Showing papers on "Organic photorefractive materials published in 2008"

Beam coupling in photorefractive liquid crystal light valves

Abstract: Photorefractive liquid crystal light valves (LCLVs) are hybrid devices that combine a nematic liquid crystal layer with a thin monocrystalline Bi12SiO20 (BSO) photorefractive crystal in the form of a cell wall. The device behaves as an optically addressed spatial light modulator, where the photoconductive layer is made of the BSO crystal. Differently from conventional types of spatial light modulators, usually working in retroreflective configuration, the photorefractive light valves work in transmission, thus allowing new applications related to the coupling of the optical beams when they pass through the liquid crystal layer. Here, we review some recent experiments of beam coupling in photorefractive LCLVs. After a characterization of the device in terms of its spatial resolution, which is related to the features of pattern formation in an optical feedback configuration, we present two-beam coupling and optical amplification in single pass experiments. Then, we develop a theoretical model by taking into account the Raman–Nath diffraction of the incoming beams over the thin liquid crystal layer. By using two or more light valves in cascade, we show, both experimentally and theoretically, that the two-wave mixing gain can be enhanced by the Talbot effect related to the multi-passage of the beams through the successive layers of the nematic liquid crystal. Finally, we show that self-pumped phase conjugation can be realized by placing the light valve in a tilted feedback configuration. In this case, the four-wave mixing is spontaneously created through the scattering of the signal beam onto the feedback induced grating.

Laser induced memory bits in photorefractive LiNbO3 and LiTaO3

Abstract: 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.

Refractive Index of Organic Systems Doped with Nano-Objects

Abstract: 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.

Fast photorefractive self-focusing in InP:Fe semiconductor at infrared wavelengths

Abstract: Photorefractive beam self-trapping is investigated in InP:Fe and is shown to occur within tens of microseconds after beam switch on. This fast response time is predicted by an analytical theoretical interpretation based on a simple photorefraction model which suggests buildups in a time range consistent with experiments.

Refractive index of Si-doped n-InGaAs

Abstract: The dependence of the refractive index of Si-doped n-InGaAs on carrier density was investigated. The shift in the refractive index from that of undoped InGaAs was found to be mainly caused by the band-filling effect, the band-gap shrinkage effect, and the plasma effect. Model calculations agreed better with the measured data when the conduction band nonparabolicity effect was taken into account. Nevertheless, the calculated refractive index values were slightly lower than the experimental values. Because the difference was larger for photon energies close to the band-gap energy of InGaAs, the shift in the refractive index is attributed to the absorption tail effect that appears near the band-gap energy due to Si doping.

Spatial modulation instability driven by light-enhanced nonlinearities in semiconductor CdZnTe:V crystals

Abstract: We present the experimental observation of spatial modulation instability in photorefractive semiconductor crystals (CdZnTe:V), where the optical nonlinear effects are enhanced by light. We find that the total refractive index change can be expressed as sum of a uniform index change, which can exceed the value of 0.003, and a local index change which is limited to ≈1.6×10−4. However, only the later, arising from the intensity-enhanced photorefractive effect, contributes to the formation of the modulation instability. Finally, we find that the refractive index change experiences large temporal fluctuations induced by the combination of uniform cw illumination and applied electric field.

The Electrooptic and Photorefractive Effects

Abstract: 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.

Influence of doping on the photorefractive properties of a polymer-dispersed liquid crystal

Abstract: The photorefractive effect is studied in a photoconducting polysiloxane containing small droplets of a low molar mass liquid crystal. In order to vary the trap density for charge carriers, the system is doped with tetramethyl-phenylendiamine. The measured energy transfer from one beam to the other (two-beam coupling) indicates a high gain coefficient, which is enhanced by adding small concentrations of the dopant. Measurements of the phase and the amplitude of the refractive index modulation confirm that this behavior can be attributed to an increase in the trap density.

Chromophore Concentration Effect on Photorefractive Performance of Organic Photorefractive Composites

Abstract: 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.

Light Intensity Effects in Photorefractive α‐Phase PE‐LiNbO3 Waveguides

Abstract: Light intensity effects on recording photorefractive gratings have been studied in α‐phase proton exchanged LiNbO3 guides. The saturating refractive index change increases as a function of the recording intensity and reaches an unstable region above a certain intensity threshold. The threshold value increases with the exchange time used for preparing the guides, showing a behavior quite similar to that observed with a single beam (non holographic) configuration. Dark decay measurements indicate that dark conductivity can be disregarded with regard to photoconductivity.

All-optical control of gain via surface-induced photorefractivity in twistable nematic

Abstract: We report the first study of SIPRE in optically twistable nematic cells, where different strategies for all-optical control of the gain coefficient have been explored.

Characterization of Photorefractive Materials Using Holographic and Photoconductivity Techniques

Abstract: Photorefractive materials exhibit photoconductive and electro‐optic properties. The first step in the optical recording process in these materials is the building up of a spatial modulation of electric charges via charge carriers excitation from localized to extended states where they diffuse and/or are drifted and then retrapped in a localized state. In this way the pattern of light onto the sample is converted into a spatial modulation of charges and its corresponding space‐charge electric field produces a volume index‐of‐refraction modulation via electro‐optic effect. The latter is the resulting volume phase hologram. The position of the photoconductive centers in the localized states in the material band gap as well as the density and properties of the donor/acceptor centers and charge carriers transport properties are therefore important parameters characterizing the materials and determining their performance as far as optical recording is concerned. For determining these parameters we use optical, ...

Photo-electromotive-force from vibrating speckled pattern of light on photorefractive CdTe:V

Abstract: We report the use of the photo‐electromotive force (photo‐emf) effect produced by a vibrating speckle pattern of light, generated by laser radiation at 1064nm, in the volume of a photorefractive vanadium doped CdTe crystal. This effect is used to measure the sample's photocondutivity and the vibration amplitude of the pattern of light. When the vibrations are much faster than the photorefractive material reponse time the photocurrent is independent of the response time. The theoretical model predicts a maximum value for the first temporal harmonic term of the photocurrent at a fixed value for the vibration amplitude‐to‐speckle size ratio. This prediction was experimentally confirmed and this maximum can be used to calibrate the setup in order to facilitate practical applications.