Kyle B. Ferrio

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.

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.

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.

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.

Photorefractive polymers with video-rate performance

A photorefractive polymer employing a fluorinated cyanotolane chromophore exhibits a stable response-time of approximately 2 ms and may support applications requiring video-rates. The achievable bandwidth is not limited by reorientational dynamics and might be increased further by improvements in photoconductivity.

4-ms response time in a photorefractive polymer

Efficient photorefractive polymers hold great potential for optical data processing and storage and so have been a topic of intense investigations in recent years. Materials with nearly 100 percent diffraction efficiencies have been developed, but these have been generally characterized by much slower response than their less efficient inorganic counterparts. In this report, we describe a 4-ms initial rise-time using a visible-transparent chromophore in a PVK- based polymer of Tg equals 39 degrees C for 48-V/micrometers bias projected along the grating vector and 250-mW/cm2 writing beams. This fast component consistently accounts for 90 percent of the steady-state response and is analyzed as a function of bias field. We also present transient ellipsometry measurements of an orientational response significantly faster than the grating development, suggesting that chromophores alignment sped does not limit transient grating formation. We describe the role of such underlying property measurements in understanding photorefractive transient behavior, and prospects for further improvements.