K. Meerholz

Ellipsometric measurements of poling birefringence, the Pockels effect, and the Kerr effect in high-performance photorefractive polymer composites

Using a simple ellipsometric technique, we measure the total refractive-index modulation as a function of the frequency of the applied electric field in low glass-transition-temperature photorefractive polymer composites. From these measurements we deduce the relative contributions of the poling birefringence and the Pockels and Kerr effects. By applying the oriented gas model we determine the microscopic properties of the nonlinear optical chromophore, including the anisotropic polarizability, and the first and the second hyperpolarizabilities. In the search for new high-performance materials, the technique provides a measure of the linear and the second- and third-order nonlinear optical properties simultaneously.

Birefringence, Pockels, and Kerr effects in photorefractive polymers

A simple frequency‐dependent ellipsometric technique for measuring the birefringence, the Pockels and Kerr properties of low glass‐transition temperature photorefractive polymers is proposed. The technique is applied to the characterization of highly efficient DMNPAA:PVK:ECZ:TNF photorefractive polymers and the determination of the microscopic properties of the DMNPAA molecule.

Optical computing by use of photorefractive polymers

A matched-filter four-wave-mixing optical correlator was built with a photorefractive polymer as the nonlinear material. Two different time scales are important for this type of device: the time to write a grating, which is limited by the response time of the photorefractive material (~100 ms for the polymer used) and the time to diffract light from the grating, which is essentially instantaneous. For the matched-filter optical correlator we have obtained correlations comparing two 5000-pixel images, using 220-fs pulses with 5-mW average power, demonstrating that the time to compare the test image and the reference image is not limited by the relatively slow response time of the photorefractive polymer. Since the photorefractive grating is erasable the device can be reprogrammed for a different reference image in less than 1 s. The results show that photorefractive polymers can be used efficiently in optical image processing applications.

Non-Bragg orders in dynamic self-diffraction on thick phase gratings in a photorefractive polymer.

We demonstrate that recording thick holographic phase gratings in photorefractive polymers can lead not only to very efficient Bragg diffraction but also to rather strong diffraction into non-Bragg orders. We show that this effect has features drastically different from those of Raman-Nath diffraction on thin gratings. We compare the experimental results with a model based on the theory of dynamic self-diffraction in a photorefractive medium. Applications of this effect in devices for optical image processing are proposed.

Study of non-Bragg orders in dynamic self-diffraction in a photorefractive polymer: experiment, theory, and applications

We present a consistent experimental and theoretical study of non-Bragg orders observed in dynamic self-diffraction on thick phase holograms in a photorefractive polymer that showed a strong refractive-index modulation amplitude. We demonstrate experimentally how the diffraction into non-Bragg orders can be used for image processing and as an independent evaluation method for the optical–photorefractive properties of the polymers.

Improved Long-Term Stability and Performance of Photorefractive Polymer Devices Containing Eutectic Mixtures of Electrooptic Chromophores

Recently, amorphous organic photorefractive (PR) materials have attracted a lot of attention. However, since most of the high-performance PR organic materials are polymers blended with large amounts of low-molecular-weight components, their shelf-life time is limited due to phase separation. One way to improve the shelf-life time, which was proposed by us recently, is to use eutectic mixtures of two electrooptically active chromophores. However, in our earlier attempt in improved life time was achieved at the cost of a reduced performance due to increased absorption. In this paper, we apply this technique again with great success, not only improving the life time by a factor of 15, but at the same time improving the steady-state holographic performance by 30%.

PHOTOREFRACTIVE POLYMERS AND THEIR APPLICATIONS

Abstract We review the basic properties of photorefractive polymer composites and discuss the performance of low glass-transition guest/host type polymers. Examples of applications using these new materials are presented.

High-performance PVK-based photorefractive polymers

Photorefractive materials have many potential photonic applications, including dynamic holographic storage and image processing. Recently, the new class of photorefractive polymers has emerged, offering wide structural flexibility, easy processability, and low cost. Progress in this field has led to nearly 100% diffraction efficiency for the readout of a hologram stored in the material and to extremely large net gain coefficients of more than 200 cm-1. These excellent properties result from refractive index modulations as large as (Delta) n approximately equals 10-2 which could be generated in these materials with low- power lasers, such as HeNe laser or laser diodes.

Azo dye-doped photorefractive polymers

The studies of the photorefractive properties of the two polymer composites based on poly(N-vinylcarbazole) (PVK) are reported. The polymers were obtained by doping PVK with 2,4,7 trinitro-9- fluorenone as a sensitizer agent and two different azo dyes as nonlinear chromophores. Best results were obtained for the polymer doped with 2,5-dimethyl-4-p-nitrophenylazoanisole, which showed a maximum diffraction efficiency of 34% at 40 V/micrometers external electric field strength in 105 micrometers thick samples. The influence of different geometries of the hologram writing- retrieval on the diffraction efficiency is described. Net amplification of one of the writing beams in two-beam coupling within the material has been observed. Recording of the holograms of 2-dimensional objects in real time has been demonstrated with good resolution and image brightness. All the results were obtained with a low power visible laser diode emitting at 675 nm.

Photorefractivity And Photoisomerization In Azo-Dye-Doped Polymers

Gratings formed by photorefractive and photoisomerization processes in an azo-dye-doped polymer, poly(vinyl carbazole):trinitrofluorenone/Disperse Red I, are investigated by polarization- and field-dependent four-wave mixing experiments. High diffraction efficiencies and long storage times are observed for the photorefractive gratings. Both types of grating are erasable, and we can select them by choosing the polarization of the reading beam. The effect of photoisomerization on photorefractivity is discussed. Our results indicate that azo-dye-doped polymers are promising candidates for reversible optical storage applications.