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.
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.
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.
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.
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 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.
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