Andre Leopold

Fast and stable photorefractive systems with compatible photoconductors and bifunctional NLO-dyes

Abstract 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 ( T g ). 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 T g 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 T g values of 13, 36 and 42 °C and they can also be mixed with polymeric photoconductors to obtain stable photorefractive systems with low T g 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 T g of the sample allows us to differentiate between the time contributions due to the fast electro-optic effect and the slow orientational diffusion processes.

Characterization of charge generation and transport in a photorefractive organic glass: comparison between conventional and holographic time-of-flight experiments

Abstract 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.

Temperature dependencies in organic photorefractive materials

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.

Temperature dependent measurements on a low-molecular-weight photorefractive glass

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 Tg, a fast one on time scales of seconds and a slower one with lifetimes around 103 s. We attribute the fast process to the electro-optic effect and the slower one to orientational diffusion processes of the glass molecules. Above Tg, 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 Tg.

Influence of the sensitizer on the dynamic behavior of an organic photorefractive material

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 C60. 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.

Influence of the Charge Transport Characteristics on the Holographic Response of 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.

Variation of the glass transition temperature in organic photorefractive materials: plasticizer versus novel synthetic approaches

Most 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 microsecond(s) at a writing intensity of 1 W/cm2. 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 degree(s)C. 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.

Investigations on the effective drift length in organic photorefractive materials

Abstract 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.

Build-up dynamics of fast organic photorefractive glasses

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 C60 we obtained a composite material, which shows extremely fast initial response times of 2.5 ms at writing beam intensities of Iwrite equals 1 W/cm2 and 450 microsecond(s) at Iwrite equals 10.8 W/cm2. Combined with high refractive index modulations of up to (Delta) n equals 6 X 10-3 and sample lifetimes of over 6 months this material exhibits an excellent overall performance.

Photoelectrical characterization of photorefractive organic systems

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.