Photorefractive Polymer Research Articles

Synthesis and characterization of bi-functional photorefractive polymers

A novel bi-functional photorefractive acrylate polymer with pendant carbazolyl groups and azo derivatives as side chains was synthesized, in which azo derivatives as electro-optic chromophores and carbazolyl as photoconductive moiety were covalently linked on the acrylate backbone. Photorefractive experiments showed that a high two-beam coupling gain coefficient of 93cm−1, diffraction efficiency of 12% and electro-optic coefficient of 26pm/V were obtained. With increasing writing beam's intensity, the two-beam coupling enhanced gradually, and then reached saturation. Using the method of four-wave mixing, the photoisomerization grating was observed.

Stabilization of the response time in photorefractive polymers

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.

Effect of the chromophore donor group and ferrocene doping on the dynamic range, gain, and phase shift in photorefractive polymers

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.

Highly Efficient Photorefractive System Based on Carbazole-Substituted Poly(Siloxane)

Photorefractive polymer composite was prepared with carbazole-substituted poly(siloxane) as a host polymer, NLO chromophore and 2,4,7-trinitro-9-fluorenone as a sensitizer. This composite exhibited excellent photorefractive property. The gain coefficient determined by 2-beam coupling measurement was 250 cm −1 at 80 V/μm. And the composite was very stable toward phase separation.

Photorefractivity of the Blend Based on Poly(N-Vinylcarbazole) and Azo Dye-Functionalized Polymer

Photorefractive polymers based on the blend of poly(N-vinylcarbazole) and azo dye-functionalized nonlinear optical polymer were prepared for the enhancement of phase stability in a photorefractive system. Polymer blends with homogeneous phase showed increased gain coefficient with increase in the content of azo dye groups, charge generator and plasticizer in the blend.

Hole transport through chromophores in a photorefractive polymer composite based on poly( N-vinylcarbazole)

The hole mobility, as measured by the linear time-of-flight technique, is higher than expected at (2±0.5)×10 −6 cm 2 (V s) −1 at an applied field of 50 V μm −1 in a photorefractive polymer composite. The components were poly( N-vinylcarbazole), the electro-optic chromophore 1-(2′-ethylhexyloxy)-2,5,dimethyl-4-(4′′nitrophenylazo)benzene together with sensitisor 2,4,7-trinitro-9-fluorenone. This mobility is unchanged within experimental accuracy after the replacement of the poly( N-vinylcarbazole) by the dielectric polycarbonate. Polycarbonate in pure form without the chromophores is an insulator. We conclude that hole transport in the photorefractive composite occurs through the network of chromophores and not through the charge-transport polymer.

Development of fully functionalized photorefractive polymers

In this article, we review recent progress in the area of photorefractive polymers. Photorefractive (PR) polymers are multifunctional materials which combine photoconductivity and electro-optic response to show a new phenomenon: light-induced reversible modulation of the refractive index. Because of their multifunctional features, design, synthesis and preparation of these materials exhibiting high performance is an intellectual challenge. Moreover, numerous applications of photorefractive materials in optical devices have been established using inorganic materials. Utilizing the unique features of organic polymeric materials to prepare useful devices is an engineering challenge. In the past several years, research in this area has gained momentum because numerous new materials which possess better characteristic photorefractive parameters than their inorganic counterparts have been synthesized. Several interesting devices have been presented. Two different approaches have been developed to synthesize and prepare PR polymers, namely composite materials and fully functionalized polymers. Both approaches have had success in identifying new materials and in gaining understanding of the design principles of better materials. This paper discusses these aspects and gives a prospective view about this field.

Synthesis and characterization of photorefractive polymers with triphenylamine unit and NLO chromophore unit on a side chain

Synthesis and characterization of photorefractive polymers with triphenylamine unit and NLO chromophore unit on a side chain

%Synthesis and Properties of Photorefractive Polymers Containing Indole-Based Multifunctional Chromophore as a Pendant Group

Novel photorefractive chromophores with indole and nitrobenzene as the push and pull groups, respectively, were synthesized and subsequently reacted with methacryloyl and acryloyl chloride to give multifunctional monomers. Polymers were obtained by copolymerization of these monomers with methyl methacrylate (P1, P2) or butyl acrylate (P3). These polymers were very soluble in organic solvents to be fabricated into optically clear polymer films and showed glass transition (Tg) at ca. 150 °C for P1, P2 and at 8 °C for P3, respectively. Large nonlinear optical coefficients (d33 as high as 49 pm/V) and photoconductive sensitivity (on the order of 10-9 S·cm-1/W·cm-2) were identified with high Tg polymers P1 and P2. Preliminary investigation of photorefractivity was carried out with low Tg polymer (P3) by two-beam coupling and four-wave mixing experiments. It was observed that P3 doped with 1% TNF showed two-beam coupling gain of 2.2 cm-1 at 75 V/μm and diffraction efficiency of 0.44% at 60 V/μm. When the photoc...

Influence of the chromophore ionization potential on speed and magnitude of photorefractive effects in poly(N-vinylcarbazole) based polymer composites

We report on the synthesis of three highly polar chromophores and their use as dopants in poly(N-vinylcarbazole) based photorefractive polymer composites sensitized with (2,4,7-trinitro-9-fluorenylidene)malononitrile. Small alterations in the amino donor group substituents were used to tune the dye’s ionization potential (IP) by 0.2 eV. At 780 nm, 5 °C above the glass transition temperature (Tg), and with an applied field of 59 V/μm, we observed complete internal diffraction and a gain coefficient of 167 cm−1. In this temperature range, diffraction efficiency, gain coefficient, and photorefractive phase shift were found to correlate with the chromophore IP. At 20 °C below Tg, the contribution from birefringence to the index modulation is insignificant, and the speed of the photorefractive effect correlates well with the chromophore IP. Analysis of the results suggests that the space–charge field is influenced by the chromophore IP.

Image amplification and novelty filtering with a photorefractive polymer

Energy transfer between two laser beams writing a volume hologram in a photorefractive polymer composite is applied to video-rate optical processing applications. A net increase in image intensity as high as a factor of 37 can be observed within one video frame time (33 ms) using a total beam intensity of 1 W cm−2 and 77 V μm−1 applied electric field. Moving object detection (novelty filtering) is also demonstrated.

Fully Functionalized Photorefractive Polymers Incorporating Transition Metal Phthalocyanine and Porphyrin Complexes as Photosensitizers

Photorefractive polymers containing transition metal porphyrin or phthalocyanine complexes (see Figure) are prepared here by Heck polycondensation, and the structural effects of these macrocyclic complexes on the photorefractive performance are investigated. It is reported that the polymers containing Zn complexes exhibit much larger photogeneration quantum yields and optical gains than those containing Cu.

Fully Functionalized Photorefractive Polymethacrylates with Net Gain at 780 nm

We report on the synthesis and characterization of three fully functionalized photorefractive polymethacrylates containing different chromophores. Carbazole- and NLO-functionalized methacrylate monomers in a 1:1 ratio were polymerized with 20 mol % of dodecyl methacrylate to obtain photorefractive polymers. The resulting polymers had glass transition temperatures of 48, 47, and 52 °C, respectively. Asymmetric two-beam-coupling and four-wave-mixing experiments at 780 nm and three different temperatures pointed out that the resulting polymers, doped with 1 wt % (2,4,7-trinitro-9-fluorenylidene)malononitrile (TNFDM), showed good photorefractive properties. A net two-beam coupling gain and a diffraction efficiency of 60% were observed at 58 V/μm.

Synthesis and Characterization of Novel Multifunctional High-Tg Photorefractive Materials Obtained via Reactive Precursor Polymers

We describe the synthesis of a new class of high-Tg multifunctional photorefractive polymers. They were obtained by radical copolymerization of methyl vinyl isocyanate and various N-substituted maleimides. Hole transporting carbazole moieties were attached through a variety of alkyl and phenyl spacers to the imide positions of the maleimides. In a polymer analogous reaction the reactive isocyanate groups of the precursor polymers were reacted with different hydroxyalkyl-terminated nonlinear optical chromophores. Two azo chromophores including dispersed red-1 and one pyrazolone dye were used. The resulting materials have been characterized by means of GPC, DSC, and UV/vis spectroscopy. All materials are amorphous and possess excellent solubility in common solvents such as chloroform and THF. Molecular weights range between 20 and 272 kg/mol. The glass-transition temperatures vary between 60 and 194 °C, depending on the length and the nature of the spacer groups between the maleimide rings and the carbazole...

Holographic index-contrast prediction in a photorefractive polymer composite based on electric-field-induced birefringence

The physical properties of a hologram written in a photorefractive polymer composite are predicted from a macroscopic model based on ellipsometry. An electric poling field increases the birefringence of the composite. The way that the bulk birefringence changes with the electric field is used to predict the holographic index contrast, and, by comparison with experiment, accurate deductions of the holographic space-charge field are made. A photorefractive polymer composite was used that contained 47.5 wt. % 1-(2′-Ethylhexyloxy)2,5-dimethyl-4-(4″ nitrophenylazo)benzene electro-optic dye. When sinusoidal modulation in optical intensity is used with high contrast, the higher spatial harmonics of the modulation of the holographic space-charge field become important. The amplitude of the first-order modulation in the space-charge field is accordingly reduced by 13% relative to the predictions of the standard model of photorefractivity in the case of a high-saturation field.

Figures of merit of push–pull molecules in photorefractive polymers

Push–pull molecules are the most widely used nonlinear optical chromophores incorporated in polymers to generate photorefractivity. They are at the origin of the Pockels and the Kerr electrooptic effects in high-Tg photorefractive polymers and give also rise to the optical birefringence in low-Tg photorefractive polymers. High-Tg and low-Tg figures of merit characterize the efficiency of push–pull molecules to generate phototrefractivity. We present, here, the two-level figures of merit of various push–pull molecules determined by using electrooptical absorption measurements in solution. Analysis of the results within the framework of the two-form two-state model provides useful guidelines for the design of push–pull molecules for photorefractive applications. This model enables us to evaluate the optimum structures as well as the corresponding maximum optical birefringence, Pockels and Kerr electrooptic responses. It also shows how these maxima are related. Tuning a molecular structure to reach an optimum one will not automatically lead to a very large figure of merit. An essential parameter that is usually overlooked is the magnitudes of these maxima. We show that the maxima are affected both by the conjugation length and by the choice of the donor/acceptor substituents. Some of the longest compounds investigated here exhibit very large high-Tg and low-Tg figures of merit.

Photorefractive holographic contrast enhancement via increased birefringence in polymer composites containing electro-optic chromophores with different alkyl substituents

The birefringence of a photorefractive polymer composite in response to an applied electric field is used to predict the holographic index contrast attainable. Predictions have been obtained for three different polymer composites based on the charge transfer complex matrix poly(N-vinyl carbazole)/2,4,7-trinitro-9-fluorenone, doped with electro-optic azo chromophores differing only in their alkyl substitutions. A comparison with experimental data indicates that unexpected large variations in the holographic contrast obtainable from different chromophores are, in most cases, accounted for by a corresponding variation in electric field-induced birefringence. This variation may be due to a combination of the chromophore number density and the composite viscosity. Where the birefringence does not correlate with holographic index contrast, a prediction based on a trap density limited space-charge field fits the data well.

Time-scale invariance of the reorientation dynamic in photorefractive polymers

Under the influence of an applied electric field, dipoles reorientate to provide most of the index contrast of holograms in photorefractive polymer composites. The poling transient is commonly described by modified exponential functions. Either a KWW stretched exponential or a bi-exponential function is required to fit adequately the observed behavior. Moreover, when the analysis was performed for data that extended over 3 orders of magnitude in time then at least 5 time constants were required. We show that the dispersive rotational dynamic is, in fact, described by a power law, and hence does not have characteristic lifetimes. The dispersive rotation of dipoles is described simply and accurately using a time-dependent diffusion coefficient. This gives an excellent agreement between theory and measurement. This has important implications for the characterization of the transient response of any electro-optic process or device based on the rotation of dipoles in a dispersive amorphous material such as a polymer or glass.

Synthesis and characterization of photorefractive polymers with triphenylamine unit and NLO chromophore unit on a side chain

Synthesis and characterization of photorefractive polymers with triphenylamine unit and NLO chromophore unit on a side chain

Photorefractive polymeric optical spatial solitons

We predict the formation of optical spatial solitons in photorefractive polymers. The orientational enhancement from the doped chromophores and the dependency of the quantum efficiency of generating mobile holes on the electric field make the polymeric solitons behave differently from other photorefractive solitons.