High T(g) photorefractive polymers: influence of the chromophores' beta tensor

In this paper we study the effect of the chromophores' beta tensor active components on the diffraction efficiency of a high T(g) photorefractive polymer. In particular, we study the two simplest structures with nonvanishing dipole moment, the one-dimension push-pull systems, and the Lambda-shaped chromophores. We have developed a model that relate the diffraction efficiency expression with experimental conditions and microscopic properties of the molecules used. Using this model we determine the optimum experimental conditions for both kinds of chromophores and the criteria for the design of chromophores with improved microscopic properties. The model was also used to evaluate the diffraction efficiency of the chromophore Disperse Red 1 (DR1) with a good agreement with experimental data present in bibliography, and of other chromophores selected with the criteria derived from the model, using quantum mechanical calculations to obtain the microscopic properties. Using the designed chromophores diffraction efficiencies more than one order of magnitude higher than that calculated for DR1 with the experimental conditions has been obtained in simulations. These chromophores also exhibit a low dependency of eta on the electric field polarization in contrast to the DR1 or the low T(g) photoreactive materials.     

Photo-fabrication of surface relief gratings on polymer films

Surface relief gratings were optically produced on a number of azobenzene-based polymer films. The surface grating formation was investigated by monitoring the diffraction efficiency and using atomic force microscopy. The effect of the structure of the chromophores on surface modulation was investigated. The surface deformation process depended on the polarization state of the writing beams. The localized variations of the light intensity and alteration of the resulting electric field polarization were essential writing conditions to the formation of the surface relief gratings. The surface pattern from straight edge diffraction established that the surface profile of the recorded gratings is proportional to negative gradient of the intensity pattern incident on the polymer film.     

Novel polyesters with amino‐sulfone azobenzene chromophores in the main chain

The synthesis and optical properties of a series of amorphous polyesters with amino‐sulfone azobenzene chromophores in the main chain are presented. The condensation of alkyl diacid chlorides with 4‐[(2‐hydroxyethyl)ethylamino]‐4′‐(2‐hydroxyethylsulfonyl)azobenzene (7) gave polymers that showed low and unstable photoinduced birefringence because of their low glass‐transition temperature. The condensation of 7 with mixtures of adipoyl chloride and terephthaloyl chloride gave a polymer with a higher glass‐transition temperature and a stable reversible photoinduced birefringence. A photostationary birefringence of 0.07 was observed. Surface profile gratings were optically inscribed and produced diffraction efficiencies less than or equal to 19%. Photostationary, long‐term stable birefringence and surface‐grating efficiency increased with polymer rigidity. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 2245–2253, 2000     

Carbazole-based bi-functional photorefractive polyphosphazene for correcting the laser wavefront distortion

Carbazole-based polyphosphazene containing nitro azo chromophore moieties was synthesized first. The photorefractive polyphosphazene was prepared to optical qualitative films and used to correct the laser wavefront distortion under the room temperature without external electric field, pre-poling and addition agent. The results indicate that the photorefractive polyphosphazene film can correct both the diffraction distortion and phase distortion of the laser wavefront. At the same time the high-resolution laser wavefront distortion correction can be achieved. Further, as the result of the effectively correction of the diffraction distortion, the polymer film may be applied in microscale direct image.     

Photorefractive performance of a CdSe/ZnS core/shell nanoparticle-sensitized polymer

We report the photorefractive performance of a polymer composite sensitized by CdSe/ZnS core/shell nanoparticles, and also comprising poly(N-vinylcarbazole) and an electro-optic chromophore. The nanoparticles are characterized by absorption and photoluminescence spectroscopy, elemental analysis, transmission electron microscopy, and powder x-ray diffraction. The electro-optic response of the composite is measured independently of the photorefractive effect by transmission ellipsometry. An asymmetric two-beam coupling gain of 30.6+/-0.4 cm(-1) is obtained, confirming photorefractivity. Degenerate four-wave mixing is used to assess photorefractive performance and, at a poling field of 70 V microm(-1), yields a diffraction efficiency of 4.21%+/-0.03%, a holographic contrast of 3.05 x 10(-4)+/-1 x 10(-6), a space-charge rise time of 25+/-2 s, and a sensitivity of 4.7 x 10(-5)+/-4 x 10(-6) cm3 J(-1). These results constitute a significant improvement on the performance of previous nanoparticle-sensitized photorefractive polymer composites.     

Layered photoconductive polymers: anisotropic morphology and correlation with photorefractive reflection grating response

We demonstrate that the mesophase morphology of the layered photorefractive polymers has a substantial influence on the photorefractive properties, especially in reflection grating geometries with a minimal grating spacing. The layered morphology of the photoconductive polymers based on poly(p-phenyleneterephthalate) (PPT) with pendent carbazole (CZ) groups can be efficiently controlled by changing their molecular weight. Photorefractive composites based on PPT-CZ polymers with different chromophores, diethylaminodicyanostyrene (DDCST) or piperidinodicyanostyrene (PDCST), show anisotropic morphology induced by the squeezing flow during sample preparation. The contributions of the highest occupied molecular orbital levels of the chromophores and of the degree and anisotropy of the layered crystalline structure to the charge transport and trapping result in a high efficiency of the PDCST composite and a similar response speed in DDCST and PDCST composites in the reflection grating geometry, although of about six times lower photoconductivity in the less-ordered PDCST composite.     

PHOTOINDUCED SURFACE RELIEF GRATINGS ON AZOCELLULOSE FILMS

Surface relief gratings (SRG) on films of azobenzene–modified cellulose (azocellulose) with ultrahigh molecular weight were holographically recorded in a single step. Surface modulation depth of the SRGs increased with the degree of substitution (DS) of the azobenzene chromophores in the cellulose as well as with the grating spacing. Large surface modulations over 500 nm on all samples were obtained with a grating spacing of 4 μm. A linear increase of photoinduced birefringence with the azobenzene chromophore concentration in these photodynamic polymers was also observed. The maximum birefringence of 0.11 was achieved for an azocellulose polymer with a DS of 0.99.

     

Orientation of native cellulose in an electric field

Native cellulose has been oriented in an ac electric field at both the macroscopic and colloidal level. Ramie fiber fragments suspended in chloroform have been shown to point along the field. Cellulose microcrystal suspensions in cyclohexane have also been allowed to evaporate in an electric field and have exhibited a high degree of orientation when further examined by TEM and electron diffraction. Similarly, cellulose whisker suspensions showed increasing birefringence with increasing field strength and displayed interference Newton colors that saturated at around 2000 V cm(-)(1). A high degree of order of this suspension was also obtained by evaluating the induced birefringence with color charts.     

Enhanced performance of photorefractive poly(N‐vinyl carbazole) composites

We present the enhanced photorefractive performance of high molecular weight poly(N‐vinyl carbazole) (PVCz)‐based composites. Higher diffraction efficiency with faster speed of grating build‐up was obtained by optimizing the composition of the PVCz composites. At relatively low applied electric field of E = 45 V μm^?1, diffraction efficiency of 26% for p‐polarized probe beam and corresponding that of 5.1% for s‐polarized probe beam were measured with faster grating build‐up speed of 48.3 s^?1 for the composite with 2,4,7‐trinitrofluorenone (TNF) as a sensitizer. Fastest speed of grating build‐up of 100 s^?1 and large optical gain up to 110 cm^?1 were measured at E = 80 V μm^?1 for the composite with fullerene derivative of PCBM as a sensitizer. These improved performances are due to a large orientational enhancement effect with faster response speed in addition to Pockels effect for the samples with appropriate glass transition temperature. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Effect of graphene doping of holographic polymer‐dispersed liquid crystals

Doping a polymer matrix with a minute amount of graphene (0.05–0.25%) had significant effects on the grating formation kinetics and electro‐optical performance of a holographic polymer‐dispersed liquid crystal. Low graphene contents (≤0.1%) reduced the viscosity and induced rapid diffusion, curing, grating formation, and high diffraction efficiency with a diffraction overshoot of 0.05%. Conversely, high graphene contents increased the compound viscosity, and the entire process proceeded slowly. Graphene increased the polymer conductivity and local electric field, reduced the operating voltage from 65 to 25–50 V, and increased the contrast ratio from 7 to 8–23 with a concomitant decrease in rise time. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

主链型偶氮聚酯的合成和光响应性研究

合成了一种主链含偶氮生色团的线性聚酯,用核磁共振、热分析、紫外-可见光谱、红外光谱、GPC等方法对其结构进行了表征.该聚合物膜在线偏振激光的作用下,光致取向形成的双折射各向异性Δn可达0.07.聚合物的旋涂膜经干涉的P偏振488nmAr⁺激光照射60min后,得到了规则的正弦表面起伏光栅,光栅的一级衍射效率约为18%.     

Photorefractive effect of ferroelectric liquid crystals

This paper reviews our recent work on the photorefractive effect of ferroelectric liquid crystals (FLCs). The photorefractive effect is defined as the optical modulation of the refractive index of a medium as a result of a variety of processes. The interference of two laser beams in a photorefractive material establishes a refractive index grating. This phenomenon enables the creation of different types of photonic applications. FLCs exhibit fast electric field response, and the orientation of the molecular axis of FLCs changes its direction according to the change in direction of the spontaneous polarization (Ps). When two laser beams interfere in a photoconductive FLC, an orientational grating is formed. The mechanism of the formation of the grating is based on the response of the Ps to the photoinduced internal electric field. The time of formation of the refractive index grating is significantly shorter in FLC materials.     

聚合物半导体光电转换的理论分析

导电聚合物半导体的最大特点是掺杂物在工作条件下可以移动。这类“可移动掺杂物”型半导体已被许多研究者用为光生伏打效应的材料, 目前所得到光电转换效率普遍较低。本文提出这类光电极的物理模型和数学模型, 并用数值解法解决了这个边界问题; 理论分析表明这类“可移动掺杂物”型半导体最大光电转换效率可以接近于传统无机半导体的光电转换效率; 数值解提出必须注意提高这类半导体的载流子迁移率和体内复合寿命。     

Photofabrication of surface relief gratings from azobenzene containing perfluorocyclobutane aryl ether polymer

An azobenzene based perfluorocyclobutane (PFCB) aryl ether polymer, poly[4‐(phenylazo)triphenylamine‐1,2‐hexafluorocyclobutyl ether], was synthesized by using a palladium catalyzed amination reaction and the 2π + 2π cyclodimerization of 4‐[(trifluorovinyl)oxy]bromobenzene. This polymer was designed and synthesized to permit azo chromophores to be incorporated, thus introducing photoinduced dynamic behavior into the PFCB containing polymer structure. The polymer exhibited a T_g of about 122 °C, and the maximum absorption of the azo chromophores was 407 nm in the film state. A thin film was prepared through a spin coating process, and the rapid growth of diffraction efficiency was then achieved by irradiation with an interference beam. The efficient formation of holographic surface relief gratings was also verified from AFM images, which show regularly defined and aligned grating structures. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 3525–3532, 2005     

Enhanced diffraction efficiency in a photorefractive liquid crystal cell with poly(9-vinylcarbazole)-infiltrated mesoporous TiO2 layers

The photorefractive effect of a layer-structured liquid crystal cell was significantly enhanced when a C60-doped poly(9-vinylcarbazole) (PVK)/TiO2 nanocomposite was used in two photoconductive layers. The C60-doped PVK/TiO2 nanocomposite film was prepared by infiltrating C60-doped PVK into a highly ordered mesoporous TiO2 layer. The addition of the TiO2 layer to the C60-doped PVK layer increased the first-order Raman-Nath diffraction efficiency from 24% to 42.9%. This enhancement of diffraction efficiency is attributed to a blocking effect of charge recombination in the composite layer. The electron transfer from the PVK layer into the TiO2 layer would decrease the recombination of photogenerated charges in the PVK layer, while charges in the PVK layer could participate in the formation of a space-charge field.     

The influence of photosensitizers on the photorefractivity in poly[methyl-3-(9-carbazolyl)propylsiloxane]-based composites

We investigated the photosensitizers effect on the photorefractive (PR) properties in five poly[methyl-3-(9-carbazolyl)propylsiloxane] (PSX-Cz)-based PR composites which were doped with various photosensitizers having each different electron affinity, such as 2,4,5,7-tetranitro-9H-fluorine-9yilden malonitrile (TeNFM), 2,4,7-trinitro-9-fluorenone (TNF), 9-dicyanomethylene-2,4,7-trinitro-fluorenone (TNFM), tetracyanoethylene (TCNE), and 7,7,8,8-tetracyanoquinnodimethane (TCNQ). At 632.8 nm, photo-charge generation efficiencies, photoconductivities, space charge field, four wave mixing diffraction efficiencies, and PR grating buildup times were measured as a function of external electric field. The photo-charge generation, which is dependent on the light absorption, was achieved through the charge transfer (CT) complexes between the PSX-Cz and each of the photosensitizers. The photon energy of the CT transition decreased with increasing electron affinity of the photosensitizer. In composites doped with TeNFM, TNF, and TNFM, the space charge field (E_sc) increased as the photo-charge generation efficiency increased; the grating buildup in these composites is rate-limited by the photo-charge generation speed. In sample doped with TCNE, and TCNQ, the hole mobility was reduced due to the larger amount of photosensitizer anion traps produced by photoreduction of the photosensitizer. Then, the grating buildup speed became hole mobility limited, and smaller buildup rates were observed. The magnitude of space charge field was sustained as the charge and trap density increased. In all composites, the refractive index modulation is increased with the magnitude of space charge field.     

Effect of the sensitizer on the properties of fully functionalized photorefractive epoxy polymers and their performance in the hologram image storage

The 3‐amino‐9‐ethyl carbazole (AEC)/Dispersed Orange 3 (DO3)/diglycidyl 1,2 cyclohexanedicarboxylate (DCD) main chain copolymers are synthesized and studied on their photorefractive (PR), photoconductive, and holographic characteristics. They are good hologram recording media because not only the stored hologram exhibits excellent fringe contrast with the resolution at about 20 μm, but also the image can be stored, erased, and overwritten. The UV/Vis spectra shows that AEC segments may form charge transfer complexes with either DO3 segments or TNF sensitizers. By incorporating the sensitizer or increasing the charge transfer component concentration, the grating growth rate can be speeded up. Applying an electric field on the polymer film containing no sensitizer helps to elevate the grating growth speed, but it shows little influence on the grating growth speed of the films with sensitizers. More nonlinear optical (NLO) segments in the copolymer result in higher diffraction efficiency. The dark decay of the film depends on the dark conductivity. Incorporating sensitizers or more charge transport segments leads to more dark decay because of increasing dark conductivity. The dark decay of the PR properties at elevated temperature can also be evaluated by the thermal stimulated discharge current (TSDC) spectroscopy technique. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1057–1068, 1999     

Field-force alignment of disc-type pi systems.

The ability of electric fields to align nonpolar semiconducting molecules was demonstrated using hexa(para-n-dodecylphenyl)hexabenzocoronene (HBC-PhC12) as a model compound. A solution of HBC-PhC12 was applied to a glass surface by drop-casting and the molecules were oriented into highly ordered structures by an electric field during solvent evaporation. Atomic force microscopy (AFM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) showed a long-range alignment where the disclike molecules were organized in columns perpendicular to the direction of the imposed electric field. The high anisotropy of the uniaxially aligned films was characterized by cross-polarized light microscopy. The birefringence of the HBC-PhC12 films was related to the presence of extended domains of unidirectionally aligned columns in which the aromatic cores of the HBC-PhC12 molecules were perpendicular to the columnar axis. The packing and the arrangement of the molecules in the field-force ordered films were proven by electron diffraction and X-ray analyses.     

Conducting polymer diffraction gratings on gold surfaces created by microcontact printing and electropolymerization at submicron length scales

Conducting polymer diffraction gratings on Au substrates have been created using microcontact printing of C18-alkanethiols, followed by electropolymerization of either poly(aniline) (PANI) or poly(3,4-ethylenedioxythiophene) (PEDOT). Soft-polymer replicas of simple diffraction grating masters (1200 lines/mm) were used to define the alkanethiol template for polymer growth. Growth of PANI and PEDOT diffraction gratings was followed in real time, through in situ tapping-mode atomic force microscopy, and by monitoring diffraction efficiency (DE) as a function of grating depth. DE increased as grating depth increased, up to a limiting efficiency (13-26%, with white light illumination), defined by the combined optical properties of the grating and the Au substrate, and ultimately limited by the loss of resolution due to coalescence of the polymer films. Grating efficiency is strongly dependent upon the grating depth and the refractive index contrast between the grating material and the surrounding solutions. Both PEDOT and PANI gratings show refractive index changes as a function of applied potential, consistent with changes in refractive index brought about by the doping/dedoping of the conducting polymer. The DE of PANI gratings are strongly dependent on the pH of the superstrate solution; the maximum sensitivity (DeltaDE/DeltapH) is achieved with PANI gratings held at +0.4 V versus Ag/AgCl, where the redox chemistry is dominated by the acid-base equilibrium between the protonated (emeraldine salt) and deprotonated (emeraldine base) forms of PANI. Simulations of DE were conducted for various combinations of conducting polymer refractive index and grating depth, to compute sensitivity parameters, which are maximized when the grating depth is ca. 50% of its maximum obtainable depth.