Organic photorefractive materials

About: Organic photorefractive materials is a research topic. Over the lifetime, 697 publications have been published within this topic receiving 13041 citations.

Demonstration of an Fe doped KNbO3 photorefractive hybrid

Abstract: Highly birefringent liquid crystals are attractive for photorefractive applications but device operation, usually restricted to the Raman-Nath regime, can be degraded by small phase shifts between the optical and refractive index gratings and coarse grating spacings with narrow beam intersection angles. Having to apply external electric fields and tilt the cell at an angle to the grating k vector cause further complications. In this letter, two-beam coupling in a hybrid photorefractive cell comprising a nematic liquid crystal layer influenced by an Fe doped KNbO3 window is described. The KNbO3 window determines photorefractive properties while the liquid crystal amplifies the overall refractive index modulation. Full Bragg matched gain and large beam intersection angles, with perfect 90° phase shifts between the optical and refractive index gratings, at normal incidence, without the need for an external field are demonstrated.

Measurement of laser-induced refractive index change of inverted ferroelectric domain LiNbO 3

Abstract: Optical nonlinearities of periodically poled LiNbO3 crystals were investigated by the single beam Z-scan technique with a continuous wave (cw) laser beam at 532 nm. The nonlinear optical absorption coefficient and refractive index change are determined to be 8.1×10−6 cm/W and 2.6×10−4 at 0.5 MW/cm2 light intensity, respectively. Both sign and magnitude of the measured refractive nonlinearity are considerably different from the Z-scan results in congruent LiNbO3. The nonlinearities in the periodically poled LiNbO3 induced by 532 nm continuous waves are believed to be mainly due to the photorefractive effect.

Photorefractive polymer dispersed liquid crystals

Abstract: Organic photorefractive materials have received increasing attention for photonic applications due to their highperformance level and their ease ofprocessing. Here, we present functionalized polymer dispersed liquid crystalsthat exhibit strong photorefractive properties. Their properties are investigated by wave-mixing experiments andtheir performance compared with photorefractive polymers and photorefractive liquid crystals.Keywords: Liquid crystal, photorefractive, polymer, organic, nonlinear optics, holography, optical processing,photoconductor 1. INTRODUCTION Photorefraclive materials arc among the most sentive nonlinear optical materials since they exhibit large refractive indexchanges when exposed to low power laser beams.' The photorefractive effect is based on the build-up of a space-chargethrough the photoexcitalion of carriers and their transport over macroscopic distances. Transport can occur by eitherdiffusion of the carriers, if the excitation is nonuniform, or by drift, if an electric field is applied to the material. Chargeseparation results in a space-charge field that changes the refractive index of the material. This refractive index change is aphase replica of the initial light distribution. Thus, photorefractive materials are used as optical recording medium.Optical media that can record a grating, that is the interference pattern of two coherent laser beams, are in high demand.For real-time optical recording or processing applitions, the material must have a dynamic response. hi other words thelight-induced gratings should be erasable or be able to accommodate any changes in the light waves that are inducing them,in real time. Materials with such optical encoding properties allow for implementanon of a wide range of optical applicationsranng from reconfigurable interconnects, dynamic holographic orage, to optical correlation, image recognition, imageprocessing; and phase conjugation. Therefore, materials where the optical encoding is dynamic and based on a periodicmodulation ofthe refractive index are the focus of intense research.During its first twenty-five years of existence, the photorefractive effect was studied mainly in inorganic Crystals,

Correlation between photorefractive index changes and optical damage thresholds in z-cut proton-exchanged-LiNbO 3 waveguides

Abstract: An interferometric Mach-Zehnder technique very recently developed has been applied to measure photorefractive index changes in different types of z-cut proton-exchanged planar waveguides in LiNbO3 These measurements are complemented by determining the intensity-threshold for the onset of optical damage with a standard single-beam setup In the intensity region just below the threshold-intensity obtained in the single-beam experiment the refractive index change is found to saturate at values around 1×10-4 Furthermore, we measure the dark conductivities of proton-exchanged waveguides by monitoring the decay of the light-induced index changes Via the time constant of the decay we obtain dark conductivities of the order of about 5 × 10-16 Ω-1 cm-1, that are negligible compared with the photoconductivity within the light intensity range used The results of the measurements compare well with the predictions of a recent work, that uses a two-center model to explain the optical damage

Effect of photoconductivity and dielectric constant of the photorefractive materials on two-beam coupling gain and phase-shifts for a single unidirectional photorefractive ring resonator

Abstract: 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.