Showing papers on "Organic photorefractive materials published in 1995"

Nonlinear surface optical waves in photorefractive crystals with a diffusion mechanism of nonlinearity.

Abstract: 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.

Photorefractive surface waves.

Abstract: 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.

Optical computing by use of photorefractive polymers.

Abstract: A matched-filter four-wave-mixing optical correlator was built with a photorefractive polymer as the nonlinear material. Two different time scales are important for this type of device: the time to write a grating, which is limited by the response time of the photorefractive material (~100 ms for the polymer used) and the time to diffract light from the grating, which is essentially instantaneous. For the matched-filter optical correlator we have obtained correlations comparing two 5000-pixel images, using 220-fs pulses with 5-mW average power, demonstrating that the time to compare the test image and the reference image is not limited by the relatively slow response time of the photorefractive polymer. Since the photorefractive grating is erasable the device can be reprogrammed for a different reference image in less than 1 s. The results show that photorefractive polymers can be used efficiently in optical image processing applications.

Bifunctional chromophore for photorefractive applications

Abstract: A bifunctional chromophore has been developed which serves as a charge transporting agent as well as a second‐order nonlinear optical compound The chromophore has been incorporated into an inert polymer to form a photorefractive material when doped with a photocharge generation sensitizer The dependence of the four‐wave mixing diffraction efficiency, holographic grating writing rate, and two‐beam coupling gain on the chromophore concentration has been studied The results confirm the photorefractive character of the composite

Nonlinear self‐channeling of a laser beam at the surface of a photorefractive fiber

Abstract: We experimentally observed the self‐channeling of the light beam in photorefractive Bi12TiO20 fiberlike sample owing to the strong fanning effect. It results in significant speed up of the photorefractive response time in waveguides and in anomalous dependence of the response time on the applied ac electric field amplitude: the higher electric field, the faster response.

Photorefractive composites with high‐band‐gap second‐order nonlinear optical chromophores

Abstract: A series of photorefractive composite materials has been developed in which an inert polymer has been doped with second‐order nonlinear optical chromophores, charge transporting agents, and photocharge generation sensitizers. The composites show high photoconductivity, large photorefractive responses, and optical transparency at many wavelengths. Photorefractivity has been demonstrated at wavelengths of 633, 514.5, and 488 nm, with net two‐beam coupling gains.

Fabrication and applications of long-lifetime, holographic gratings in photorefractive materials

Abstract: Substantial improvements in photorefractive device lifetimes are provided by control of electron migration which results in the decay of gratings in photorefractive materials due to diffusion and other effects. A new class of photorefractive devices using compensating electronic and ionic gratings having relatively low efficiency but nonetheless usable gratings is provided by arranging the gratings to be reflective in a wavelength band outside the photo-excitation band of the photorefractive material, as by using an infrared operating wavelength. Longer lifetimes in high efficiency gratings are achieved by constant or periodic illumination of photorefractive materials to assure uniform charged distribution of electrons and maintenance of the ionic backbone grating.

Surface-strain effects on photorefractive gratings.

Abstract: Taking the crystal surface to be traction free, we calculate the lattice distortions and associated elasto-optic refractive-index changes induced by a photorefractive grating. The surface effects are significant and result in refractive-index changes near the surface that differ considerably from bulk values. Such surface effects should be of importance in thin-film photorefractive devices, in thin samples, and in certain situations in bulk crystals.

Nonelectrooptic nonlocal photorefractive effect in a polymer composite

Abstract: The grating spacing dependence of photorefractive diffraction efficiency in an organic polymer composite is presented and discussed. The analysis of the grating spacing dependence of the refractive index modulation with the standard model of photorefractive grating dynamics is shown to lead to inconsistencies in determination of the maximum attainable index change. It is shown that the effects resulting from orienting of the composite’s optically second‐order nonlinear chromophores in the periodic local field affect the formation of the refractive index grating.

Photorefractivity in new organic polymeric materials

Abstract: Two experimental studies of new photorefractive polymer materials are described. First, an examination of the fabrication, stability, and photorefractive behavior of a recently-reported high-efficiency polymeric composite 33%PVK:50%DMNPAA:16%ECZ:1%TNF (K. Merrholz et at., Nature 371, 497) shows that this material is metastable, crystallizing on time scales from hours to days depending upon preparation. The grating phase shift also varies considerably from sample to sample. In the second part of this paper, first measurements and analysis of the properties of a new photorefractive polymer composite which contains a liquid nonlinear optical chromophore are presented. This new composite, which is immune to crystallization even at high chromophore loading levels, shows a useful diffraction efficiency of several percent at high fields, but the index of refraction grating is shown to consist of two components; a smaller photorefractive index grating and a grating which is not photorefractive in origin as shown by grating translation measurements.

Photorefractive polymers-A status report

Abstract: 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.

Comparison of photorefractive-index changes in annealed-proton-exchanged channel waveguides in MgO-doped and congruent LiNbO(3).

Abstract: A quantitative comparison of the photorefractive effect in annealed proton-exchanged channel waveguides in MgO-doped and congruent LiNbO(3) at the wavelengths of 633 and 830 nm is presented. An accurate measurement technique is described to measure the refractive-index change as a function of time and the guided mode intensity for different wavelengths. The results show that doping with 7% MgO reduces the photorefractive effect at a wavelength of lambda = 633 nm by 2 orders of magnitude. The photorefractive effect in the doped substrate shows only a weak dependence on the guided power. Doping with 4 mol.% MgO has only little effect on the photorefractive effect compared with that on the congruent material. A reduced photovoltaic current is responsible for the small photorefractive effect in the 7 mol. %-doped substrate.

KNbO3 crystals for photorefractive applications

Abstract: Characterization and optimization of KNbO 3 crystals for photorefractive applications over an extended spectral range is described. Intrinsic properties are used to describe the refractive index response to space-charge field gratings. Extrinsic properties that are important for the photorefractive effect, such as absorption, photoconductivity, charge transport, and trapping, are discussed. Some results on the extended IR response of crystals doped with Fe, Ni, Cu, Ce, Mn, and Co are presented. Post growth annealing at temperatures of 400 to 900°C in a controlled atmosphere is shown to modify the charge-transport parameters, therefore also modifying the response time of photoinduced refractive index changes.

Influence of annealing treatments on photorefractive properties of KTa1-xNbxO3:Fe crystals

Abstract: 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.

Modification of Photorefractive Waveguides in Lithium Niobate by Guided Beam for Optical Dynamic Interconnection

Abstract: We investigate experimentally and numerically a novel method of modifying photorefractive waveguides in a lithium niobate crystal by a guided beam. The guided beam with a strong optical power can change the refractive index of the photorefractive waveguide because of the photorefractive effect. The modification of waveguide structure can be utilized for optical dynamic interconnection. Changes of optical characteristics of a probe beam caused by a modification beam are investigated. Experimental results show that the transmitted power or the peak intensity of the probe beam decreases exponentially as a function of total exposure energy of the modification beam. We also numerically analyze the refractive index change caused by the modification beam in the photorefractive waveguide. The numerical results show good agreement with the experimental results.

Applicability of the band transport (Kukhtarev) model to photorefractive polymers

Abstract: The band transport model of space-charge production has proved very successful in describing photorefractive phenomena in a wide range of inorganic crystals, ranging from high-mobility semiconductors like GaAs, to highly insulating ferroelectrics like BaTiO3. This success is primarily due to the applicability of a generic picture of charge transport in extrinsic crystalline semiconductors, a picture that becomes cloudy in noncrystalline systems like the new photorefractive polymers. Photorefractive polymers exhibit the classic photorefractive behaviors such as photoconduction, electro-optic response, hologram formation and storage, and two-beam energy coupling, all without benefit of a crystalline lattice or well defined conduction band. In this report, I will outline how the band transport model can be adapted to describe photorefraction in noncrystalline materials by the dual expediencies of renaming certain embarrassing 'constants' and admitting that they are strongly dependent on the total electric field.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Role of photoconductivity in molecularly doped photorefractive polymer

Abstract: The photorefractive effect is a reversible mechanism of holographic grating formation in electro-optic materials, that has potential applications in integrated optics, optical data storage, optical computing, and several other areas. This effect was recently observed in photorefractive polymers doped with charge transport agents. The grating formation is initialized by the photoconductive response which includes charge generation, mobility, and trapping, occurring in successive order. Finally, the grating is formed as a result of modulation of the refractive index by the resulting space charge field, via the electro-optic (Pockels) effect. We present the results of photoconductive measurements as a function of temperature, applied electric field, and illuminated intensity. The investigation is focused on the nonlinear optical polymer bisphenol A 4-4-nitroaminostilbene mixed with 30 weight % of the hole transport agent diethylamino-benzaldehyde diphenyl hydrazone. We observed that the photoconductivity of the photorefractive polymer has the form exp[ aE1/2 + bE1/2/T2 - c/T2] in agreement with the diorder theory of the well- known hopping model developed for charge-transport in molecularly doped polymers. We also observed that the apparent hopping distribution bandwidth increases with increased intensity, consistent with an increasing density of states.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A method of producing a photorefractive effect in optical devices and optical devices formed by that method

Abstract: A method for producing a refractive effect in an optical device and an optical device formed by the method are disclosed in which a material from which the device is to be formed is illuminated to induce solarisation involving a photoinitiated REDOX reaction in the material so that the material exhibits a permanent or quasi-permanent photorefractive effect. The mechanism of forming the photorefractive effect in the material relates to the photon induced oxidation of a species present in the material, either as a dopant, co-dopant or as a main component. The photo-oxidation of the species liberates an electron which, after a brief lifetime, usually finds a suitable repository by combining with some reducible species in the material. The method enables photorefractive effects to be produced in materials and, in particular, glass materials other than standard germanium-doped and silicon-based materials.

Temperature Dependence of the Photorefractive Effect in Proton-Exchanged Optical Waveguides Formed on Lithium Tantalate Crystals

Abstract: Temperature dependence of the photorefractive effect in proton-exchanged waveguides formed on lithium tantalate crystal is investigated. The index change induced by the photorefraction is reduced by the annealing process. Resistance to the photorefractive effect is greatly improved when the samples are heated at temperatures as high as 80° C. In the low-intensity region, the dark conductivity seems to be dominant in the photorefractive effect.

High-performance PVK-based photorefractive polymers

Abstract: Photorefractive materials have many potential photonic applications, including dynamic holographic storage and image processing. Recently, the new class of photorefractive polymers has emerged, offering wide structural flexibility, easy processability, and low cost. Progress in this field has led to nearly 100% diffraction efficiency for the readout of a hologram stored in the material and to extremely large net gain coefficients of more than 200 cm-1. These excellent properties result from refractive index modulations as large as (Delta) n approximately equals 10-2 which could be generated in these materials with low- power lasers, such as HeNe laser or laser diodes.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Increased photorefractive effect in Ti-diffused Fe:LiNbO3 Planar waveguide

Abstract: Ti:Fe:LiNbO3 planar waveguide has been fabricated by Ti-diffusion in Fe:LiNbO3 substrate. Its photorefractive parameters were measured by holographic method. The results indicate that Ti:Fe:LiNbO3 waveguide has higher photorefractive sensitivity, space charge field, and, consequently, larger light induced refractive index change than Ti:LiNbO3. But the transport property of carriers in the waveguide is the same as in bulk.

Self-trapping of weak optical beams in photorefractive materials

Abstract: Intensity-independent self-trapping of optical beams in photorefractive crystals was found recently. But we show that due to the existence of dark conductivity in some photorefractive materials or coherent and non-coherent background irradiance, the self-trapping of weak optical beams in photorefractive materials, such as SBN, becomes intensity dependent. The threshold condition of the applied external de electric field for self-trapping is determined as function of the material parameters as well as the initial transverse sizes and relative intensities of the beams.

Constructively and destructively interfering diffraction in a photorefractive medium

Abstract: Diffraction from a photorefractive grating in the presence of two input beams is studied both theoretically and experimentally. The results are extended to the control of the interference of diffracted beams from two photorefractive gratings, which adopts a nondegenerate pulsed wave coupling technique.Keywords: photorefractive effect, grating, optical storage, two-beam coupling, nondegenerate wave mixing,acousto-optic modulation, hologram, angular multiplexing 1. INTRODUCTION The photorefractive effect has attracted much attention since its discovery not only for scientific interests butalso for optical signal processing and image storage1 . In the use of the photorefractive effect for optical signalprocessing or image storage, the typical ways of angular multiplexing of laser beams toward a photorefractivemedium are mechanical methods such as rotating mirrors or a base on which the photorefractive medium sits.Recently, we proved a method of electrical control of the angular multiplexing of mode-locked laser beams towardthe photorefractive crystal by acousto-optic modulation with an external synchronization technique2. That waspossible by enabling nondegenerate wave coupling in a photorefractive medium.In this paper we study diffraction from a photorefractive grating in the presence of two input beams. Generalizeddiffracted beam intensity formulae are suggested. This result could have applications in making a writing-timingschedule ofmany photorefractive gratings.Adopting the technique forming photorefractive gratings with acousto-optically deflected nondegenerate pulsedlasers, we also probe the feasibility of conirolling interferences of diffracted beams from two photorefractive gratings,which could have applications in optical signal processing or optical computing.In section 2, constructively interfering diffraction from a photorefractive grating in the presence of two inputbeams is discussed. In section 3, the control of interference of diffracted beams is discussed adopting a nondegeneratewave coupling technique with acousto-optic angular multiplexing of laser beams.

Photorefractive Mode Conversion for Fault-Tolerant Source-Fiber Coupling.

Abstract: : The purpose of this contract to study the potential applications of nonlinear optical phenomena in photorefractive materials in various geometries (such as waveguides, fibers, and bulks) for efficient and fault-tolerant laser-to-fiber coupling The study focuses on the development of new application concepts, theoretical modeling and experimental demonstration/characterization of novel devices based on energy coupling among different optical modes via dynamic holograms in c-axis photorefractive waveguides and fibers; and on optical resonators incorporating photorefractive dynamic holograms in bulk crystals The holograms are self-generated by the interaction of various optical modes (waves) propagating through the photorefractive material The final objective is to identify an optimum material and configuration and to demonstrate a higher coupling efficiency and better fault-tolerance in coupling light from a laser to an optical fiber or waveguide than that achievable by the conventional method (AN)

Modelling of Semi-Insulating Photorefractive Quantum Well Devices

Abstract: The photorefractive effect in electrooptic materials provides a very promising tool for image processing applications. Semiconductor multiple quantum well (MQW) structures are attractive materials due to the short response time and the high value for the electrooptic coefficient. If the electric field is applied perpendicular to the MQW planes, than the electrooptic behaviour is associated with the quantum confined Stark effect for excitons in the wells. In these photorefractive multiple quantum well devices, a complicated interaction takes place between the electrons, holes, excitons, photons and deep-level charges. For a better understanding of the internal behaviour of these photorefractive multiple quantum well devices, a numerical simulator was built.

A new class of optical materials for holographic storage and beam steering

Abstract: Photorefractive materials have often been proposed as optical storage as well as optical interconnection media but attempts to build practical systems have been hindered by fundamental limitations of the materials themselves. Recently we have reported a new class of local photorefractive materials based on the properties of deep electron traps known as DX centers which occur in some doped compound semiconductors. These materials overcome many of the problems associated with the conventional materials showing 30 times larger refractive index changes, 60 times better sensitivity to exposing light, grating-period independent sensitivity, and an absence of erasure which normally occurs during multiple-grating exposures in conventional photorefractive materials. The price of all of these advantages is, at least for the present, that the effect in the new materials is persistent only at cryogenic temperatures. Work is underway to identify materials exhibiting these properties at room temperature.

Spectroscopic and photorefractive properties of high efficient cerium-doped barium titanate crystals

Abstract: An array of four diffraction gratings, each tilted at a slightly different angle near Littrow, disperses the sidebands in each segment. The gratings are tilted in a “V” configuration to reduce temporal delay across the beam. As the dispersed beam passes through a relay telescope, a focal plane slit removes all but one sideband from each segment. The tilt of each grating is adjusted so that a different sideband passes through the slit, and what emerges from the telescope is four adjacent segments each with a different single wavelength (Fig. 3). The gratings can be easily tuned to select any wavelength for any segment. At the target chamber, a special foursegment frequency conversion array with independently tunable crystals converts the light to the third harmonic. This system has been used in high energy target shots, achieving x% uniformity between segments on target. To achieve this energy balance, segmented absorption filters near the grating array compensate for sideband imbalance in the oscillator, gain narrowing, and other spectral effects in the laser chain. The ”four color” system has supported two series of target experiments to investigate the effects of broadband illumination of a plasma.