Eric Hendrickx

Direct observation of orientation limit in a fast photorefractive polymer composite

We report on a photorefractive polymer with a 4-ms-response time in transient four-wave mixing experiments at 0.5 W/cm2 writing irradiance, 95 V/μm applied electric field, and a grating period of 3.1 μm. Complementary transient ellipsometry, however, reveals orientational birefringence response which leads the four-wave mixing response all the way to its saturation, despite complex dynamics in these processes. Orientation does not limit the dynamic formation of photorefractive gratings in this polymer, which suggests that even faster photorefractive responses are possible for polymer composites with improved charge generation and transport properties.

Photoconductive properties of PVK-based photorefractive polymer composites doped with fluorinated styrene chromophores

We have synthesized nine anisotropic chromophores, with different degrees of fluorination, and studied the effect of the chromophores ionization potential on charge-transfer complexation, photoconductivity, and response time in photorefractive polymer mixtures based on poly(vinylcarbazole). (2,4,7-Trinitrofluoren-9-ylidene)malononitrile (TNFDM) or C 60 provided the sensitization. We have found evidence of strong complexation between TNFDM and the chromophore. At high electric fields, the photoconductivity decays during illumination and reaches a limiting value that correlates with the chromophores ionization potential. A buildup of C 60 – radical anions is observed simultaneously. The strong decline in photoconductivity correlates with an increase in the photorefractive grating buildup time.

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.

Thermally stable high-gain photorefractive polymer composites based on a tri-functional chromophore

We report on the photorefractive properties of thermally stable polymer composites based on the dye 2, N, N-dihexylamino-7-dicyanomethylidenyl-3,4,5,6,10-pentahydronaphthalene. At an applied field of 50 V/μm, we have achieved a dynamic range of Δn=8.5×10−3 and a net two-beam coupling gain of 202 cm−1. The diffraction efficiency peaks at an applied field of 28 V/μm, giving an external diffraction efficiency of 71%.

High photogeneration efficiency of charge-transfer complexes formed between low ionization potential arylamines and C60

Photogeneration of carriers was measured at 633 nm in a series of polymer composites based on polystyrene, doped with a triphenyldiamine derivative as hole transport molecule and C60 as sensitizer. Light absorption and generation of mobile carriers was achieved through the charge-transfer (CT) complex formed between the hole transport molecule and the sensitizer. We discuss the influence of the ionization potential of the hole-transport molecule on the photogeneration efficiency of the CT complex it forms with the sensitizer C60. Photogeneration efficiency of unity at an applied electric field of 55 V/μm is found in one composite.

Stabilization of the response time in photorefractive polymers

The optical and photoconductive fatigue of fast photorefractive polymers have been studied in a family of C60-sensitized polymer composites containing styrene-based chromophores with varying ionization potential. Changes in response time and in photoconductivity were studied for exposures up to 104 J/cm2. Increasing the chromophore ionization potential beyond that of the polyvinylcarbazole host was found to stabilize the response time. Studies of the electric-field dependence of the steady-state diffraction efficiency in various samples confirm the role of C60 anions as possible traps.

Photorefractive polymers sensitized by two-photon absorption

We demonstrate the recording of holograms and their nondestructive readout in a photorefractive polymer, using two-photon absorption. Sensitivity is provided by the excitation of the electroactive chromophore with femtosecond pulses, followed by charge injection into the photoconducting poly(N -vinylcarbazole) matrix. The holograms can be fully erased with a pulsed laser source but are insensitive to cw laser beams with the same wavelength. Studies of the field and intensity dependence of the diffraction efficiency indicate that the holograms are formed through the photorefractive effect.

Ellipsometric determination of the electric-field-induced birefringence of photorefractive dyes in a liquid carbazole derivative

Abstract We have used ellipsometric measurements on solutions of 2-dihexylamino-7-dicyanomethylidene-3,4,4a,5,6-pentahydronaphthalene and related polyenes in liquid 9-(2′-ethylhexyl)carbazole to measure the electric-field-induced birefringence at wavelengths of 690 nm and 830 nm. This electric-field-induced birefringence is related to the product of dipole moment squared and the polarizability anisotropy, μ 2 Δ α , and to a large extent determines the dyes photorefractive figure-of-merit (FOM). We show that μ 2 Δ α increases with the conjugation length and report one of the highest μ 2 Δ α values observed so far.

Progress in organic photorefractive material development

The refractive index modulation in photorefractive polymers with a low glass transition temperature is dominated by orientational birefringence effects. To take advantage of these effects we developed several photorefractive polymers that contain: (i) chromophores designed to have simultaneously a large dipole moment and a high linear polarizability anisotropy, (ii) nematic phase liquid crystal droplets, (iii) transparent molecules generally used for liquid crystal applications. We discuss recent advances in these three different classes materials and emphasize their merits and trade-off.

New advances in organic photorefractive material development

The rapid improvement of the performance of photorefractive polymers over the past few years has generated a strong technological interest for these new materials. The areas of application include holographic storage, image processing, optical correlation, and phase conjugation. This paper reviews some of our recent advances in the field of organic photorefractive materials. We will first present several new polymeric composites that combine high efficiency and long shelf lifetimes. Then we will discuss the performance of a new class of organic photorefractive materials: photorefractive polymer dispersed liquid crystals. Finally, we will present two examples of applications based on photorefractive polymers: (i) optical correlators for security verification, and (ii) imaging through scattering media in the near infra-red.