Showing papers on "Organic photorefractive materials published in 2001"
TL;DR: A nonlinear optical chromophore for photorefractive applications containing a 2-dicyanomethylen-3-cyano-2,5-dihydrofuran acceptor group is presented in this article.
Abstract: A nonlinear optical chromophore for photorefractive applications containing a 2-dicyanomethylen- 3-cyano-2,5-dihydrofuran acceptor group is presented. When doped into a plasticized composite of poly(n-vinylcarbazole), large gain coefficients (Γ) are observed with photorefractive speed similar to the best composites reported in the literature while maintaining low sample absorption (∼15 cm−1).
42 citations
TL;DR: In this article, the main goal was to verify if some advantages could be obtained by the replacement of poly(1-vinylcarbazole, a component commonly employed for organic photorefractive materials, with various polymers containing side-chain heteroaromatic moieties.
Abstract: The main goal of this research was to verify if some advantages could be obtained by the replacement of poly(1-vinylcarbazole), a component commonly employed for organic photorefractive materials, with various polymers containing side-chain heteroaromatic moieties. For this purpose, poly(1-vinylpyrrole), poly(1-vinylindole), and some methyl-substituted compounds of poly(1-vinylindole) were considered. The best conditions for both monomer synthesis and polymerization were found. A first possible advantage of the new polymeric substrates resided in the values of the glass-transition temperature, which, as expected, was constantly lower than that of poly(1-vinylcarbazole). This could lead to a material that requires the introduction of a lower quantity of plasticizer in the final photorefractive blend to display photorefractive behavior at room temperature. In addition, the verified higher electric dipole moments of the pyrrole and indole derivatives could improve the compatibility of the optically nonlinear component required in the system, typically an azo-molecule, by increasing its solubility inside the blend. All the synthesized vinyl monomers and polymers gave clear spectroscopic evidence of the formation of charge-transfer complexes with 2,4,7-trinitrofluorenylidenmalonitrile, an efficient sensitizer for photoconductivity. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 253–262, 2001
41 citations
TL;DR: The photorefractive properties of nominally pure near-stoichiometric LiNbO(3) crystal that is strongly reduced in vacuum are reported, and to the authors' knowledge this is the first experimental evidence of a subsecond photoreFractive response in pure LiNBO( 3) crystals.
Abstract: The photorefractive response time of LiNbO3 crystal is of the order of minutes, and such a long response time limits the crystal’s practical applications. We report the photorefractive properties of nominally pure near-stoichiometric LiNbO3 crystal that is strongly reduced in vacuum. A short photorefractive response time of the order of 100 ms is measured at a wavelength of 514.5 nm, with incident light intensity of 1.6 W/cm2, and possible corresponding mechanisms are discussed. To our knowledge this is the first experimental evidence of a subsecond photorefractive response in pure LiNbO3 crystals. The diffraction efficiency of a holographic grating written in this reduced crystal is low but can be enhanced by an externally applied electric field.
40 citations
TL;DR: In this paper, a series of molecular materials that show efficient photorefractive effect was reported, which consist of monolithic methine dye molecules and can form amorphous films.
Abstract: This letter reports a series of molecular materials that shows efficient photorefractive effect. The materials which consist of monolithic methine dye molecules can form amorphous films. A net photorefractive gain of 215 cm−1 and a diffraction efficiency of 87.6% at a semiconductor laser wavelength of 780 nm are observed. These materials are the simplest molecular materials and possess the best photorefractive performances. Due to the large photorefractive effect, optical self-defocusing and instability of the laser beam are observed in these materials when the applied electric field is above a threshold.
29 citations
TL;DR: In this article, the photorefractive effect of a ferroelectric liquid crystal doped with a photoconductive compound was investigated, and it was shown that the refractive index grating formation time was measured to be ∼30 ms.
Abstract: The photorefractive effect of a ferroelectric liquid crystal doped with a photoconductive compound was investigated. The photorefractive effect appeared only at the temperature at which the sample exhibits ferroelectricity. The refractive index grating formation time was measured to be ∼30 ms. In the ferroelectric phase, reorientation of the spontaneous polarization vector was found to be induced by the internal space-charge field.
24 citations
TL;DR: In this article, the authors introduced holographic scattering as a technique to determine the activation energy for thermal fixing of refractive index patterns in photorefractive crystals, and measured the time dependence of the transmitted intensity at the fixing temperature.
Abstract: We introduce holographic scattering as a technique to determine the activation energy for thermal fixing of refractive index patterns in photorefractive crystals. After recording a parasitic hologram at ambient temperature, we measured the time dependence of the transmitted intensity at the fixing temperature, to determine the time constant. The temperature dependence of the latter allowed us to evaluate the thermal activation energy. For comparison, we performed an equivalent experiment employing the standard two-wave mixing method. The values obtained using the two techniques agree very well.
23 citations
IBM1
TL;DR: Binary information in the form of a 256 x 256 pixel array has been stored holographically within a polymeric photorefractive composite of 130-mum thickness and the high TNF concentration leads to short digital hologram recording times of 2 s without detriment in optical quality.
Abstract: Binary information in the form of a 256 × 256 pixel array has been stored holographically within a polymeric photorefractive composite of 130-µm thickness. Devices used consisted of 55-wt. % organic chromophore 1-(2′-ethylhexyloxy)-2,5-dimethyl-4-(4″nitrophenylazo) benzene and up to 11.2-wt. % 2,4,7-trinitro-9-fluorenone (TNF) dispersed within a poly(N-vinyl carbazole) matrix. In a degenerate four-wave mixing arrangement, the refractive-index modulation and speed were 10-3 and 200 ms, respectively. The high TNF concentration leads to short digital hologram recording times of 2 s without detriment in optical quality. Although not yet optimized, a figure of merit, M#, for holographic storage in this composite has been measured to be 0.017, and this is estimated to reach 0.18 in a revised experimental geometry.
21 citations
TL;DR: In this article, a theoretical model for a photorefractive running hologram in bulk-absorbing materials in the presence of self-diffraction is presented, and a nonperturbative technique that allows one to measure at the same time the diffraction efficiency and the output beam's phase shift is reported, and its advantages are discussed.
Abstract: We report a theoretical model for a photorefractive running hologram in bulk-absorbing materials in the presence of self-diffraction and use this model to analyze experiments for photorefractive materials characterization. A nonperturbative technique that allows one to measure at the same time the diffraction efficiency and the output beam’s phase shift is reported, and its advantages are discussed. We use this technique and apply the theoretical model to compute some parameters for the electron-charge carriers (Debye screening length ls≈0.03 µm, diffusion length LD≈0.14 µm, and photoexcitation quantum efficiency Φ≈0.45) at the 514.5-nm-wavelength laser line for a nominally undoped Bi12TiO20 crystal. Particular experimental features are detected and assumed to be consequences of hole–electron competition in this sample.
20 citations
TL;DR: In this article, the photorefractive properties of a chiral smectic A liquid-crystalline phase with two photoconducting layers are investigated. But the effect due to the dielectric anisotropy typical of nematic phases, the refractive-index modulation is due to electroclinic effect.
Abstract: We present results of the photorefractive properties of a chiral smectic A liquid-crystalline phase. In our approach, the mesophase is sandwiched between two photoconducting layers. In contrast to the effect due to the dielectric anisotropy typical of nematic phases, the refractive-index modulation is due to the electroclinic effect. We measured the grating spacing and the field dependence of the gain coefficient and obtained a net two-beam coupling gain of 600 cm−1 at 35 V/μm.
18 citations
TL;DR: This work fabricated photorefractive InGaAs/GaAs multiple quantum wells that are sensitive at wavelengths near 1.06 mum for what is believed to be the first time and measured four-wave-mixing diffraction efficiency, using a Nd:YAG laser.
Abstract: We have fabricated photorefractive InGaAs/GaAs multiple quantum wells that are sensitive at wavelengths near 1.06 mum for what is believed to be the first time. We have measured four-wave-mixing diffraction efficiency, using a Nd:YAG laser. A maximum diffraction efficiency of 7 x 10(-4) and a cutoff grating period of ~2 mum are obtained.
17 citations
TL;DR: It is demonstrated, for what is believed to be the first time, that in a photorefractive crystal with positive refractive-index perturbation a two-dimensional dark or bright photovoltaic spatial soliton can be formed in the same crystal by a signal beam and a background beam of different wavelengths.
Abstract: We have demonstrated, for what is believed to be the first time, that in a photorefractive crystal with positive refractive-index perturbation a two-dimensional dark or bright photovoltaic spatial soliton can be formed in the same crystal by a signal beam and a background beam of different wavelengths. We discuss the conditions on the effective Glass constants and the absorption coefficients of the background and signal beams in determining the formation of the spatial soliton.
TL;DR: In this article, the reorientational dynamics of chromophores in a photorefractive polymer composite are characterized by use of a new, analytical, nonexponential expression.
Abstract: The reorientational dynamics of chromophores in a photorefractive polymer composite are characterized by use of a new, analytical, nonexponential expression. A single material parameter, the diffusion constant, is used to describe reorientation in the material. The effect of plasticization on the diffusion constant is studied. The relationship between space-charge field growth and the biexponential fit to holographic growth is investigated. For typical parameters the exponential space-charge field’s rise time may be as much as 10 times less than the holographic growth time constant obtained conventionally.
TL;DR: An all-optical tracking novelty filter based on the dynamic holographic properties of an efficient and fast infrared-sensitive photorefractive polymer is constructed and images of a swinging pendulum observed through the novelty filter are shown.
Abstract: We have constructed an all-optical tracking novelty filter based on
the dynamic holographic properties of an efficient and fast
infrared-sensitive photorefractive polymer. The photorefractive
polymer was used in a two-beam coupling geometry. The polymer had a
gain coefficient of 175 cm-1 at a wavelength of 780 nm and
an applied field of 72 V/µm. In contrast to what has
been observed in photorefractive crystals, the gain coefficient and the
filter contrast are largely independent of the writing beam’s
intensity ratio. We show images of a swinging pendulum observed
through the novelty filter.
TL;DR: In this paper, the Braun model for field-dependent photogeneration efficiency is incorporated into the standard model of photorefractivity, and the effect on the space-charge field, and thereby holographic index contrast, is calculated directly without the need for empirically found parameters.
Abstract: The field-dependence of the charge photogeneration efficiency reduces the holographic contrast in an organic photorefractive composite. In this article, the Braun model for field-dependent photogeneration efficiency is incorporated into the standard model of photorefractivity. Using this formalism, the effect on the space-charge field, and thereby holographic index contrast, is calculated directly without the need for empirically found parameters. The dependence of the space-charge field on experimental geometry is also analyzed. A powerful description of the effective space-charge field is thus derived and is experimentally verified. This analysis is then used to compare the observed holographic contrast to that predicted from transmission ellipsometry measurements.
TL;DR: In this article, a degenerate four-wave mixing experiment enabled measurements of phase conjugate reflectance as a function of input beam intensities, applied electric field, and grating spacing.
Abstract: Optical phase conjugation in a high-performance photorefractive polymer-dissolved liquid crystal composite has been observed with a cw frequency-doubled Nd-YAG laser (532 nm) or He–Ne laser (633 nm). The degenerate four-wave mixing experiment enabled measurements of phase conjugate reflectance as a function of input beam intensities, applied electric field, and grating spacing. The refractive index modulation was estimated from the phase conjugate reflectance on the basis of the coupled wave theory including the absorption loss of the photorefractive media. Small input powers were sufficient for inducing a large refractive index modulation around 4.0 ×10-3.
TL;DR: In this paper, the authors show that photorefractive materials having large electro-optic coefficients such as BaTiO 3 require relatively small electric field to induce sufficient phase mismatch for the reduction of the diffraction efficiency to a large extent.
TL;DR: In this paper, the photorefractive effect in a cell filled with nematic liquid crystal under electric-field application is reported, which is caused by formation of a spatial inhomogeneous charge distribution due to the Helfrich-Carr effect in the absence of light-excited charge carriers.
Abstract: In the present work the observation of the photorefractive effect in a cell filled with nematic liquid crystal under electric-field application is reported. Photorefraction is caused by formation of a spatial inhomogeneous charge distribution due to the Helfrich-Carr effect in the absence of light-excited charge carriers.
14 Aug 2001
TL;DR: The photorefractive effect is a phenomenon in which the local index of refraction is changed by the spatial variation of the light intensity, and such an effect was referred to an 'optical damage'.
Abstract: The photorefractive effect is a phenomenon in which the local index of refraction is changed by the spatial variation of the light intensity. Such an effect was first discovered in 1966. The spatial index variation leads to the distortion of the wavefront, and such an effect was referred to an 'optical damage'. The photorefractive effect has since been observed in many electro-optic crystals, including LiNbO3, BaTiO3, SBN, BSO, BGO GaAs, InP, etc. Photorefractive materials are, by far, the most efficient media for the recording of dynamic holograms. In these media, information can be stored, retrieved and erased by the illumination of light. In addition to the holographic properties, energy coupling occurs between the recording beams and also between the reading beam and the scattered beam. In this Lecture, we first briefly describe the photorefractive effect. The band transport mode is introduced to analyze the process involved in the photo- induced index variation. This is followed by a more detailed analysis of the dynamics of grating formation. We then describe the interaction between electromagnetic waves propagating inside photorefractive media. Nonlinear optical processes including two-wave mixing, four-wave mixing and phase conjugation are discussed. We also point out some fundamental including two-wave mixing, four-wave mixing and phase conjugation are discussed. We also point out some fundamental properties of grating diffraction. Then we demonstrate the applications of the photorefractive effect including volume holographic data storage, image processing, optical interconnections, computing and neural networks. Finally, we discuss some recent developments in photorefractive materials and applications. As an example, we describe an application of photopolymers in a flat-topped tunable filter for optical fiber communication.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.
TL;DR: In this article, the photorefractive properties of the Sn2P2S6 crystals are improved by growth conditions (change of the transport agent) and such modified samples exhibit significantly increased two-beam gain coefficient.
Abstract: The Sn2P2S6 crystals are photorefractive materials suitable for red and near infra-red spectral range. Photorefractive properties of the Sn2P2S6 crystals are improved by growth conditions (change of the transport agent). Such modified samples exhibit significantly increased two-beam gain coefficient. Some of the microscopic parameters responsible for the photorefraction (Debye screening length, diffusion length, effective electrooptic coefficients etc.) were evaluated. The accompanied photorefractive scattering effects are also studied.
TL;DR: In this article, the photorefractive optical damage in annealed proton-exchanged LiNbO3 planar waveguides is studied and the pfotorefraction thresholds for the wavelengths 5145 and 6471 nm are determined.
Abstract: The photorefractive optical damage in annealed proton-exchanged LiNbO3 planar waveguides is studied The pfotorefractive thresholds for the wavelengths 5145 and 6471 nm are determined Also, the light-induced refractive index change in a wide range of powers is determined An interesting and unexpected photorefractive behaviour of the samples in q-state (metastable at room temperature) and s-state (steady-state) is described An analysis of the light self action in s-type waveguides is performed and used for estimating the waveguide conductivity
08 Jul 2001
TL;DR: In this article, the resonance peak characteristic for both the electron and the holephotoactive centers in a Bi12Ti020 photorefractive crystal were detected in a nearly degenerate two-wave mixing experiment under applied electric field.
Abstract: The resonance peak characteristic for both the electron- and the holephotoactive centers in a Bi12Ti020 photorefractive crystal were detected in a nearly degenerate two-wave mixing experiment under applied electric field. The general features in this experiment were analyzed using a simplified mathematical model describing the effect of hole-electron competition upon running holograms where wave mixing as well as the electric coupling between electrons and holes are considered.
11 May 2001
TL;DR: In this article, the authors report the experimental observation of the dynamical evolution of the soliton beam when the beam is launched into a biased SrBaNb/sub 2/O/sub 6/ photorefractive crystal.
Abstract: Summary form only given. Recently, incoherent-soliton-related phenomena, such as partially incoherent spatial solitons, modulation instability, and pattern formation in spatially incoherent light beams, have aroused many research interests in nonlinear optics. One thing in common of the these phenomena is that the material must be noninstantaneous, i.e., the material gives rise to the nonlinear refractive index depending on the optical intensity averaged over a period of time longer than the material response time. Biased photorefractive material can support optical spatial solitons. Due to its charge redistribution nature, photorefractive nonlinearity is of course noninstantaneous. To understand how a varying optical spatial soliton beam behave in such a noninstantaneous nonlinear medium, here we report the experimental observation of the dynamical evolution of the soliton beam. The observation is done when the soliton beam is launched into a biased SrBaNb/sub 2/O/sub 6/ photorefractive crystal.
11 May 2001
TL;DR: In this article, the resonant self-focusing effects in photorefractive semiconductors are described. But the authors focus on a model that relies on two donors levels, one deep trap level and conduction of both electrons and holes.
Abstract: Summary form only given. Self-trapping of 1D and 2D optical beams in photorefractive InP has been experimentally observed back in 1996. Here we present the first theory describing the resonant self-focusing effects in photorefractive semiconductors. The theory is based on a models that relies on two donors levels, one deep trap level and conduction of both electrons and holes.
08 Nov 2001
TL;DR: In this article, a photorefractive material was put into the interference field, another reading-laser beam would be diffracted, and then the reading laser beam went into an encoder to demodulate signals.
Abstract: During the high speed processing, the intensity of the light beam must be detected and converted into some digital signals without electronic device for system controlling. The A/D converting should be in full optics way. The interferometer may make some fringes correspond the input light, while the fringe intensity contains some important information. When a photorefractive material was put into the interference field, another reading-laser beam would be diffracted. The order of the diffraction can be controlled because the nonlinear reactivity of photorefractive materials. Then, the reading laser beam goes into an encoder to demodulate signals. Through the demodulating processing in light speed, the group-codes may be output as the input signals of other control system. The speed of the modulating should be tied from the photorefractive materials. The important step is how to change the time domain information into space domain information. If the responding speed of the photorefractive material is suitable to react on the input signal light, the conversion is successful. In that process, organic photorefractive materials or crystal photorefractive materials have different application features.
08 Jul 2001
TL;DR: In this paper, a periodically phase modulated two-wave mixing technique was applied to obtain simultaneously the absorption and refractive index contributions to the diffraction efficiency, and the amplitude and phase of the refractive and absorption index changes were measured.
Abstract: We apply a periodically phase modulated two-wave mixing technique to obtain simultaneously the absorption and refractive index contributions to the diffraction efficiency. The technique allows us to measure the amplitude and phase of the refractive and absorption index changes, and therefore their relative contributions to the diffraction efficiency with high sensitivity.
11 May 2001
TL;DR: In this paper, it was shown that spontaneous subharmonics may occur when two plane waves with a small frequency detuning intersect inside the photorefractive Bi/sub 12/SiO/sub 20/ crystal in the presence of a DC electric field.
Abstract: Summary form only given. Subharmonics generation is a phenomenon originating from the excitation of so-called space charge waves (SCW) and instability of the photorefractive grating against these SCWs. It was found experimentally that spontaneous beams may occur when two plane waves with a small frequency detuning intersect inside the photorefractive Bi/sub 12/SiO/sub 20/ crystal in the presence of a DC electric field. These beams propagate in the directions corresponding to the diffraction from the gratings with grating vectors K/sub N/=K/N, K being the grating vector of the principal light-induced grating, with integer N, which is why the phenomenon was called spatial subharmonic generation. It was shown also that subharmonics may be generated with an AC electric field applied to the sample illuminated by a stationary interference pattern. While the theory predicts subharmonics excitation for photorefractive semiconductors, they were never observed before in crystals other than various sillenites. We report on first experimental observation of the spatial subharmonics in germanium doped cadmium telluride and show in that this phenomenon is quite general for photorefractive materials and is inherent not only to the sillenite-type crystals.
12 Nov 2001
TL;DR: In this article, the phase conjugate beam of the signal beam itself as the other writing beam was used to record real-time holographic interferograms using photorefractive crystals.
Abstract: Photorefractive crystals are one of the most efficient nonlinear optical media for recording real-time holograms with relatively low intensity requirements. This is because of their practically unlimited recyclability and they can be recorded and erased in this medium with sufficiently high sensitivity. Innovative geometries have been proposed by various authors to record holograms in these crystals with and without using applied electric field across the crystal in both two and four wave mixing geometries respectively. But, in all these cases the diffraction efficiency was not comparable with the conventional holography using photographic emulsions and this will be evident when one records real-time holographic interferograms using photorefractive crystals. In this paper we propose a new geometry for recording high efficient holograms in the photorefractive media using the phase conjugate beam of the signal beam itself as the other writing beam. Diffraction efficiencies increase by four times and PR gain increases two times in this case.
15 Jul 2001
TL;DR: In this article, a two-beam coupling was used to investigate the photorefractive properties of Bi12TiO20 single crystals doped with different concentrations of Ru and optical transmission spectra were measured.
Abstract: Bi12TiO20 single crystals doped with different concentrations of Ru were grown. Optical transmission spectra were measured. Photorefractive properties were experimentally investigated using two-beam coupling. Applications for optical information processing will be investigated.
TL;DR: In this paper, the correlation between the optical response time and photoelectric quantities, such as the hole mobility and the dispersivity of charge carrier transport, was investigated for organic photorefractive materials.
Abstract: Organic photorefractive materials have attracted a lot of interest recently. Their optical response times of a few milliseconds, however, are not yet adequate for the desired commercial applications. We present an investigation on the correlation between the optical response time and photoelectric quantities, such as the hole mobility and the dispersivity of charge carrier transport.
12 Nov 2001
TL;DR: By doping nonlinear optical chromophores into liquid crystals, the threshold voltage could be reduced significantly and the mechanism involved was investigated in this article, which is attributed to reorientation of liquid crystal molecules by light induced space-charge field.
Abstract: Photorefractive effect in nematic liquid crystals is attributed to reorientation of liquid crystal molecules by light induced space-charge field. In the absence of external field, nematic liquid crystals form domain structure, which is responsible for high threshold voltage. By doping nonlinear optical chromophores into liquid crystals, the threshold voltage could be reduced significantly and the mechanism involved was investigated.