under the influence of a coherent light. An interesting phenomenon which occurs at this effect is optical phase conjugation (PC). Photorefractive oscillators, that is photorefractive mirrors present important decides in photorefractive optics and their function is based on photorefractive effect. In these oscillators, a phase-conjugated light beam occurs. The basic characteristics of photorefractive oscillators, such as reflectivity, the existance of the oscillation threshold and the threshold of the coupling strength are explained by the so-called grating-action method. This is analysed on a ring oscillator, semilinear mirror and linear mirror.

The phase shift between interference pattern and index grating in photorefractive materials affects the coupling between the input beams and hence changes the output. The effect of this phase shift is being analyzed on the redistribution of energy among higher diffraction orders in photorefractive sillenite crystals. The direction of energy transfer with the higher-order beams can be controlled by varying the phase shift. The case of two-wave mixing in transmission geometry is considered. The generation of higher-order beams are also being affected by the ratio of intensities of the two incident beams. The generation of higher orders controlled by various parameters could be utilized for the application of optical interconnects.

The dynamic refractive-index-change behavior of the light-induced process in an Mn:Fe:KTN crystal illuminated by a focused light sheet can be observed experimentally by digital holographic interferometry. By numerically retrieving a series of sequential phase maps from recording digital holograms, the spatial and temporal evolution of the light-induced refractive-index-change distribution inside the material is visualized in situ and monitored in a quantitative and in full field way. With this technique, the effect of recording parameters, such as writing laser power and polarization, bias voltage, temperature, and writing time, on the Mn:Fe:KTN crystal in the photorefractive effect can be explored. Therefore, optimized recording parameters will be achieved according to the dynamic behavior. The method provides an access to explore the evolution of the photorefractive (PR) effect of electrooptic crystal under various situations.

In organic optical semiconductors, it is rather challenging to achieve precise control of photoconductivity and charge trapping, which determines the device performance. This paper reports on enhanced photorefractive response rate through control of the photoconductivity and trapping rate in organic triphenylamine-based photorefractive materials by means of bulk state tuning. The bulk state in organic triphenylamine-based photorefractive composites was controlled through a rapid cooling process from various melting temperatures during sample fabrication. The photoconductivity and trapping rate were determined from photocurrent measurements. Fabrication at lower melting temperatures enhanced the trapping rate for deep traps, whereas it reduced the trapping rate for shallow traps. As a result, a faster photorefractive response was obtained.

In this paper, the measurement of grating period in photorefractive anisotropic self diffraction by using digital holography technique is proposed. In our experimental setup, He –Ne laser beam with wavelength of 632.8 nm has been separated and then incident on photorefractive cerium doped barium titanate crystal to produce photorefractive index grating. The transmitted probe beam, which contains phase and amplitude has been expanded and recorded on digital camera. To explore the grating periods, both phase and amplitude of the images are reconstructed by numerical process using computer. Then the grating periods in photorefractive anisotropic self diffraction have been measured. The results show grating periods, holograms and their reconstruction.

In this paper, we review recent progress of research on the photorefractive effect of ferroelectric liquid crystals. The photorefractive effect is a phenomenon that forms a dynamic hologram in a material. The interference of two laser beams in a photorefractive material establishes a refractive index grating. This phenomenon is applicable to a wide range of devices related to diffraction optics including 3D displays, optical amplification, optical tomography, novelty filters, and phase conjugate wave generators. Ferroelectric liquid crystals are considered as a candidate for practical photorefractive materials. A refractive index grating formation time of 8–10 ms and a large gain coefficient are easily obtained in photorefractive ferroelectric liquid crystals.

Photochromic effect in nominally pure and doped Sn 2 P 2 S 6 photorefractive crystals is investigated in the temperature range 120–310 K. This effect determines a mechanism of the amplitude hologram formation at low temperatures, and we show that a competition between the photorefractive (phase) and the amplitude gratings occurs at increasing temperature.

Hot-electron transport in photorefractive semiconductors strongly affects the recording of photorefractive grating. However, the theoretical description of this behavior has been always made under small signal approximation. In this paper rigorous numerical investigation of stationary and transient behavior of photorefractive grating induced in the semiconductor quantum wells structure is presented. The calculations were performed in the framework of the band transport model for the case of high modulation depth. The comparison between numerical and approximated analytical solutions is given as well as between linear and nonlinear transport model. Obtained results allow to verify some statement formulated in few earlier works concerning beneficial effects of electron transport nonlinearity.

Photorefractive response time and photorefractive sensitivity are two important parameters characterizing photorefractive materials. We study theoretically the effect of the grating spacing on these parameters at large modulation depth in the absence and presence of an applied electric field for some of the most promising photorefractive materials, namely, LiNbO3, KNSBN, SPS, BGO, and GaAs. We find that the response time increases with increasing grating spacing for LiNbO3 and KNSBN, whereas the response time decreases with increasing grating spacing for SPS, BGO, and GaAs. The photorefractive sensitivity is mainly affected by the mobility–lifetime product in the presence of an applied electric field.

We provide a theoretical investigation of temperature effects on one-dimensional modulation instability of broad optical beams in photorefractive crystals. Those physical factors related to the temperature effect, i.e., the dielectric constant, diffusion process and the dark irradiance, are considered. Our results show that, for non-centrosymmetric crystals, the dark irradiance dominates the temperature dependence of modulation instability; however, the dielectric constant temperature-dependence dominates the process in centrosymmetric photorefractive crystals. Relevant examples are provided.

Introduction Classification of Materials Crystalline Materials Ceramics, Metals and Alloys, and Polymers Functional Classifi cation of Materials Crystal Structures Directions and Planes in Crystal Structures Interstitial Sites or Holes in Crystal Structures Coordination Numbers Radius Ratio Concept Crystal Structures of Different Materials Defects in Materials Point Defects in Ceramic Materials Kroger-Vink Notation for Point Defects Dislocations Stacking Faults and Grain Boundaries Microstructure-Property Relationships Amorphous Materials Nanostructured Materials Defects in Materials: Good News or Bad News? Electrical Conduction in Metals and Alloys Ohm's Law Sheet Resistance (Rs) Classical Theory of Electrical Conduction Drift, Mobility, and Conductivity Electronic and Ionic Conductors Limitations of the Classical Theory of Conductivity Resistivity of Metallic Materials Joule Heating or I2R Losses Dependence of Resistivity on Thickness Chemical Composition-Microstructure-Conductivity Relationships in Metals Resistivity of Metallic Alloys The Quantum Mechanical Approach to Conductivity Electrons in an Atom Electrons in a Solid Band Structure of Solids Concept of the Fermi Energy Level Fundamentals of Semiconductor Materials Intrinsic Semiconductors Temperature Dependence of Carrier Concentrations Band Structure of Semiconductors Direct- and Indirect-Bandgap Semiconductors Applications of Direct-Bandgap Materials Motions of Electrons and Holes Extrinsic Semiconductors Donor-Doped (n-Type) Semiconductors Acceptor-Doped (p-Type) Semiconductors Amphoteric Dopants, Compensation, and Isoelectronic Dopants Dopant Ionization Conductivity of Intrinsic and Extrinsic Semiconductors Effect of Temperature on the Mobility of Carriers The Effect of Dopant Concentration on Mobility Temperature Dependence of Conductivity Effect of Partial Dopant Ionization Effect of Temperature on the Bandgap The Effect of Dopant Concentration on the Bandgap (Eg) The Effect of Crystallite Size on the Bandgap Quantum Dots Semiconductivity in Ceramic Materials Fermi Energy Levels in Semiconductors Fermi Energy Levels in Metals Fermi Energy Levels in Semiconductors Electron and Hole Concentrations Fermi Energy Levels in Intrinsic Semiconductors Carrier Concentrations in Intrinsic Semiconductors Fermi Energy Levels in n-Type and p-Type Semiconductors Fermi Energy as a Function of the Temperature Fermi Energy Positions and the Fermi-Dirac Distribution Degenerate or Heavily-Doped Semiconductors Fermi Energy Levels across Materials and Interfaces Semiconductor p-n Junctions Formation of a p-n Junction Drift and Diffusion of Carriers Constructing the Band Diagram for a p-n Junction Calculation of Contact Potential Space Charge at the p-n Junction Electric Field Variation across the Depletion Region Variation of Electric Potential Width of the Depletion Region and Penetration Depths Reverse-Biased p-n Junction Diffusion Currents in a Forward-Biased p-n Junction Drift Currents in a p-n Junction Diode Based on a p-n Junction Reverse-Bias Breakdown Zener Diodes Semiconductor Devices Metal-Semiconductor Contacts Schottky Contacts Ohmic Contacts Solar Cells Light-Emitting Diodes Bipolar Junction Transistor Field-Effect Transistors Types of Field-Effect Transistors MESFET I-V Characteristics Metal Insulator Field-Effect Transistors Metal Oxide Semiconductor Field-Effect Transistors Linear Dielectric Materials Dielectric Materials Capacitance and Dielectric Constant Dielectric Polarization Local Electric Field (Elocal) Polarization Mechanisms-Overview Electronic or Optical Polarization Ionic, Atomic, or Vibrational Polarization Shannon's Polarizability Approach for Predicting Dielectric Constants Dipolar or Orientational Polarization Interfacial, Space Charge, or Maxwell-Wagner Polarization Spontaneous or Ferroelectric Polarization Dependence of the Dielectric Constant on Frequency Complex Dielectric Constant and Dielectric Losses Equivalent Circuit of a Real Dielectric Impedance (Z) and Admittance (Y) Power Loss in a Real Dielectric Material Equivalent Series Resistance and Equivalent Series Capacitance Ferroelectrics, Piezoelectrics, and Pyroelectrics Ferroelectric Materials Relationship of Ferroelectrics and Piezoelectrics to Crystal Symmetry Electrostriction Ferroelectric Hysteresis Loop Piezoelectricity Direct and Converse Piezoelectric Effects Piezoelectric Behavior of Ferroelectrics Piezoelectric Coefficients Tensor Nature of Piezoelectric Coefficients Relationship between Piezoelectric Coefficients Applications of Piezoelectrics Devices Based on Piezoelectrics Technologically Important Piezoelectrics Lead Zirconium Titanate Applications and Properties of Hard and Soft Lead Zirconium Titanate Ceramics Electromechanical Coupling Coefficient Illustration of an Application: Piezoelectric Igniter Recent Developments Piezoelectric Composites Pyroelectric Materials and Devices Magnetic Materials Origin of Magnetism Magnetization (M), Flux Density (B), Magnetic Susceptibility (chim), Permeability (mu), and Relative Magnetic Permeability (mur) Classification of Magnetic Materials Ferromagnetic and Ferrimagnetic Materials Other Properties of Magnetic Materials Magnetostriction Soft and Hard Magnetic Materials Hard Magnetic Materials Isotropic, Textured (Oriented), and Bonded Magnets Soft Magnetic Materials Magnetic Data-Storage Materials Index

Organic photorefractive (PR) materials have been studied during the last quarter-century, and they have recently received much attention due to their updatable features that allow them to be used in dynamic holographic devices. However, understanding bulk trap sites that drive the PR effect (by inducing a space-charge field) remains a critical issue. In general, the trap site behavior can be controlled from the energetic point of view; however, bulk devices contain not only the energy trap sites but also structural neutral ones, which decrease the carrier mobility. Organic PR devices are bulk optoelectronics devices, and the investigation of their trap tuning mechanism is still ongoing. In this thesis, carrier transport and trapping properties of the PR materials (represented by triphenylamine-based polymers) with induced space-charge fields are investigated. The effects of plasticizers and wavelengths of interference beams on the PR performance of the resulting holographic devices are studied via photoconductivity measurements. In order to solve a critical challenge of trap tuning, a modified fabrication technique is proposed, which enhances the photoconductivity of the PR materials by suppressing shallow traps. The trap site properties are controlled by studying bulk dipolar molecules with a photoemission yield spectroscopy in air technique, which provides the distribution of both the transport and trap sites. Finally, dynamic holographic images were produced by using organic PR composite films.

The orientational contribution pointed out the major contribution of the molecular first order susceptibility to the index modulation in organic photorefractive materials. This discovery leads the way to the use of liquid crystals as PR materials due to their spontaneous birefringence and to the many possible electric field-induced reorientational effects. In this chapter, we are focusing on the structure and electro-optic properties of the smectic phases. After a brief introduction to the different phases (A and C), we will discuss the specific properties each phase contribute to in a photorefractive liquid crystal system. The theory of the molecular alignment into the photo-generated space-charge field is detailed incorporating the particularities due to liquid crystal phases and light polarization. Some specific cases are analyzed in detail such as the application of an AC field and the use of bistable devices which bring both long-term stability and nondestructive readout.

Photorefractive phenomenon is based on the refractive index modulation due to the redistribution of charge carriers generated upon the illumination of the interference of laser beams in the materials possessing both photoconductivity and optical nonlinearity. Photogeneration of charge carriers in the bright regions of the interferencing beams initiates the photorefractive phenomenon. Thus the photogeneration of charge carrier upon photoexcitation is first step for the photorefractive phenomenon. Through several photophysical processes, positive and negative charge carriers are periodically distributed to form the refractive index modulation via the first-order electro-optics effect of the Pockels effect. In this chapter, the role and the influence of photoconductive properties in organic photorefractive materials on the photorefractive performance are discussed, and the strategy for obtaining the better and the best photorefractive performance, higher diffraction efficiency, and faster speed of the grating formation of the corresponding materials is discussed from the point of view of the photoconductivity.

In the paper results of the investigation of the influence of electric properties of the environment surrounding LiNbO3 crystals on photorefractive effect induced in these crystals by Gaussian Ar+ laser beam with various intensities are presented. We show spatial and temporal dependences of changes of the refractive index obtained experimentally in LiNbO3: Fe and LiNbO3: Fe:Mn samples surrounded by media with different electric conductivities and different permittivities (water, air, water solution of CaCl2). The space and time dependences of the refractive index changes induced by the Ar+ laser beam are observed by means of the Mach–Zehnder interferometer using light from HeNe laser. The experimentally obtained results are in a good agreement with those following from numerical calculations using the manifold mirroring method. The agreement between calculated and experimental results indicates that the polarization charge at the photorefractive crystal/surrounding medium boundary significantly influences the photorefractive process in the crystal. The experimentally observed slow spontaneous decrease of the refractive index change in a sample placed into a slightly conducting medium (air) after switching off the beam also indicates that the polarization charge in the sample's surroundings affects the photorefraction.

Photorefractive surface waves propagating in a stationary regime along the surface of the metal — photorefractive crystal (SBN-75) interface are considered. The transverse structure of the optical field distribution is calculated in the near- and far-field zones, depending on the angle of incidence of the exciting wave. The calculation results are in good agreement with the published experimental results. It is shown that the photorefractive effect leads to a splitting of the spectrum of a surface plasmon polariton excited at the metal — photorefractive crystal interface.

We consider a steady-state nonlinear photorefractive surface wave (PR SW) with TE or TM polarisation when the refractive index of the photorefractive crystal (PRC) depends on the strength of the diffusion crystal electric field emerging upon the wave propagation. We have determined the phase trajectory and transverse structure of the PR SW intensity distribution for different values of the diffusion photorefractive nonlinearity. We have investigated a photorefractive diffraction grating, which arises in the surface PRC layer during propagation of the nonlinear PR SW.

The influence of photoconductivity on the photorefractive effect of ferroelectric liquid crystals (FLCs) was investigated. Mixtures of phenylpyrimidine derivatives, a chiral dopant, and a viscosity reducer were used as host FLC materials and two types of photoconductive compounds and an electron acceptor compound were added. The photorefractive effect was evaluated using two-beam coupling experiments. It was found that the magnitude of the gain coefficient was affected by the difference between the mobility of the photoconductive compound and that of the electron acceptor.

This review describes recent advances and applications in the field of organic photorefractive materials, an interesting area in the field of organic electronics and promising candidate for various aspects of photonic applications. We describe the current state of knowledge about the processes involved in the formation of photorefractive gratings in organic materials and focus on the chemical and photo-physical aspects of the material structures employed in low glass-transition temperature amorphous composites and organic photorefractive glasses. State-of-the art materials are highlighted and recent demonstrations of photonic applications relying on the reversible holographic nature of the photorefractive materials are discussed.

In this work we report the results of the experimental investigation of the refractive index change due to the photorefractive effect in periodically poled lithium niobate (PPLN) Ti-diffused waveguides in dependence on light intensity and sample temperature. We show an experimental method that allows for the direct determination of the refractive index change. The saturation state of the change of the refractive index is reached at 775 nm with input power of 50 μW.

Using the methods of photorefractive and Raman light scattering, we study subtle features of the structure of LiNbO3 single crystals with different Li/Nb ratios (pure and doped with nonphotorefractive cations) grown by different methods. We reveal that, upon the irradiation of a single crystal with visible laser light, locally fluctuating micro- and nanostructures are initially formed in it, with their physical parameters being different from the corresponding parameters of the single crystal in the absence of the photorefractive effect. Upon an increase in the irradiation intensity and in the course of time, more and more such micro-structures are formed, and they are transformed into static micro- and nanoformations, which are subsequently converted into a continuous laser track. The speckle structure of photorefractive scattering is studied in detail. We show that the photorefractive effect in single crystals of the stoichiometric composition is fairly strong for their use as materials for recording and storing information.

Refractive index grating recording is achieved in the infrared in a nematic liquid crystal placed between semiconductor CdTe substrates. Light-induced space-charge field created in photorefractive semiconductor substrates varies an alignment of liquid crystal molecules inhomogeneously in space forming a spatial modulation of the refractive index in liquid crystal. Two-beam coupling is studied in the hybrid cell, in which a gain factor Γ=16 cm−1 is achieved in the layer of liquid crystal of the sandwich.

The relationship between the thermal properties and the photorefractive effect of various ferroelectric liquid-crystal compositions was investigated. Mixtures of phenyl pirimidine derivatives were used as host liquid-crystal materials to which chiral dopants and photoconductive compounds were added. The photorefractive two-beam coupling gain coefficient was strongly dependent on the magnitude of spontaneous polarization and the uniformity of the surface-stabilized state.

We theoretically studied photorefractive surface waves supported by polymers with orientationally enhanced photorefractive effect. We find that the orientational enhancement and the dependency of the quantum efficiency of generating mobile holes on the electric field make the diffusion nonlinearity in photorefractive polymers dependent not on only the variation of light intensity but also on external electric field, which is very different from the external electric field independent diffusion nonlinearity in inorganic photorefractive crystals. Consequently the profile, frequency and especially the penetration depth of polymeric photorefractive surface waves strongly depend on external electric field, which makes it more controllable than that in photorefractive inorganic crystals. The stability of polymeric photorefractive surface waves, the effects of birefringence component and electro-optic component of orientationally enhanced photorefractive nonlinearity on the formation of x or y-polarized polymeric photorefractive surface waves are also discussed.

Photoconductive dependence of two-beam coupling between the pump beam and the signal beam in photorefractive materials have been analyzed in case of non-degenerate wave mixing under the undepleted pump approximation method. During the two-wave mixing in photorefractive materials, steady state amplification of the signal beam and oscillation characteristics of a single unidirectional ring resonator has been studied. The domination of the two-beam coupling gain over the combined absorption and resonator losses such as Fresnel reflections from the crystal and imperfect mirrors builds up unidirectional oscillation. The buildup of such an oscillation leads to a saturation of the gain, which can be explained in terms of the photorefractive phase-shift. The existence of this phase-shift between the photorefractive index grating and the illumination intensity pattern, which is of characteristic of the photorefractive effect, leads to an energy transfer between the two beams. For a single unidirectional ring resonators, the effects of photoconductivity of the materials, two-beam energy coupling coefficient, dielectric constant, crystal thickness, and material's absorption coefficient on amplification of the two-beam coupling gain and photorefractive phase-shifts of the signal beam have also been studied in detail. It has been found that amplification of the signal beam and phase-shift can be enhanced by taking the photorefractive crystal having higher photoconductivity and lower dielectric constant, which improves performance of the resonators.

Comparative studies are made of the photorefractive scattering of light in nonlinear single crystals of lithium niobate with congruent compositions (LiNbO3) doped with "photorefractive" Cu [0.015 mass %] and "nonphotorefractive" Zn [0.5 mass %] cations. For the first time it is found that single crystals doped with "photorefractive" and "nonphotorefractive" cations have different indicatrices for photorefractive light scattering. The aperture angle for photorefractive scattering reaches its steady state value more rapidly with high laser powers than with low. However, at high powers laser induced heating of the crystal is greater, and this leads to a narrowing of the scattering indicatrix. It is also found that photorefractive scattering in these single crystals depends on the region of the boule from which a sample has been cut. This indicates that there is a nonuniform distribution over the boule of the imperfections with localized electrons which determine the magnitude of the photorefractive effect.

Photocurrents in liquid crystal cells of different structures were experimentally studied. The results of the study confirm the photorefractive mechanism of the light beam self-action in nematic liquid crystals, which consists in a decrease in the density of surface charges screening an external electric field upon light exposure. The data obtained from photoelectric and optical measurements suggest that the photorefractive effect appears in cells with different orientants and in the absence of orientant. In a planar cell oriented by rubbed polyimide, the crucial role is played by the interface between the liquid crystal and polyimide deposited on the anode.

We investigated the effect of chromophore (DB-IP-DC) on the photorefractive performance in organic photorefractive composites. We prepared several photorefractive composites; PSX-Cz:TNF:DB-IP-DC:BBP = 69:1:x:(30 − x) wt% and measured photocurrent and PR grating build-up time. The photocurrent was measured by the competition between the hole trapping and detrapping rates from DB-IP-DC and PSX-Cz. The PR grating build-up rate increased with the increment of photocurrent.

Laser-induced change in the refractive index in fullerene- and nanotubes-doped conjugated structures have been investigated by the dynamic holographic technique. Nonlinear refractive index n 2 and nonlinear optical susceptibility χ(3) of the compounds have been calculated. The role of the high frequency Kerr photorefractive effect in the holographic grating recording under nanosecond laser irradiation has been discussed. A niche of the organic materials doped with nano-objects has been found among other nonlinear optical systems. The nano-object-stimulated improvement of laser strength of materials has been shown.

We study experimentally the formation of refractive index voxels (volume elements) in photorefractive LiNbO3 and LiTaO3 crystals illuminated with high irradiance femtosecond laser pulses. We used 150 fs pulses at 800 nm wavelength (energy 6–50 nJ) tightly focused inside the crystals in a single shot regime. This resulted in a formation of a micrometer size region of elevated refractive index, which may be used as memory bits in information storage/retrieval application. The maximum refractive index change of 5×10−4 was recorded in undoped LiNbO3 at an average light intensity of ∼TW/cm2 that is close to the breakdown threshold. A simple setup for photorefractive recording and in situ monitoring of the refractive index changes has been proposed.

This review article presents the concept of photorefractive beam coupling in hybrid photorefractive cells comprising a liquid crystal layer adjacent to inorganic photorefractive windows. The roles of the liquid crystal layer and the photorefractive windows, as well as the overall advantages of this architecture, are explained. The mechanism involved and the required window parameters to break the local alignment symmetry in order to achieve Bragg-matched two-beam coupling in a liquid crystal layer are discussed.

We consider the modes of surface waves with the diffusion mechanism by the band transport model of the photorefractive effect and the nonlinear equation for the first time. We demonstrate the relationship between the modes of surface waves and the angles of incidence. The conditions for the realization of surface modes at the interface between photorefractive crystal and a metal, a linear dielectric with a lower refractive index, or another photorefractive crystal are discussed.

Photorefractive surface waves on the surface of Sr0.6Ba0.4NbO3 with a diffusion mechanism of nonlinearity have been observed. The formation time of the photorefractive surface waves is related to the intensity of the incident beam and the background illumination. The formation time of the photorefractive surface waves is about 40 min with a beam intensity of 3 mW, 25 min with 10 mW, and 30 s with 10 mW when an additional background illumination is applied. The experimental results showed that an appropriate background illumination will shorten the photorefractive response time.

In this paper, results of studies regarding the photorefractive effect in bistable surface stabilized ferroelectric liquid crystals are reported. In particular, an innovative approach toward the optical control of polarization switching in C70-doped bistable devices by means of a photogenerated space-charge field is described. The photorefractive nature of the resulting refractive index gratings is confirmed by two-beam coupling and direct phase shift measurements. The influence of an applied field and of light intensity on grating formation is also studied. The photorefractive gratings obtained by using this approach are stable, easily erasable, and rewritable. The described devices are “binary” systems, in which the refractive index switches only between two possible values. Bistable systems constitute a completely new class of photorefractive materials, with peculiar properties which distinguish them from previously developed media.

We report some photorefractive higher-order diffraction phenomena of a liquid crystal polymer composite. The photorefractive composite consists of the polymer poly[N-vinylcarbazole] (PVK) doped with the chromophore 4,4′-n-entylcyanobiphenyl (5CB) and the sensitizer C60. In two-wave coupling experiments, the signal beam was interfered by a mutually coherent reference beam within composite materials at a small incident angle. Results are given when the composite was placed behind the focal plane, in front of the focal plane, and in the focal plane. Higher-order diffraction images were obtained by amplification and reduction. A theory of the properties of higher-order diffraction images in a photorefractive liquid crystal polymer composite is developed. The theory is in good agreement with the results of experimental work. The experiments demonstrate the feasibility of optical image processing.

The “optical-damage” effect, or random changes of local refractive index in photorefractive media, is unique phenomenon for creation of the generic dynamic singular vector speckle fields. They were realized in the photorefractive LiNbO3:Fe crystal under illumination by He-Ne laser beam. This paper is devoted to investigation of topological dynamics of the developing singular vector fields and its thorough analysis by the Hurst exponent firstly used in dynamic singular optics.

We show experimental results for the propagation of a diffracted beam by a straight edge like initial condition in a photorefractive crystal under drift nonlinearity. We have observed that the diffracted pattern and propagation is enhanced in the form of multiple waveguides.

Existence of surface gratings associated with volume gratings in a photorefractive material is proved by analyzing reflected diffraction orders of an incident reading beam. A novel technique is used to numerically eliminate the Fabry-Perot effect from the diffracted orders.

The ultraviolet photorefractive effect of Mg-doped near-stoichiometric LiNbO 3 crystals prepared by vapor transport equilibration (VTE) technique was studied at 351 nm. It was found in the near-stoichiometric LiNbO 3 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.

Influence of the external field on photorefractive surface waves is studied theoretically and numerically. We demonstrate that photorefractive surface waves cannot be formed without the diffusion nonlinearity. An externally applied electric field can change the profiles of the photorefractive surface waves, especially the spatial frequency etc., and even destructs photorefractive surface waves upon a threshold value. This provides a possible method to control the formation and the shape of photorefractive surface wave. The influence of dark intensity is also discussed.

It is, well know that some, optical materials to change his refractive index when these exposed to light, depending on particular conditions, they can generate several distributions of refractive index. In this work, an experimental periodic arrangement of refractive index generated on the Bi12TiO20 photorrefractive crystal is presented, the initial condition used was generated by superposing two He-Ne laser linearly polarized beams in a perpendicular direction to the external field applied to the crystal at 632.8 nm. The diagrams of bands generated with this arrangement is studied, based on the similarities presented by the periodic refractive index with the photonic bandgap (PBG) diagrams one-dimensional superlattices composed of alternating layers of two distinct materials. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)