Showing papers on "Organic photorefractive materials published in 1997"

High net gain at 514 nm in a photorefractive polymer doped with a chalcone derivative

Abstract: We report on the photorefractive properties of a low Tg composite consisting of functionalized polysiloxane doped with a chalcone derivative. The high transparency of this doping molecule enabled the observation of high net gain at 514 nm. Orientational and Pockels contributions to the total refractive index variations were measured by frequency-dependent ellipsometry experiments. Finally, the field dependence of the gain coefficient is described using Kukhtarev’s model for the space charge field with an effective trap density as a single fitting parameter.

Self-trapping of two-dimensional optical beams and light-induced waveguiding in photorefractive InP at telecommunication wavelengths

Abstract: We demonstrated an experimental observation of self-trapping and self-deflection of a two-dimensional optical beam by the photorefractive effect at telecommunication wavelengths under an applied dc field. Self-trapping is effective for an intensity range related to the intensity-temperature resonance known for two-wave mixing in InP:Fe. The photorefractive index change giving rise to the trapping is measured at 10−4, while the photorefractive space-charge field is measured at about 50 kV/cm, ten times higher than the applied field. We show experimentally that this index change creates a waveguide that can be used to guide a second beam at 1.55 μm.

Temperature-dependence studies of photorefractive effect in a low glass-transition-temperature polymer composite

Abstract: The temperature dependence of the photorefractive effect in a polymer composite containing poly(9-vinycarbazole), tricresyl phosphate, buckminsterfullerene, and 4-(N,N- diethylamino)-β-nitrostyrene is presented. The photoconductive, electro-optic and photorefractive properties of the material have been studied in the temperature range of 22–61 °C. An apparent increase of electro-optic modulation with temperature and its eventual saturation is observed. This behavior is attributed to the temperature activated orientational mobility of the second-order nonlinear chromophores. The polarization anisotropy between the p- and s-polarized readouts is consistent with what would be expected on the basis of directly measured effective electro-optic coefficients. By correlating the electro-optic value with the diffraction efficiency, the temperature dependence of the space-charge field is obtained and explained by temperature dependencies of the dark conductivity and the photoconductivity of the material.

High intensity illumination effects in LiNbO3 and KTiOPO4 waveguides

Abstract: Quasi-phase-matched waveguides are known to degrade when generating high intensity short wavelength light. Photoinduced changes in the refractive indices can lead to reduced efficiency by broadening phase-matching curves or inducing power dependent losses. In this work a pump-probe technique was used to investigate photorefractive effects in nonlinear optical waveguides. A strong two photon photorefractive effect in single domain and Ti-poled LiNbO3 was found that is considerably reduced in electric field periodically poled LiNbO3 and is absent in Rb-exchanged KTiOPO4.

Thermal fixing of refractive index gratings in a photorefractive polymer

Abstract: A significant increase in storage time by thermal fixing of photorefractive (PR) gratings in a polymeric composite of poly-9-vinycarbazole, tricresyl phosphate, fullerene, and 4-N,N-diethylamino-β-nitrostyrene is demonstrated. The fixing of the volume holograms is created by quasifreezing the orientation of the linearly anisotropic chromophore molecules. The fixed PR gratings can be easily erased by elevating the sample’s temperature to the glass transition temperature of the composite. The readout of the thermally fixed first-order grating does not require the presence of an external electric field.

Optical and Photorefractive Characterization of BTO Crystals Doped with Cd, Ca, Ga, and V

Abstract: Single crystals of Bi12TiO20 (BTO) are grown by a modified Czochralski technique. Doping with cadmium, calcium, gallium, and vanadium is performed by adding corresponding oxides to the melt. Optical parameters like refractive index, optical absorption, optical activity, light induced absorption, electrooptic coefficient, and photoconductivity are determined. Photorefractive properties of the samples are characterized with the help of a conventional two-beam interference setup. Diffraction efficiency and decay time constant of elementary holograms are determined. The influence of the utilized dopands is discussed.

High photorefractive sensitivity at 860 nm in reduced rhodium-doped KNbO(3).

Abstract: We show that high-temperature reduction in a CO–CO2 atmosphere increases the photorefractive sensitivity of KNbO3:Rh at 860 nm by 4 orders of magnitude compared with that of the as-grown crystal. The effective trap density is increased by a factor of 3, and the photoconductivity by a factor of 30, and the photorefractive response at a grating spacing of 0.15 µm is accelerated by a factor of 400. The grating buildup time at a grating spacing of 0.7 µm and an intensity of 1 W cm-2 is 0.5 s, a value comparable with that of as-grown KNbO3:Fe at visible wavelengths. The optical and photorefractive parameters of Rh-doped KNbO3 subjected to reduction treatment are characterized for wavelengths of 0.48–1.064 µm.

Optical dispersion of the refractive index modulation in low Tg photorefractive polymers

Abstract: We measure at different wavelengths the linear and nonlinear optical properties of a new chromophore for photorefractive applications by means of frequency-dependent ellipsometry experiments. We show that the optical dispersion of the refractive index modulation responsible for the high diffraction efficiency in low Tg polymers can be described by a two-level model.

Charge-carrier diffusion length in photorefractive crystals computed from the initial hologram phase shift

Abstract: The phase shift between the pattern of light onto a photorefractive crystal and the resulting hologram at the very beginning of the recording process in two-wave mixing is analyzed and measured as a function of the applied electric field. These data allow one to compute the diffusion length of photoexcited charge carriers and to evaluate the actual electric field inside the crystal. A diffusion length of 0.14 μm is measured in a nominally undoped photorefractive Bi12TiO20 crystal using a 532 nm wavelength laser illumination, in agreement with results obtained from other methods.

Edge effects in photorefractive thin films

Abstract: We have used a two-dimensional charge transport model to study edge effects in photorefractive thin films. Our result shows the presence of a large surface charge layer that causes saturation of trap-limited-field in the large fringe spacing limit. Quadratic electro-optic materials were used as an example to quantitatively study the contribution of the surface-charge-layer field to the total photorefractive grating. We clearly demonstrate that the device performance can be dominated by either surface charge or bulk trap charges.

Low-field hole mobility in a photorefractive polymer

Abstract: We present the time-of-flight measurements of hole mobility in a photorefractive polymer composite as a function of temperature and applied electric field. The analysis shows that the temperature dependence of the low-field mobility is in apparent disagreement with the predictions of the Gaussian disorder model and also with polaron models.

Z-scan measurements of photorefractive nonlinearities for a SBN: Ce crystal

Abstract: In this paper, we derived the Z-scan formula for a photorefractive crystal sample under an external applied electric field. The far-field diffraction pattern of a Gaussian beam wavefront through a photorefractive crystal is calculated by considering spatial-phase perturbation induced by the space-charge field. The photorefractive drift nonlinearities and correspondingly, the electro-optic coefficients r 33, r 13, (r 23) and the characteristic ratio n 3 e r 33/n 3 o r 13 for a cerium-doped Sr0.61Ba0.39Nb2O6 crystal are determined from the Z-scan experiments. Z-scan with enhanced sensitivity is also realized by measuring the normalized transmittance at the off-axis position in the far field. A deviation between the off-axis Z-scan experimental results and the theoretical prediction is discussed too.

Optical Limiting in a Photorefractive Polymer

Abstract: Optical limiting has been suggested as a possible device application of high gain photorefractive (PR) polymers. The principle of operation relies on the existence of amplified scattering (beam fanning) to deplete the intensity of the incident coherent optical beam. The newest photorefractive polymers show a combination of high gain and reasonable speed, and we show how these materials behave as a beam fanning optical limiter and as a limiter based on self-pumped phase conjugation. A prototype cascaded beam fanning device is demonstrated and its salient features are discussed.

Off-Bragg analysis of the diffraction efficiency of reflection photorefractive holograms.

Abstract: Formulas are given for the calculation of diffraction efficiency of reflection-type gratings recorded in a photorefractive medium. The analysis incorporates the coupled-wave theory that was developed for photorefractive hologram gratings. This analysis takes into account grating slant with respect to the medium surface, light absorption during reconstruction, any incident angle of the reference beam, and any photorefractive phase shift. General solutions for signal and reference wave functions are given in a closed-form expression by use of a hypergeometric function. The optimum media parameters and recording conditions for high diffraction efficiency are obtained by the derived formulas. The diffraction properties for off-Bragg conditions are also discussed.

Improvement of photorefractive properties of proton-exchanged LiTaO3 waveguides

Abstract: are fabricated by both a proton exchange and a successive copper exchange. The influence of different fabrication steps on the refractive index profile and optical absorption is investigated. The holographic efficiency is determined together with the dark- and photo-conductivity. We show that the photorefractive properties of proton-exchanged LiTaO3 waveguides are considerably improved by the additional copper exchange. It is established that the holographic efficiency depends on the copper content, the phase of the proton-exchanged layer and the light intensity.

Liquid crystal/polymer composite with high optical anisotropy and low dielectric anisotropy to lower hysteresis exhibiting transparent and light scattering states

Abstract: A composition for use in a liquid crystal optical element, such as a liquid crystal display or laser shutter device, comprised of a liquid crystal material and a solidified polymer material. The liquid crystal is such that its refractive index is changed depending on states of applying a voltage. In one state, the refractive index of the liquid crystal substantially coincides with the refractive index of the solidified matrix to thereby pass light. While in the other state, the refractive index of the liquid crystal does not coincide with the refractive index of the solidified matrix to thereby cause the scattering of light. Specifically, the liquid crystal material used in the composition has a refractive index anisotropy (Δn) is greater than 0.18 and the dielectric anisotropy (Δe) satisfies the relation of 5<Δe<11.6.

Writing a grating in the photorefractive polymers with no applied field

Abstract: Writing a refractive index grating by anomalous carrier diffusion without applying an external electric field has been achieved in a photorefractive polymer whose D/μ value is larger than that derived from Einstein's law. A grating was written in a photorefractive polymer by anomalous diffusion.

Recent advances in high gain photorefractive polymers

Abstract: This talk focuses on the development and characterization of new high gain photorefractive polymers, with emphasis on a particular stable composite composed of the photoconductor poly(n-vinyl carbazole), PVK, the charge generator C/sub 60/, the nonlinear optical chromophore 4-piperidino-benzylidene-malononitrile (PDCST), and the liquid plasticizer butyl benzyl phthalate, BBP.

New advances in organic photorefractive material development

Abstract: The rapid improvement of the performance of photorefractive polymers over the past few years has generated a strong technological interest for these new materials. The areas of application include holographic storage, image processing, optical correlation, and phase conjugation. This paper reviews some of our recent advances in the field of organic photorefractive materials. We will first present several new polymeric composites that combine high efficiency and long shelf lifetimes. Then we will discuss the performance of a new class of organic photorefractive materials: photorefractive polymer dispersed liquid crystals. Finally, we will present two examples of applications based on photorefractive polymers: (i) optical correlators for security verification, and (ii) imaging through scattering media in the near infra-red.

Current status and future of photorefractive polymers for photonic applications

Abstract: Photorefractive polymer are emerging as promising photonic materials for a variety of applications including holographic storage, real-time optical processing, imaging, non-destructive testing, and phase conjugation. These materials combine the low cost and ease of processing of polymers with the highly sensitive optical encoding mechanism of photorefractive materials in general. This paper will introduce the basic concepts of photorefractivity, organic nonlinear optical and amorphous organic photoconductors. Then we will review the physics and the chemistry of photorefractive polymers and give a survey of different classes of materials that have been proposed during the relatively short history of organic photorefractive materials. Finally, we will discuss some of the potential and future challenges of these materials in the context of photonic applications.

High re-orientational mobility and crystallization resistance in photorefractive polymer composites

Abstract: Organic photorefractive materials’ have attracted a lot of attention in recent years because of their high performance and great potential in photonic applications, including holographic storage, optical correlation,’ dynamic holography, image processing, nondestructive testing, novelty filtering, and phase conjugation. Photorefractive polymers look particularly promising because of their high efficien~y,~ low cost, and ability to be processed into large area films. In recent years, polymers have reached a level of performance where they compete with inorganic photorefractive crystals. High refractive index modulation amplitudes (An = 7 X have been demonstrated3 in low glass transition temperature polymers as a result of orientational effects. However, the high electric field needed to pole the chromophores is a limitation. To reduce that field, photorefractive nematic liquid crystals have been proposed and demon~trated.4.~ In these materials, an internal spacecharge field reorients the director of the nematic liquid crystalline phase. However, the macroscopic coherence length of the nematic liquid crystalline phase limits the resolution of these materials and high efficiency is observed only for large values of the grating spacing (A >10 pm). Here, we present a new class of organic photorefractive materials: polymer dispersed liquid crystals (PDLC). These new materials combine the high resolution of photorefractive polymers, and the high refractive index changes associated with field-induced reorientation of nematic liquid crystals. We demonstrate that high refractive index modulation amplitudes can be achieved at lower applied field. Such materials are prepared by dispersing liquid crystal droplets of almost spherical shape in a photoconducting solid organic polymer matrix. The sensitizing, photoconducting, and trapping properties necessary for the buildup of a space-charge field are provided by the polymer matrix and the refractive index changes are due to the field-induced reorientation of the liquid crystal droplets as illustrated in Fig. l. For our experiments, we

Photorefractive polymers with improved efficiency

Abstract: Recently, photorefractive polymers have emerged as promising recording media for holographic storage and optical processing such as optical correlation devices.',' Because of their structural flexibility, these new materials could be optimized and materials with fully reversible refractive index modulation of An = 7 X 10K3 have been developed leading to nearly 100% diffraction efficiency in 105-pmthick samples.' However, the high electric field that needs to be applied (50-100 V/pm) to these polymers to reach that level of performance is a limitation. Thus there is a need for photorefractive polymers doped with more efficient chromophores. Here, we present a new photorefractive polymer composite based on a PVK matrix and doped with the new chromophore HR-254. The photorefractive properties of these new composites are studied by four-wave mixing and two-beam coupling experiments. The chromophore, whose structure is shown in the inset of Fig. 1, has been designed to lead to an efficient orientational effect in low glass transition temperature ( T,) photorefractive polymers. The chromophore has an absorption maximum at 500 nm as shown in Fig. 1. The structure of the molecule has been carefully designed to prevent photoisomerization and to ensure good solubility in the photoconducting matrix and to prevent crystallization. The nonlinear response of the chromophore was measured with the frequency-dependent ellip-

Holographic grating oscillations in photorefractive materials

Abstract: tween interacting beams at the initial stage of their coupling in the case ofthe drift-type nonlinearity.' We have experimentally observed that such a transient coupling is enough to create the mutually pumped phase-conjugate mirror in a Bi,,TiO,, crystal. In this work the nonstationary conditions were implemented by applying a rising front ofthe dc electric field to the sample while both pump beams were derived from cw He-Ne lasers at A = 633 nm. Transient fanning effect of the pulse form was observed under these circumstances at smaller (few degrees) angles of scattering. When we adjust the pump-beams incidence so that formation of both fanning gratings coincides both in space and in time, two transient phaseconjugate waves of the pulse form were generated from the opposite crystal's end faces. Both the response time and the conversion efficiency of transient MPPC depend on the applied electric field as shown in Fig. 1. The curve (a) shows that the higher electric field is applied to the crystal the faster MPPC response time is observed. This contradicts usual behavior of photorefractive media, where the response time grows up with the increasing electric field. However, we have reported recently the anomalous response-time behavior for the fanning effect in a thin Bi,,TiO,, crystal.' It is attributed with the generation of photorefractive surface waves, which appear owing to the internal reflections of fanned beams on the crystal surface and from the photorefractive gratings recorded near this s ~ r f a c e . ~ , ~ As a result, the light energy is self-confined near the crystal surface without any prefabricated waveguide. As fast response time as 4.5 ms was obtained for the external electric field of 61 kV/cm and the pump beam power 10 mW. To the best of our knowledge, this is the fastest MPPC observed in photorefractive crystals under the cw pumping. Note that the conversion efficiency of the transient phase conjugate mirror also grows up while the electric field is increased, as shown in Fig. 1b. Maximal conversion efficiency is measured to be 25%, which corresponds to 39% after the correction to Fresnel reflections.

Photorefractive gap waves

Abstract: The experimental investigations of the grating oscillations were performed for thin Bi,,TiO,, samples taken from different sources. An Ar-ion laser was used. The investigations of U, and also two-wave mixing signals as a function of R (for the frequencies R ranging from 0.1 Hz to lo4 Hz), incident light intensity, and external electric field were carried out. It was found that the U, dependence on CL exhibits a resonance-like and, under certain conditions, even a periodic behavior. At least two resonance-like peaks with different amplitudes were observed for different frequencies R, which can be attributed to two different relaxation processes with T 3 s. These values were found to depend on illumination of the sample and magnitude of applied electric field. A detailed theoretical analysis was performed by solving the Kukhtarev’s equation for the case when a hologram is recorded by an oscillation interference pattern. It was found that U,, is determined mainly by oscillations of holographic grating. The obtained theoretical results qualitatively describe the main experimental facts including the dependence of U, on n, on the intensity of illuminating light, and the external electric field magnitude. We should note that the use of ITWM could also improve the sensitivity of the adaptive holographic interferometric technique because it allows a significant reduction in the noise caused by the instability of the output laser power. *National Technical University, Physics Department, 15780 Zografou, Athens, Greece ““Universitet Osnabrueck, 0-49069 Osnabrueck, Germany 1. M. Petrov et al., J. Appl. Phys. 79, 2846 (1996). s and T

Visualization of photo-induced waveguide formation in photorefractive LiNbO/sub 3/ crystal

Abstract: In summary, we have developed a new imaging technique for photo-induced index change in photorefractive crystals Formation of waveguide structures were successfully visualized both spatially and temporally This technique will be an useful tool for monitoring versatile photorefractive phenomena, including photorefractive grating formation