In many fields of nature, a system reacts to sudden changes of an external parameter in the form of two (or more) cascaded relaxation processes, where the first step depends directly on the external parameter and the second process, which is connected with the experimental observable, is determined by the state of relaxation of the first one. It follows from linear-response theory that in this case the experiment yields a behavior which distinctly deviates from a single-exponential decay, even if each of the processes is linear and follows an exponential law. The relaxation starts with time derivative zero, which is most pronounced when the two time constants are of similar magnitude. If the experimental data are fitted with a Kohlrausch-Williams-Watts (KWW) function, the fit will therefore tend to overestimate the KWW exponent beta. Even beta values larger than one can be obtained. As an example, the diffracted light signal in a photorefractive polymer is analyzed.

Predictions made by two models describing dynamic processes in organic photorefractive materials are verified for a photorefractive polymer, consisting of a Poly-TPD, a dicyano dye and the sensitizer C-60. One of the models can describe our measurements under the assumption of optically active traps as dominant trap species. This is confirmed by altering the sensitizer concentration, which has an influence on the charge transport properties of the material. Also, increasing the sensitizer concentration yielded a significant increase of the grating erasure rate in our material.

Polymeric photoconductors carrying triphenyldiamine units and soft oligomethylene spacers were synthesized. Due to the flexible spacers, the polymers are highly soluble and could be obtained as film-forming materials with appreciably high molecular weights and low glass transition temperature (Tg). They possess HOMO value of about −5.1 eV as determined from cyclic voltammetry. These photoconductors can be mixed with nonlinear optical dyes in any proportion to obtain stable photorefractive samples. The Tg can be tuned about room temperature by just changing the composition of the guest–host photorefractive system and requires no additional amount of plasticizer. Fast photorefractive response time of about 1 ms was obtained for these new guest–host systems from degenerate four-wave mixing and two-beam coupling measurements. Further, a series of well-defined low molecular weight bifunctional compounds, TPA–Azos were synthesized in which the photoconductor moiety, triphenylamine (TPA) is covalently linked to an azo dye. The TPA–Azos exhibit Tg values of 13, 36 and 42 °C and they can also be mixed with polymeric photoconductors to obtain stable photorefractive systems with low Tg values. With a new setup to control and vary the sample temperature precisely, the temperature dependence of erasing dynamics of holographic gratings in these systems was studied. It reveals change of response time by three orders of magnitude over a temperature range of 13 K. The difference in behaviour of the photorefractive system above and below the Tg of the sample allows us to differentiate between the time contributions due to the fast electro-optic effect and the slow orientational diffusion processes.

Altering the sample temperature in a photorefractive material changes the rotational mobility of the chromophores. A change of three orders of magnitude in the response times over a temperature change of 12 K has been observed. In the photorefractive experiment, however, the chromophore orientation is induced by the non-instantaneous change of the space charge field. The finite speed of the latter causes the chromophore answer to be different from their normal relaxation behaviour to an instantaneous change. This effect is most pronounced when both time constants are in the same range.

A series of photoconducting poly( tetraphenyldiaminobiphenylene alkyl ether) s in which tetraphenyldiaminobiphenyl (TPD) units are covalently linked through flexible oligomethylene glycol spacers in the main chain were synthesized and the thermal, optical and electrochemical properties were studied. Due to the introduction of flexible spacers, the polymers are highly soluble and could be obtained as film- forming materials with appreciably high molecular weights. The polymers exhibit glass transition temperatures between 92 degreesC and 128 degreesC which is about 100 degreesC less than those main chain polymeric bis( triphenylamine) s without such spacers. The HOMO value as determined from cyclic voltammetry is about -5.1 eV. The glass transition temperature of the photorefractive composites prepared by mixing the different polymers with an electro- optic chromophore, 1-( 2- ethylhexyloxy)- 2,5- dimethyl- 4-( 4- nitrophenylazo) benzene, EHDNPB, could be tuned over a wide range about room temperature by just changing the photoconductor and without the need of any additional amount of plasticizer. Degenerate four- wave mixing and two- beam coupling in composites with the composition, photoconductor : EHDNPB : C-60 (60 : 39 : 1 by wt/wt%) results in refractive index modulations of 10(-3) with corresponding response time similar to 10 ms and a photorefractive gain of Gamma = 13 cm(-1) for a writing beam intensity of 1 W cm(-2) (645 nm) under an external electric field of 60 V mum(-1) .

The erasing dynamics of holographic gratings in a low molecular weight photorefractive glass depending on the sample temperature were investigated. Changes in the overall speed of the material by three orders of magnitude over a temperature range of 13 K were observed. We identified two distinct processes below the glass transition temperature T-g, a fast one on time scales of seconds and a slower one with lifetimes around 10(3) S. We attribute the fast process to the electrooptic effect and the slower one to orientational diffusion processes of the glass molecules. Above T-g, the fast process vanishes, whereas the diffusional processes accelerate up to time constants in the range of seconds. This study shows, that an accurate temperature control is indispensable when measuring photorefractive dynamics, especially in the temperature range around T-g.

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.

Photoaddressable polymers based on azobenzene side-chains have recently drawn a lot of industrial attention due to their possible application as data storage media. From the scientific point of view an additional effect, the occurrence of light induced surface relief gratings, is nowadays extensively studied. This article describes the behavior of side-chain and guest–host polymers (with and without azobenzene groups) in holographic experiments with pulse lasers. We observe surface relief gratings also for polymers, which only contain an absorbing, but nonisomerizing chromophore. Relief formation in pulsed holographic experiments is shown to arise from thermal effects, but not from trans–cis isomerization.

Some organic photorefractive materials are too dispersive in order to be investigated by means of time-of-flight measurements. We present the determination of the hole mobility of two of these materials using a holographic technique. Additionally, attempts to determine the effective drift length of the charge carriers for one of the materials are described and their results discussed.

Recently, Ramanujam and co-workers described for the first time that surface relief gratings can be inscribed into azobenzene copolymers with a single pair of nanosecond laser pulses. We performed a detailed investigation of the involved processes. Two contributions to the diffraction efficiency are observed, one arising from a surface relief and one from a transient cis–trans grating which decays on millisecond time scales. The stable surface relief, in contrast, is caused by a thermal effect which sets in at a well-defined threshold value of the pulse energy. © 2000 American Institute of Physics.

Many photorefractive (PR) materials require plasticizers in order to decrease the glass transition temperature, allowing for orientational enhancement by the chromophores. Introduction of the plasticizer, however, alters not only the viscosity but also the photoconductive properties of the material.This can be shown by comparing two different plasticizers which were introduced into a bifunctional low-molecular-weight PR glass and into a polyfluorene guest-host polymer. The latter reaches response times down to 600 mus at a writing intensity of 1 W/cm(2).We have recently improved the concept of low-molecular-weight PR glasses. A suitable photoconducting unit allows the synthesis of a bifunctional system with a glass transition of 22.6 degreesC. Therefore, no plasticizer is needed. The material is based on a triphenyldiamine (TPD) moiety to which a nonlinear-optical chromophore is directly attached. The system is the first representative of a whole class of TPD molecules and polymers for photorefractive applications.

The drift length of charge carriers has a significant influence on the dynamics of the space-charge field in organic photorefractive materials. This letter introduces a relatively simple method for the determination of the drift length, which takes into account that the charge carrier mobility depends on the sample thickness. By combining results of time-of-flight and holographic time-of-flight experiments using the stochastic transport model of Scher and Montroll, the effective drift length can be determined as 2.4 μm in the investigated photorefractive glass.

We report on photoelectric and holographic investigations of an organic photorefractive material based on a low molar mass glass with both photoconductive and nonlinear optical properties. By implementing a suitable plasticizer we obtained a composite system which shows extremely fast initial response times down to 450 mu s at writing beam intensities of I-write = 10.8 W/cm(2) and 2.5 ms at the canonical intensity of I-write = 1 W/cm(2). Furthermore, high refractive index modulations up to Delta n = 6.1 x 10(-3), long lifetimes and high optical quality of the samples are observed. In comparison to a second similar composite system, which was plasticized by a more polar dopand, we demonstrate the crucial role of this functional constituent on the photoelectric properties. Time-of-flight measurements show a major impact of the plasticizer on the dispersivity of charge-carrier transport. Subsequently the buildup and decay dynamics of the photorefractive grating are Substantially affected. Holographic time-of-flight measurements confirm these observations.

Photorefractive organic systems presently show much longer response times than their inorganic counterparts. The origin of this limitation is not yet fully understood. We present a detailed investigation of the photoelectric processes involved in the photorefractive effect, namely charge generation and charge transport. A comparison between conventional (TOF) and holographic time-of-flight (HTOF) experiments, which were used to determine the charge carrier mobility, is presented. The mobility determined by TOF is shown to depend on the sample thickness. The results show that charge carrier generation and transport are not the limiting factors for the cw-response time of holographic experiments on this system.

One challenge for photorefractive organic materials is to overcome the problem of their relatively low speed. The latter may be caused by either the photoelectric properties of the material or by the reorientation of nonlinear optical chromophores in a space-charge field. This contribution focuses on the first aspect. Using holographic and pulsed experiments, we determine the charge carrier mobility in a bifunctional glass. A comparison of both techniques shows the critical influence of several parameters, such as the concentration of the photoconductor and the sample thickness, on the photoelectric properties of the material.

We present a detailed study of the photoelectric as well as the holographic properties of a novel organic photorefractive glass based on triphenylamine. We studied the quantum efficiency Φ of the photogeneration of charges by means of photoinduced discharge measurements. The photoconductivity σ and the charge carrier mobility μ were obtained via dc photoconduction and pulsed time-of-flight experiments, respectively. The holographic characterization was performed by two-wave and degenerate four-wave mixing experiments allowing for the determination of properties such as diffraction efficiency η, modulation of the refractive index Δn, gain coefficient Γ, and phase-shift φp of the investigated system. The experimental data for Φ could be successfully described by the Onsager formalism with a thermalization radius of r0=24 Å and a primary quantum yield of Φ0=40%. We evaluated the E field and temperature-dependent measurements of μ using the Bässler formalism yielding a width of the density of states of σ=0.13 eV and a disorder parameter Σ=3.6. On this basis the lifetime and the average drift length of the charge carriers could be estimated from the dc photoconduction experiments. From the photoelectric measurements we also calculated the holographic response time that matched very well to the measured response time and described the E-field dependence satisfactorily. The presented photorefractive system shows outstanding optical properties and stability with respect to degradation. We measured a gain coefficient of Γ=90 cm-1, and a diffraction efficiency of η=27% at a response time of 30 ms for only 40-μm-thick samples. Orientational enhancement was observed and evaluated quantitatively. To our knowledge, this work presents the first determination of each of the above quantities all in one single organic photorefractive material.

Charge transport is a basic process for the photorefractive (PR) effect and has a strong influence on the grating formation speed. We investigate the transient hole transport in three organic low-molecular PR glasses by the well known time-of-flight technique. We determine the energetic parameters in terms of the empirical Gill formalism. The characteristic depth of the trapping sites correlates with the response time of the PR effect in a holographic experiment. Thus, the introduction of deeper traps, i.e., a broadening of the density of transport states, leads to faster PR response of our systems by spreading the release-time distribution and causing dispersive transport. Consequently not only the absolute value of the mobility but also the transport mechanism - providing adequate immobilization of the charge carriers - influences the dynamics of the grating formation. A detailed analysis of the transport mechanism confirms the predictions of the stochastic model of Scher and Montroll: The trace of the current transients indicates dispersive transport and the dependence of the transit time on the applied electric field and the sample thickness obeys the same nonlinear scaling law. [S0163-1829(99)02147-5].

One of the reasons for the missing technological implementation of photorefractive organic materials is their relatively low speed as compared to inorganic systems. The origin of the speed limit can be related to either the photoelectric properties of the materials or to reorientation of nonlinear optical chromophores in a space-charge field. This contribution focuses on the first aspect. Using holographic and pulsed experiments, we determine the charge carrier mobility in two bifunctional glasses. The charge generation efficiency is measured by a xerographic technique. Both properties are related to a holographic characterization of the sample by cw two-beam coupling and degenerate four-wave mixing experiments. We discuss the limiting factors for the holographic response and ways how to overcome them.

We present an organic photorefractive material based on a low molar mass glass with both photoconductive and nonlinear optical properties. By implementing a novel plasticizer and doping with the well known sensitizer C-60 we obtained a composite material, which shows extremely fast initial response times of 2.5 ms at writing beam intensities of I-write = 1 W/cm(2) and 450 mu s at I-write = 10.8 W/cm(2). Combined with high refractive index modulations of up to Delta n = 6 . 10(-3) and sample lifetimes of over 6 months this material exhibits an excellent overall performance.In comparison to another similar composite system which was plasticized by a more polar dopand, we demonstrate the crucial role of this functionality in the dispersivity of charge transport and the photorefractive grating dynamics. The complicated temporal behavior of grating build-up and decay which was investigated by degenerate four-wave mixing experiments is discussed in detail. We propose a new way to describe these processes in a more general manner as it is done up to date, using an inverse Laplace transform (ILT) analysis.

Organic photorefractive materials react quickly to an incident light intensity modulation; their optical response times can be as low as a few milliseconds. This time scale is determined by the interplay of a number of processes which are primarily related to the photoconductivity of the material. The present contribution focuses on charge carrier dynamics, i.e. charge transport and trapping, in organic photorefractive materials. Starting from a discussion of the transport mechanisms, the relevant parameters and length scales are determined. At the end, a connection between photoconductive properties and optical response times is presented.