We present a summary of the underlying mechanisms which govern the appearance and dynamics of the photorefractive effect in polymeric materials. Charge transport properties in polymers and their influence on the build-up of space- charge fields are discussed, and the beneficial effects from orientational enhancement of in situ room-temperature poling of nonlinear optical chromophores are reviewed. We introduce a new high performance low-glass-transition-temperature polymer composite and discuss its photorefractive properties as characterized by a thorough investigation of photoconductivity, two-wave beam coupling gain, grating phase shift, and diffraction efficiency as a function of intensity, applied electric field, and grating spacing.

Photorefractive polymers which incorporate azo-dyes as the nonlinear chromophore element, can be used not only for generating gratings by the photorefractive effect, but also by photoisomerization of the azo-dye. In the latter mechanism, repeated trans-cis isomerization causes the chromophore molecules to become aligned at right angles to the laser polarization direction, thereby making the material birefringent. These two phenomena are to a large degree independent, and can be studied separately, by appropriate choice of polarization direction of the interacting beams. Furthermore, the diffraction efficiency of the photorefractive gratings is a very sensitive function of the poling field strength, while that of the photoisomerization gratings is less so. In this work, we investigate the components diffracted from each of these gratings formed in a hybrid photorefractive polymer material PVK:TNF:DEACST:disperse red 1. We then explore the possibility of performing some simple optical processing applications, exploiting the flexibility provided by this multiple grating process. A scheme for producing a novelty filter, which displays only the moving parts of a scene is considered. The limitations of these films for such processing applications are discussed.

In this paper we discuss the space charge field formation process in poly(N-vinylcarbazole)-based photorefractive polymers and more specifically its relation to charge transport. In the steady state, the experimental results are found to be in qualitative agreement with the predictions of the standard model. A large saturation field, that corresponds to a trap density of the order of 5 multiplied by 10¹⁶ cm^-3 is estimated. In the transient regime however, the space charge field departs from its expected behavior: this is shown to arise due to the influence of disorder on the charge transport process.

The photorefractive BisA-NitroAminoStilbene polymer (BisA- NAS:DEH) has drawn much attention in recent years due to its potential applications such as terabyte optical data storage, real time optical processing, image amplification, and dynamic holography. Extensive optical investigation for this polymer was performed mostly at room temperature. On the other hand less attention was given to the behavior of this polymer at much higher or lower temperatures than the room temperature. Using the electron paramagnetic resonance technique and a home built temperature controller (minus 190 to 500 degrees Celsius) we have been able to study the temperature effect on the spin concentration of BisA-NAS:DEH in the temperature range from 100 to minus 190 degrees Celsius. An anomalous behavior for the EPR signal was observed near 70 degrees Celsius as the spin density increases drastically especially around 85 degrees Celsius. Also, around minus 25 degrees Celsius the main EPR signal broadened then splits into a doublets which are quenched at higher temperatures. Preliminary interpretation of this signal is given on the basis of behavior of the NAS dye molecule at higher temperature and the transport agent DEH which transfers the charge generated at the dye molecule to the other part of the material, and thus coupling the process of photoconduction.

We present measurements of the hole mobility in a photorefractive polymer composite as a function of temperature and applied electric field. The material is a composite, non-linear optical polymer bisphenol A 4-4'- nitroaminostilbene (bisA-NAS) mixed with 30 wt % of the hole transport agent diethylamino-benzaldehyde diphenyl hydrazone (DEH). The electric field and temperature dependencies of the hole mobility in the photorefractive polymer can be described at high fields by exp((beta) E^1/2) and exp [-(T₀/T)²] respectively, in agreement with the disorder theory of the well-known hopping model developed for charge-transport in molecularly doped polymers. The mobilities at all temperatures decrease ((beta) less than 0) with increasing fields up to a certain field, but increase again ((beta) greater than 0) at higher fields.

Orientational confinement of NLO molecules, both for a maximum EO effect and as a low barrier to photo-induced charge generation and transport, is considered a key principle in the design of potentially efficient organic photorefractive crystals. A supramolecular approach to space-charge induced photorefractive effects is discussed, on the basis of an inclusion lattice formed by perhydrotriphenylene (PHTP) and optically non-linear guest molecules exhibiting pronounced terminal functional group interactions along polar molecular strings.

We report a new photorefractive polymer that contains an ionic tri(bispyridyl) ruthenium complex as the charge generating species, a conjugated polymer backbone as the charge transporting channel and a nonlinear optical chromophore. The ruthenium complex was introduced to utilize its efficient light induced metal-to-ligand charge transfer process. This polymer shows greatly enhanced photorefractive performance; a large net optical gain of about 200 cm^-1 was obtained at a zero external electric field.

A novel photorefractive polymer is presented, based on a bifunctional molecule which is a derivative of the well known charge transport molecule N,N'-diphenyl-N,N'-bis(3- methylphenyl)-[1,1'-biphenyl]-4,4'-diamine (TPD). In this material the low intrinsic tap density causes the rather low value for the gain coefficient as well as the 90 degree phase shift between the refractive index grating and the illumination pattern. The trap density can effectively be modified by adding small amounts of 1,4-bis-(N,N- dimethylamino)benzene, resulting in a large increase in gain coefficient and a lower phase shift.

Experimental studies of high density holographic digital data storage in organic photorefractive films are described. The results of experiments involving various photorefractive polymer materials are used to assess the relative importance of various material properties. A new class of organic photorefractive materials composed almost entirely of active chromophore molecules, with no inert polymer host, is introduced. The photorefractive properties of these organic glasses are discussed in relation to holographic storage applications.

We present results of temperature dependence studies of photorefractive effect in a polymeric composite PVK:TCP:C₆₀:DEANST. At elevated temperature, decreased rigidity of the host matrix leading to increased orientational mobility of the second-order nonlinear chromophores results in enhanced photorefractive performance. Diffraction efficiency approaching 100% and enhanced two-beam coupling gain coefficients are reported. The influence of temperature on the speed of photorefractive response is measured and discussed. We also report on a novel photorefractive polymeric composite. A new chromophore, 4-[N-(2- hydroxyethyl)-N-(methyl)amino phenyl]-4'-(6-hydroxyhexyl sulfonyl)stilbene, abbreviated as APSS, possessing high transparency over a broad wavelength, is used together with a PVK:TCP:C₆₀ matrix. This composite shows high photorefractive figures of merit at 488, 514.5 and 632.8 nm. Diffraction efficiencies up to 40%, as well as net two-beam coupling gains of 60 cm^-1 have been achieved at these wavelengths.

We report on novel high-performance photorefractive polymer composites based on poly(Nvinylcarbazole) using mixtures of two isomeric electro-optically active chromophores. We demonstrate that the shelf life time of devices using the eutectic mixture is considerably improvedcompared with devices containing just one isomer. Keywords: photorefractive polymers, four-wave mixing, materials stability, aging

The similarity of charge transport in a wide variety of molecularly doped polymers implies that the key unifying feature in these materials is their inherent disorder. Much attention has recently been focused on the role of permanent dipole moments of the dopants, and of the host polymer repeat unit. Theoretical considerations show that energy fluctuations arising from the interaction of a photoinjected charge with randomly placed dipole moments are approximately characterized by a Gaussian distribution, had have long- range spatial correlations. Furthermore, a theoretical link between correlated Gaussian site energies and the Poole- Frenkel has recently been established. This talk will provide an overview of recent progress in understanding transport in molecularly doped polymers in terms of a model of correlated Gaussian disorder.

Recent extensions of the standard Gaussian disorder model are discussed on the basis of Monte Carlo simulations. It is shown that spatial correlation of energetic disorder has a profound effect on the field-dependent mobility. This correlation could be responsible for the frequently observed Poole-Frenkel like behavior. It is also demonstrated that energetic and geometric disorder can act synergetically on hopping motion at high fields. Contrary to the usual assumption, geometric disorder can significantly affect the activation energy of high-field transport.

Time-of-flight measurements on a wide variety of molecularly-doped polymers reveal carrier mobilities that exhibit an exponential dependence on the square root of the applied electric field. Recent attempts to explain the observed field dependence have focused on the role played by spatial and energetic disorder. It as also been conjectured that the charge-dipole interactions often identified as the source of energetic disorder could be of sufficient range to lead to correlations in the energies of neighboring hopping sites. We have analytically explored the effect of such correlations on high field carrier transport in random potentials, and discuss how particular features of the correlations associated with charge-dipole interactions might lead to behavior similar to that seen in experiment.

Photocurrents j(t) in molecularly doped polymers (MDPs) have been extensively modeled as hopping transport through a Gaussian distribution of states, g((epsilon) , (sigma) ), whose width (sigma) on the order of 1000 K is taken from experiment. We analyze j(t) and the mobility, (mu) (E,T), in terms of the deepest sites encountered in crossing a film of thickness L. Many steps per net release from deep sites rationalize how M approximately equals 1 - 10 trapping events in MDPs control transits of over 10⁶ steps, as inferred from j(t) fits. Field-dependent energies and multi- step releases from deep sites lead to ln(mu) (E,T) proportional to (root)E at lower E than in direct Monte Carlo simulations. The microscopic picture of backsliding and equilibration with rare deep sites follows directly from the thermodynamics of g((epsilon) , (sigma) ), holds for a activated hopping rates, and has interesting contrasts to dispersive transport.

We present results of the Monte Carlo simulation of charge carrier transport in a simple model of randomly oriented and orientationally noncorrelated dipoles and compare our results with those obtained for standard Gaussian disorder model. The most significant difference is a shift of the Poole-Frenkel region of the mobility field dependence to lower fields and widening of this region. We have found that in some particular cases the mean carrier velocity is a decreasing function of the electric field.

Three issues concerning phonon-assisted hopping in molecularly doped polymers are considered. The first issue is whether Arrhenius jump rates in the vicinity of room temperature arise from single-phonon or small-polaronic hopping. It is concluded that Arrhenius hopping only occurs above low temperatures through small-polaronic hopping. Second, hopping in molecularly doped polymers is compared with small-polaronic hopping of other systems. Small- polaronic hopping typically occurs between similar chemical structures whose energies are relatively insensitive to their surroundings. Thus, disorder energies experienced by carriers are often modest, values of several hundredths of an eV are common. Nonetheless, the effects of large electric fields on carrier mobilities differ significantly among disordered systems. Data reported for molecularly doped polymers is unlike that for either transition-metal-oxide or chalcogenide glasses. In no case is high-field transport well understood. Finally, I stress that steady-state flow is driven by differences in sites' quasielectrochemical potentials (QECPs). With disorder, differences of QECPs are not simply related to the driving emf. Solution of the (nonlinear) stochastic equations for the QECPs shows that bottlenecks produced by disorder result in nonohmic conduction. Solving the linearized stochastic (disordered resistor network) equations underestimates bottleneck effects. Linearization is inappropriate when intersite differences in the QECPs exceed (kappa) T.

The activation energy for charge transport in molecularly doped polymer films containing p-diethylaminobenzaldehyde diphenylhydrazone (DEH) is measured before and after systematic UV irradiation. UV exposure has been shown to induce a photochemical reaction of the DEH molecule which dramatically reduces the molecule's capability to transport charge. Presumably, the reduction in mobility is associated with the substantial increase (above 1 eV) in the ionization potential of the DEH molecule after photocyclization. The increase in ionization potential effectively removes the photochemically modified molecule from the primary transport manifold. In previous work, we have demonstrated that systematic UV irradiation of molecularly doped polymer films containing DEH provides a novel approach for diluting the dopant concentration and effectively increasing the intersite separation between 'active' dopant molecules in situ. Furthermore, ab initio calculations suggest that the dipole moment of the photoproduct is significantly lower than that of the unconverted DEH molecule. We exploit this photochemical process to prove the hopping activation energy, (Delta) , in DEH doped polycarbonate films parametric in UV irradiation time. The principal observation of this work is that despite a systematic and substantial elimination of active hopping sites and despite the systematic increase in the population of lower dipole moment neighbors, the hopping activation energy for the DEH/polycarbonate system remains constant.

Spectral sensitivity is an important consideration in photoconductor formulation, depending on the particular type of electrophotographic device in which the photoconductor is to be used. High sensitivity in both the near-IR and visible spectral regions would be desirable for a photoconductor used with a wide variety of exposure sources. One possible way to achieve this goal is to use a mixture of charge generation pigments with absorptions in the desired spectral regions. The absorption spectra (and the spectral sensitivity) of phthalocyanine pigments vary according to the metal coordinating the phthalocyanine, and with coordination of different axial ligands (0^2-, Cl^-, OH^-, et al.). In this paper, pthalocyanine pigment composites with different sensitometric properties are prepared by coprecipitation of titanyl phthalocyanine with another phthalocyanine pigment from an organic solvent/trifluoroacetic acid solution. The phthalocyanine pigments used in the composites were chosen on the basis of their structures relative to titanyl phthalocyanine (i.e., presence or absence of an axial ligand). The crystal structure of the composites largely follows the structure of the majority component of the composite, and the photosensitivity of the O equals Ti(pc)/ClIn(pc) composites is high even at low O equals Ti(pc) concentration.

Hole mobilities have been measured in N,N'-bis(2,2- diphenylvinyl)-N,N'-diphenylbenzidine (ENA) doped into a series of different polymers. At room temperature, the mobilities vary by over four orders of magnitude. The results are described within the framework of a formalism based on disorder. The formalism is based on the assumption that charge transport occurs by hopping through a manifold of localized states that are distributed in energy. The key parameter of the formalism is the energy width of the hopping site manifold or DOS (density-of-states). For ENA doped polymers, the widths are between 0.084 and 0.118 eV. The width is described by an argument based on dipolar disorder. The argument is based on the assumption that the width is comprised of a dipolar component and a van der Waals component. The dipolar component is comprised of a dopant contribution due to the ENA and a polymer contribution. The differences in mobilities of these materials are attributed to differences in the polymer dipolar components.

Hole mobilities have been measured in poly(styrene) (PS) doped with N,N-bis(2,2-diphenylvinyl)-N,N'-diphenylbenzidine (ENA). ENA is a weakly polar donor molecule with a dipole moment of 0.86 Debye. The mobilities were unexpectedly high, exceeding 10^-3 cm²/Vs at high fields. The results are described within the framework of a formalism based on disorder, due to Bassler and coworkers. The formalism is based on the argument that charge transport occurs by hopping through a manifold of localized states that are distributed in energy. The key parameters of the formalism are the energy width of the hoping site manifold, the degree of positional disorder, and the prefactor mobility. For ENA doped PS, the widths are between 0.079 and 0.090 eV, increasing with increasing dilution. The widths are described by an argument based on dipolar disorder. The argument is based on the assumption that the total width is comprised of a dipolar component and a van der Waals component. Due to the very low dipole moment of ENA, the dipolar component is near-zero and the total width determined by the van der Waals component. The degree of positional disorder is between 1.6 and 4.8, increasing with increasing dilution. The prefactor mobilities are between 4.2 multiplied by 10^-5 and 1.9 multiplied by 10^-2 cm²/Vs, and can be described by a wavefunction decay constant (rho) ₀ of 1.0 angstrom.