B. L. Volodin

Photorefractive polymer-dispersed liquid crystals

We report observation of the photorefractive effect in functionalized polymer-dispersed liquid crystals. The photoconductive properties are provided by the polymer matrix, and the field-dependent refractive-index changes are generated by the dispersed nematic liquid-crystal droplets. A high diffraction efficiency (8%) and a high refractive-index modulation amplitude Δn=2×10-3 are obtained in 53-µm-thick samples with an applied field of 22  V/µm.

Phase stability of guest/host photorefractive polymers studied by light scattering experiments

We report on light scattering measurements in guest/host photorefractive polymers doped with different polar dyes as a function of temperature and sample composition. Crystallization processes of the polar dye are found to follow a nucleation and growth mechanism. The structure and melting point of the polar dye, the storage temperature, and the amount of plasticizer are shown to exert a major influence on the shelf lifetime of the samples. New composites with high efficiency and long shelf lifetime are presented.

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.

Transillumination imaging through scattering media by use of photorefractive polymers

We demonstrate the use of a near-infrared-sensitive photorefractive polymer with high efficiency for imaging through scattering media, using an all-optical holographic time gate. Imaging through nine scattering mean free paths is performed at 800nm with a mode-locked continuous-wave Ti:sapphire laser.

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.

Compact, low power polymer-based optical correlator

A low cost, low power compact optical correlator is demonstrated. It operates interchangeably in the matched-filter or joint-transform configurations. The active medium is a high diffraction efficiency photorefractive host–guest polymer system. Successful image correlation has been achieved.

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.