Showing papers on "Organic photorefractive materials published in 2000"
TL;DR: The photorefractive effect in both non-centrosymmetric and centrosyMMetric azo-dye-doped silica glasses, in which refractive index gratings that are spatially phase-shifted with respect to the incident light intensity pattern are observed, enables the transfer of energy between two interfering light beams (asymmetric two-beam coupling).
Abstract: Photorefractive materials1 exhibit a spatial modulation of the refractive index due to redistribution of photogenerated charges in an optically nonlinear medium. As such, they have the ability to manipulate light and are potentially important for optical applications1 including image processing, optical storage, programmable optical interconnects and simulation of neural networks. Photorefractive materials are generally crystals, polymers and glasses with electro-optic or birefringent properties and non-centrosymmetric structure2. Here we report the photorefractive effect in both non-centrosymmetric and centrosymmetric azo-dye-doped silica glasses, in which refractive index gratings that are spatially phase-shifted with respect to the incident light intensity pattern are observed. The effect results from a non-local response of the material to optical illumination, and enables the transfer of energy between two interfering light beams (asymmetric two-beam coupling). Although the writing time for the present grating is relatively slow, we have achieved a two-beam coupling optical gain of 188 cm-1 in the centrosymmetric glasses, and a gain of 444 cm-1 in the non-centrosymmetric structures. The latter are fabricated using a corona discharge process3 to induce a permanent arrangement of azo-dye chromophores.
125 citations
TL;DR: This article reviews the rapid progress of organic photorefractive materials in the last decade and focuses on the different chemical concepts and pathways for further optimization.
Abstract: Photorefractivity is an intriguing phenomenon which, however, was initially perceived as a nuisance: Laser illumination generates large internal electric fields which, in turn, modify the optical properties of the material. Useful applications in holographic systems were soon recognized and photorefractivity was subsequently demonstrated in many different materials. These materials span from inorganic crystals to semiconductors, from organic liquid crystals to polymers. Nevertheless, nearly 25 years passed from the discovery of inorganic photorefractive materials in 1966 to reports of the first organic systems. This article reviews the rapid progress of organic photorefractive materials in the last decade and focuses on the different chemical concepts and pathways for further optimization. Advanced systems are required to move photorefractivity out of research laboratories into the market. Possible applications and their first demonstration will conclude the article.
87 citations
TL;DR: In this paper, permanent photorefractive gratings with considerable refractive index modulations were written in silica optical fibers doped with very low levels of SnO2(~015 mol %) using ultraviolet (UV) laser radiation at 248 nm.
Abstract: Permanent photorefractive gratings with considerable refractive index modulations (~3×10–4) were written in silica optical fibers doped with very low levels of SnO2(~015 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 (ie, 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 %
71 citations
TL;DR: A new method is demonstrated by which the optical power in the ordinarily discarded component is used as the pump to amplify the retained component through photorefractive two-beam coupling to achieve greater than 50% throughput.
Abstract: All known polarizers operate through a separation of orthogonal electric field components, one of which is subsequently discarded. As a result, 50% of the unpolarized incident light is wasted in the process of conversion to polarized light. We demonstrate a new method by which we use the optical power in the ordinarily discarded component as the pump to amplify the retained component through photorefractive two-beam coupling to achieve greater than 50% throughput.
68 citations
TL;DR: In this paper, 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.
Abstract: 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.
39 citations
TL;DR: In this paper, the effect of rigid boundary conditions on the photorefractive response of parallel cells was investigated under a single-elastic-constant approximation with various configurations of externally applied electric and magnetic fields and of the nematic liquid crystal director.
Abstract: Light-induced space-charge fields and the related orientational photorefractive effects in nematic liquid crystal cells are studied both theoretically and experimentally. Analytical solutions for the refractive index change distribution are obtained under the single-elastic-constant approximation with various configurations of externally applied electric and magnetic fields and of the nematic liquid crystal director. The effects of the rigid boundary condition are investigated in detail. The grating spacing for which the refractive index change is maximized depends on the strength of external electric and magnetic fields. The responses of parallel cells differ from those of homeotropic cells, both upon the dependencies on the grating spacing and on the external fields. Under the same conditions, the orientational photorefractive effects are stronger in parallel cells than in homeotropic ones. It is also shown that the investigated effects will be greatly enhanced near the Freedericksz transition. Experimental verifications of the photorefractive response in parallel aligned cells confirm the necessity to consider the boundary conditions explicitly.
30 citations
TL;DR: In this paper, the photorefractive performance of poly(N-vinylcarbazole)-based composites doped with various concentrations of two structurally related dipolar chromophores, at 780 nm.
Abstract: We have studied the photorefractive performance of poly(N-vinylcarbazole)-based composites doped with various concentrations of two structurally related dipolar chromophores, at 780 nm. The two chromophores had different electron donor groups, N,N-diethylamine and julolidine, respectively. Complete internal diffraction and gain coefficients >130 cm−1 were obtained for polymers doped with these chromophores. The polymers prepared with the chromophore having the strongest electron donor group, the julolidine group, had the largest dynamic range, but proved to be slower and had a smaller photorefractive phase shift.
19 citations
TL;DR: To the authors' knowledge, this is the first experimental evidence of a subsecond response in the infrared for a nonsemiconductor photorefractive material.
Abstract: The polarization self-modulation effect was applied for effective measurement of the characteristic response time of nominally pure Bi12SiO20 (BSO) at wavelengths of 810 and 980 nm. Owing to oxygen deficiency in the crystal lattice, the BSO crystals showed unusual photorefractive sensitivity and remarkable operation speed in the near-infrared spectral region. A response time of 130 ms was measured at 810 nm, and a response time of 540 ms was measured at 980 nm, with incident intensities of 110 and 200 mW/cm2, respectively. To our knowledge, this is the first experimental evidence of a subsecond response in the infrared for a nonsemiconductor photorefractive material.
18 citations
TL;DR: In this article, the authors measured photorefractive recording and light and dark erasure in unannealed α-phase proton-exchanged LiNbO3 waveguides.
Abstract: Photorefractive recording and light and dark erasures have been measured in unannealed α-phase proton-exchanged LiNbO3 waveguides. The saturation index change, Δns≅9×10-6, is independent of the light intensity within the studied range, 0.3–50 W/cm2. The time dependencies are well represented by the sum of two exponential components. After complete optical erasure, diffraction efficiency η increases in the dark (i.e., dark developing) up to ∼17% of the saturation value ηs≅0.12 and then decays to zero in ∼4 h. All experimental results are reasonably well simulated by a model in which the Fe2+/Fe3+ light-induced charge distribution is compensated for by a light-insensitive species (ionic charges or holes) that is mobile at room temperature.
17 citations
TL;DR: An organic photorefractive glass that shows high diffraction efficiencies of as great as 40% with 25-mum-thick samples and fast response times of 4.3 ms under irradiation at 790 nm is presented.
Abstract: Infrared imaging materials with high speed and dynamic range are
required for a variety of imaging and diagnostic techniques. We
present an organic photorefractive glass that shows high diffraction
efficiencies of as great as 40% with 25-µm-thick samples and
fast response times of 4.3 ms under irradiation at 790
nm. Furthermore, the intensity dependence and field dependence of
the grating decay are investigated.
14 citations
TL;DR: In this article, an approximate analytic expression of two wave mixing for absorption and phase gratings in photorefractive materials taking into account self-diffraction effect of two writing beams was developed.
TL;DR: In this paper, the authors proposed a mathematical model for the movement in absorbing materials of photorefractive holograms under feedback constraints, and used this model to analyze the speed of a fringe-locked running hologram.
Abstract: We propose a mathematical model for the movement in absorbing materials of photorefractive holograms under feedback constraints. We use this model to analyze the speed of a fringe-locked running hologram in photorefractive sillenite crystals that usually exhibit a strong absorption effect. Fringe-locked experiments permit us to compute the quantum efficiency for the photogeneration of charge carriers in photorefractive crystals if the effect of bulk absorption and the effective value of the externally applied field are adequately taken into consideration. A Bi12TiO20 sample was measured with the 532-nm laser wavelength, and a quantum efficiency of Φ=0.37 was obtained. Disregarding absorption leads to large errors in Φ.
01 Jan 2000
TL;DR: In this article, a new and unexpected form of orientational photorefractivity was discovered; consequently high dynamic range was obtained with low power semiconductor laser diodes; and the polymers were successfully used in different applications, including holographic storage.
Abstract: While photorefractivity during the 1970s and 1980s was studied in many different classes of inorganic materials [1], organic photorefractive (PR) materials emerged only in the 1990s. The quest for organic photorefractive materials was driven by their electronic optical nonlinearity, which leads to materials that combine high electro-optic coefficient and low dielectric constant, a property that improves the figure of merit for photorefractivity Q = n 3 r/e, where n is the refractive index, r is the electro-optic (or Pockels) coefficient, and e is the low-frequency dielectric constant. During recent years, this new field of research experienced several milestones: a new and unexpected form of orientational photorefractivity was discovered; consequently high dynamic range was obtained with low-power semiconductor laser diodes; and the polymers were successfully used in different applications, including holographic storage. With their plasticity and other unique properties, they constitute a strategic class of materials because they enable the mass production at low cost of new devices with low weight, and high performance. Thus such materials are expected to have a significant impact on dynamic holographic technologies.
TL;DR: In this paper, 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, is introduced. But it is not suitable for time-of-flight and holographic simulations.
Abstract: 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.
TL;DR: In this paper, the linear and the circular bulk photovoltaic and photorefractive effects in a piezoelectric La3Ga5SiO14:Fe crystal have been studied.
Abstract: 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.
TL;DR: In this article, the photorefractive response of a bismuth silicate crystal placed in an external meander electric field was studied experimentally and theoretically, and the experimental data were shown to be in good agreement with a model, taking into account the influence of the second harmonic of the space-charge field on a photore-fractive grating.
Abstract: Nonlinearity of the photorefractive response is studied experimentally and theoretically upon two-beam interaction of light waves in a bismuth silicate crystal placed in an external meander electric field. The experimental data are shown to be in good agreement with a model, taking into account the influence of the second harmonic of the space-charge field on a photorefractive grating. The concentration of acceptors in a crystal and a product of the mobility of charge carriers by their recombination time are estimated from a comparison of the experimental and calculated data.
TL;DR: In this paper, a theory of resonant photorefractive wave coupling in cubic photore-fractive crystals that are in general optically active is presented, including the resonance enhancement of the photoreFractive response in a constant applied electric field, the influence of photoelasticity, and the spatial inhomogeneity of the light intensity.
Abstract: We present a theory of resonant photorefractive wave coupling in cubic photorefractive crystals that are in general optically active. This theory includes the resonance enhancement of the photorefractive response in a constant applied electric field, the influence of photoelasticity, and the spatial inhomogeneity of the light intensity. In a unified manner, it allows one to describe the polarization and energy properties of two-wave coupling for the optical configurations relevant to experiment. Applications of the theory are given to photorefractive crystals of the sillenite family.
TL;DR: In this paper, the threshold effect of incident light intensity for photorefractive light-induced scattering and signal beam amplification in doped LiNbO3 crystals was studied experimentally.
Abstract: In this article, we have studied experimentally the threshold effect of incident light intensity for the photorefractive light-induced scattering and signal beam amplification in doped LiNbO3 crystals. Our results show that the signal beam can be most effectively amplified for a specific pump light intensity when the threshold effect of incident light intensity for photorefractive light-induced scattering is taken into account in doped LiNbO3 crystals. Doping the LiNbO3 crystal with suitable concentration of Fe and damage-resistant dopants will provide an effective way to control the photorefractive properties of LiNbO3 crystal. We demonstrated the advantage of the fanning-noise-free double-doped photorefractive LiNbO3 crystals for three-dimensional storage. This method to suppress the fanning noise is very simple and convenient to practice.
TL;DR: In this article, the photorefractive properties of a 0.05-wt. % Ce2O3-doped Pb0.5Ba0.6 crystal are investigated at visible wavelengths.
Abstract: Photorefractive properties of a 0.05-wt. % Ce2O3-doped Pb0.5Ba0.5Nb2O6 crystal are investigated at visible wavelengths. Compared with the performance of the undoped crystal, sensitivities as high as 3 cm/J represent an increase of almost two orders of magnitude, and a photorefractive gain of 25 cm-1 for extraordinary polarization exhibits a fourfold increase. The saturated diffraction efficiency varies with the intensity ratio of the incident beams owing to strong beam coupling. Photoconductivity at 514.5 nm and dark conductivity are also measured. The intensity dependence of these light-induced parameters suggests a nonsingle-photon behavior. We use a two-center model that involves both a shallow level and a deep level to explain the measured results.
TL;DR: A cw-probe Z-scan technique was employed to measure the photoinduced index change in a photorefractive SBN:60 crystal and a theoretical simulation of the Z scan based on a band-transport model of photoreFractive-index variation provided reasonable agreement with the experimental results.
Abstract: A cw-probe Z-scan technique was employed to measure the photoinduced index change in a photorefractive SBN:60 crystal. For this experiment a three-detector data-acquisition system was used to account for temporal changes in the laser. The effects of various beam parameters such as intensity, polarization, and wavelength were studied. A theoretical simulation of the Z scan based on a band-transport model of photorefractive-index variation was also developed. This model provides reasonable agreement with the experimental results.
TL;DR: In this paper, photoinduced variations in the refractive index of nitrogen-doped silica are directly measured in the wavelength range from 350 to 2500 nm, and it is found that the change in the photorefractive 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).
Abstract: 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.
TL;DR: In this paper, the photorefractive signal beam gain is measured as a function of the pump beam intensity, beam ratio intensity, spatial frequency of the grating, and angle between grating vector and c axis of the crystal.
Abstract: photorefractive two-beam coupling and energy transfer in BaTiO3 are studied at multiple wavelengths using He-Cd, He-Ne, and diode lasers. The photorefractive signal beam gain is measured as a function of the pump beam intensity, beam ratio intensity, spatial frequency of the grating, and angle between the grating vector and c axis of the crystal. The exponential gain coefficient is calculated from signal beam gain, and its dependence on the same parameters is studied. The dependence of the signal beam gain and exponential gain coefficient on spatial frequency are also evaluated theoretically and found to be in good agreement with the experimental data. Also, the signal beam gain is studied as a function of wavelength, and a very high value is obtained at 441.6 nm. Figure-of-merit parameters such as the maximum change in the refractive index, the space charge field at saturation, the trap density of the charge carriers, and the photorefractive sensitivity of the crystal are calculated from the experimental data. since the absorption in BaTiO3 varies strongly with wavelength, its influence has been included in the calculations of the figure-of-merit parameters. The results are discussed in the light of the present understanding of photorefractive phenomena in crystals.
06 Aug 2000
TL;DR: In this article, single beam polarization interferometry was introduced to measure the Pockels coefficients in a BSO single crystal and the linear superposition principle of the induced birefringence and the optical activity was employed in the analysis of the PPI.
Abstract: Single beam polarization interferometry was introduced to measure the Pockels coefficients in a BSO single crystal. The linear superposition principle of the induced birefringence and the optical activity was employed in the analysis of the Pockels effect measurement.
01 Jan 2000
TL;DR: In this article, the photorefractive process is used to transform temporally and spatially varying light intensity patterns into physical changes in the optical properties of the material, matching the intensity patterns.
Abstract: Publisher Summary This chapter provides an introduction to photorefractive semiconductor nanostructures. They are nonlinear optical devices that alter their optical properties in response to time- and space-varying light intensity patterns. They are used to perform dynamic holography under ultra low-light intensities much smaller than those used for traditional nonlinear optical materials. These devices contain nanometer-scale features that enhance their optical properties and alter their electronic transport relative to bulk behavior. They also contain high densities of deep level defects that make the devices semi-insulating. These defects trap and store photogenerated carriers that accumulate into space-charge densities that match the intensity patterns. The trapped charge produces electric fields that modify the optical properties of the nanostructure. The translation of temporally and spatially varying light intensity patterns into physical changes in the optical properties of the material, matching the intensity patterns, is called the photorefractive process. There are several ways that electric fields can be applied to quantum-well structures, and several ways that optical beams can be incident on the devices to write holograms. The various configurations can be summarized by three basic fields and grating geometries. Two of the structures are transmission structures with the optical beams incident on the same face, and one is reflection geometry with the beams incident from opposite faces.
TL;DR: In this article, the authors present experimental results of four-wave mixing in a BaTiO3 crystal, which exhibits a large increase in the diffraction efficiency for the first few seconds after the writing beams are switched off.
Abstract: Grating erasure in the dark has different temporal characteristics for different crystals. We present experimental results of four-wave mixing in a BaTiO3 crystal, which exhibits a large increase in the diffraction efficiency for the first few seconds after the writing beams are switched off. It is clear from these results that multiple photorefractive processes that have different time constants are involved. We study two different models, including two types of charge recombination center, one with a smaller and the other with larger time constant. The rising phenomenon is explained by use of one of these models. Experimental results for three crystals that exhibit different temporal behaviors are shown, and properties of these three crystals are discussed based on calculations for this model.
TL;DR: In this article, the hole mobility of two photorefractive materials using a holographic technique was determined using a time-of-flight measurement of the holes in the materials.
28 Feb 2000
TL;DR: In this paper, the theoretical model of a photorefractive surface grating in Z-cut of BaTiO/sub 3/crystal at steady state was presented, where the crystal surface was taken to be traction free and two cases: (a) dielectric boundary and (b) short-circuited (metallized) boundary.
Abstract: We present the theoretical model of a photorefractive surface grating in Z-cut of BaTiO/sub 3/ crystal at steady state. Taking the crystal surface to be traction free we consider two cases: (a) dielectric boundary and (b) short-circuited (metallized) boundary. The analysis shows that the photorefractive grating near the crystal boundary has a complex structure. It can be shown that the crystal boundary and the electric boundary conditions have a large influence on the changes in the dielectric tensor induced by a photorefractive grating. Near the crystal surfaces, the changes in the dielectric tensor are different from their volume values, it gives rise to diffraction processes which are forbidden for a bulk grating. One should take elastic contribution into account when calculating the gain expected in photorefractive waveguides or in any case in which light beams travel near the surface of a photorefractive crystal.
TL;DR: Raman and fluorescence spectra are used to establish that the upconverted green emission of Er(3+) clusters acts as a gating source, significantly increasing the two-photon photorefractive effect of a monochromatic red or near-infrared pump source.
Abstract: It is demonstrated that Er doping increases the photorefractive
effect in LiTaO3. Raman and fluorescence spectra are
used to establish that the upconverted green emission of
Er3+ clusters acts as a gating source, significantly
increasing the two-photon photorefractive effect of a monochromatic red
or near-infrared pump source.
TL;DR: In this article, the photorefractive and dielectric properties of a novel system: nematic liquid crystal mixture doped with 1-(5-methylfuran-2-yl)-3-(4-dimethylaminophenyl)-propenon dye are examined.
Abstract: We examine here photorefractive and dielectric properties of a novel system: nematic liquid crystal mixture doped with 1-(5-methylfuran-2-yl)-3-(4-dimethylaminophenyl)-propenon dye Kinetics of formation of refractive index gratings induced by light in a two-wave mixing experiment in this dye-doped nematic liquid crystal together with dielectric relaxation and ionic conductivity are investigated We perform a degenerate four-wave mixing experiment, enabling us to observe the generation of an optical phase conjugation signal by the studied system, and report on some optical microscope observations of hydrodynamic instabilities related to charge injection from the electrodes and ionic current flow through liquid crystal layer We discuss the results in reference to other similar liquid crystalline materials Copyright © 2000 John Wiley & Sons, Ltd
TL;DR: In this article, the authors derived the coupled equations of guided modes in a photorefractive crystal waveguide and derived the modal power in core and cladding for all HEνm and EHνm modes for cylindrical fibers made of uniaxial crystals.
Abstract: The mode coupling among guided modes in a photorefractive crystal waveguide is investigated. We derive the coupled equations of guided modes in a photorefractive crystal waveguide. Analytical solutions of the coupled equations are obtained for a two-mode photorefractive crystal waveguide. We also numerically analyze the coupled equations of guided modes in a multimode photorefractive crystal fiber. As a result of the coupling, power may flow to the fundamental mode or the highest-order mode, depending on the direction of the crystal optical axis. In addition, we report an analytical solution of the modal power in core and cladding for all guided HEνm and EHνm modes for cylindrical fibers made of uniaxial crystals.