Jon A. Herlocker

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.

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%.

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 polymer composites fabricated by injection molding

We report on the fabrication of bulk samples of photorefractive polymers using the injection molding technique. The photorefractive properties of these materials are evaluated by four-wave mixing and two-beam coupling experiments. Samples with good optical quality, high diffraction efficiency, and net optical gain are obtained.

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.

Photorefractive properties of polymer composites fabricated by injection molding

Summary form only given. Optical organic photonic materials are emerging as an important class of new optoelectronic materials. Polymers and organic molecules with photoconductive, electro-active, photorefractive, and electroluminescent properties have been developed in recent years and have been inserted into numerous devices including organic field-effect transistors, lasers, electroluminescent diodes, holographic recording films, and photovoltaic cells. A major advantage of these materials over their inorganic counterparts is that they can be processed from solution into thin films or into objects of various shapes using industrial processing techniques such as extrusion and injection molding. These techniques can lead to large scale manufacturing at low temperature and at low cost of light-weight devices. We demonstrate the true industrial process capability of photorefractive polymers. These materials belong to the class of organic multifunctional organic materials. With their high dynamic range and high sensitivity, photorefractive polymers have become promising materials for real-time holographic and imaging applications. Using a Morgan Industries G55-T 22 ton vertical travel press, we fabricated by injection molding samples of photorefractive polymers with various shapes and thicknesses. For our proof-of-principle experiments we formulated a photorefractive composite that was known to have good phase stability properties and a high dynamic range.

Optimization of photorefractive polymers doped with styrene-based chromophores

We present a study on the effects of the dark conductivity on the photorefractive performance of polymers doped with styrene-based chromophores. We find that reducing dark conductivity in such composites increases diffraction efficiency and at the same time decreases the response time. We use a polymer composite based on a polyvinylcarbazole matrix doped with 4-Homopiperidinobenzylidenemalononitrile (7- DCST), sensitized with C60, and plasticized with N- ethylcarbazole (ECZ) and butyl benzyl phthalate (BBP). The reduction of the dark conductivity is achieved by coating one of the electrodes with a SiO blocking layer.

Stabilized response-time in a photorefractive polymer composite doped with a styrene chromophore and C/sub 60/

Summary form only given. The response times of photorefractive polymers have been reduced by an order of magnitude, to milliseconds, over the past two years. It is generally agreed that photoconductivity and not orientational dynamics are the chief obstacle to even faster materials. Photogeneration efficiency, which is of order 1% in many materials described to date, is therefore a critical area of research. C/sub 60/, for instance, is a good candidate to enhance photogeneration. It has been found, however, that chromophores with ionization potentials smaller than that of the transport manifold can stabilize a population of C/sub 60//sup -/ and it has even been suggested that a photorefractive grating may be written in the distribution of C/sub 60//sup -/ and C/sub 60/. The accumulation of C/sub 60//sup -/ has been documented to occur under simultaneous electrical bias and optical excitation. We report measurements that show that the build-up of C/sub 60//sup s/ accompanied by large increases in the response-time of index-modulation in transient-grating experiments. This observation suggests that the response-time may be stabilized by increasing the chromophore ionization potential, and we have experimentally confirmed this expectation.

Charge transport and chromophore orientation in a new photorefractive polymer composite with response-time in the millisecond-regime

Summary form only given. Since the first observation of photorefractivity in a polymer composite, applications to information-processing and dynamic holography have spurred rapid development. A critical step was the realization that index-modulation is enhanced by reorientation of birefringent chromophores in composites with low glass-transition temperatures, T/sub g/. Now attention has turned to understanding and improving response-times. A composite with a 4-ms component has been reported recently, with persuasive evidence that photoconductivity limits speed. We report an even faster 1.8-ms component in a very different composite a fluorinated cyanotolane chromophore.