Kaleemullah Khand

Photorefractive polymer composite trapping properties and a link with chromophore structure

The photorefractive properties and the phase stability of polymer composites are dependent on the detail of the alkyl chain substituent attached to the electro-optic dye within the composite. Photorefractive composites based on poly (N-vinylcarbazole) (PVK), sensitized with trinitrofluorenone (TNF) and mixed with a concentration of 47.5 wt. % of electro-optic dye have been tested for photorefractive performance. Two alternative azo dyes of identical molecular weight have been used to produce alternative composites; both dyes were modified to suppress spatial isomerism and incorporated an eight carbon alkyl chain at the electropositive end of the chromophore: either a straight octyl chain or a branched ethylhexyl chain was substituted. The reorientational enhancement of photorefractive performance is similar in the composites resulting from these dyes. The dye with a straight octyl chain led to a composite with improved holographic performance. The dye with a branched ethylhexyl chain led to a composite ex...

Effect of field-dependent photogeneration on holographic contrast in photorefractive polymers

The field-dependence of the charge photogeneration efficiency reduces the holographic contrast in an organic photorefractive composite. In this article, the Braun model for field-dependent photogeneration efficiency is incorporated into the standard model of photorefractivity. Using this formalism, the effect on the space-charge field, and thereby holographic index contrast, is calculated directly without the need for empirically found parameters. The dependence of the space-charge field on experimental geometry is also analyzed. A powerful description of the effective space-charge field is thus derived and is experimentally verified. This analysis is then used to compare the observed holographic contrast to that predicted from transmission ellipsometry measurements.

Poling and relaxation of dipoles in dispersive media

The poling of dipoles in dispersive media under the influence of an applied electric field and the subsequent relaxation after the removal of the applied field have been analyzed. Both the poling and relaxation processes are described by simple and exact analytical solutions to the rotational diffusion equation. The resulting expressions for the order parameters are power laws with respect to time and hence have no characteristic lifetimes. The poling and relaxation rates are, in fact, determined by a single material parameter: the diffusion constant, D0. These theoretical results have been experimentally verified for dipolar molecules doped into an amorphous polymer host. An excellent agreement between theory and measurement was found.

Photorefractive trapping and the correlation between recording and erasure dynamics in a polymer composite

Abstract The fast time constants of holographic contrast growth and erasure for various photorefractive polymer composites were measured under a wide range of experimental conditions. Either of two sensitizers, 2,4,7-trinitro-9-fluorenone (TNF) or (2,4,7-trinitro-9-fluorenylidene)malonodinitrile (TNFDM), were used in a degenerate four-wave mixing experiment under varying conditions of field, wavelength and intensity. Collected together, the fast growth and erasure time constants were found to be correlated with a coefficient of 0.96. From this correlation we conclude that the charge generation process during holographic recording and the optical de-trapping process during hologram erasure are indistinguishable. Hence in accordance with the photorefractive crystal model, hole recombination with the sensitizer anions forms the primary charge trapping mechanism in the majority of photorefractive polymer composites.

Hole transport is through dye in a PVK-matrix photorefractive polymer

Summary form only given.Most of the efficient photorefractive polymers that have been reported to date are based on a composite of an electro-optic dye and an electronegative sensitizer doped into a charge transport polymer, usually poly(N-vinylcarbazole), (PVK). One early variation of this was to use a passive host such as polymethylmethacrylate together with a charge transport dopant, in place of the charge transport polymer, which illustrates that hole transport by hopping between donor molecules present at 30 wt%. or more may be efficient. There is evidence from IR spectroscopy that the electro-optic dye has an additional role as a compensator, storing positive charge. There have also been reports that the high dipole moment of the electro-optic dye leads to a reduction in the mobility of hole transport through the PVK matrix. We report on time of flight measurements on a known photorefractive polymer composite incorporating PVK and an analogous composite in which the host matrix is polycarbonate, which is a dielectric. Surprisingly, the composite with the polycarbonate host yields an improved hole mobility relative to the PVK-based composite.

Quantification of the physical parameters of the stored hologram in photorefractive polymer composites

The local field within a reorientational photorefractive polymer composite has been estimated to be 2.2 times the applied poling field. Using a transmission ellipsometric technique the birefringence within a pulsed photorefractive polymer material has been measured as a function of applied field. The birefringence at an internal poling field of 110 Vmicrometers -1 is 4 X 10-3. By equating the sum of the poling field and the space-charge field formed within a hologram to the birefringence dependency on field and the holographic contrast obtained in a four wave mixing experiment, the amplitude of the space-charge field has been found. Its values are between 7.8 and 15 Vmicrometers -1, depending on the ratio of reorientation and the Pockels contributions to the birefringence. In this regime the holographic contrast is limited by the trap density which is found to be between 7.4 X 1015 and 15.7 X 1015 cm-3.