Volume 92, Issue 4, 15 August 2002
Index of content:
- LASERS, OPTICS, AND OPTOELECTRONICS (PACS 42)
92(2002); http://dx.doi.org/10.1063/1.1491279View Description Hide Description
The model of space-charge formation in photorefractive polymers due to Schildkraut and Buettner has been modified to include thermally accessible deep traps as well as shallow traps. The dynamic equations have been solved semiempirically using independent measurements of photoconductive properties to predict photorefractive dynamics. Dependencies of the dynamics on charge generation, mobility, trap density, acceptor density, ionized acceptor density, as well as their associated rates are examined. The magnitude of the fast time constant of photorefractive development is successfully predicted. The introduction of deep traps into the model has allowed us to qualitatively predict the reduction in speed due to deep trap filling and ionized acceptor growth. Experimental studies of photoconductivity and photorefraction (PR) in several polyvinyl carbazole photorefractive composites are carried out to demonstrate the applicability of the model. By choosing chromophores with different ionization potentials and by varying the chromophore concentrations, we investigate the influence of the chromophore ionization potential on the photoelectric and PR properties and reveal the nature of deep traps in the composites and their contribution to both photoconductivity and PR dynamics. Effects of plasticizer components are also discussed.
Fully leaky guided wave determination of the original alignment direction for the directors at the walls in a twisted nematic liquid crystal cell92(2002); http://dx.doi.org/10.1063/1.1491025View Description Hide Description
Fully leaky guided optical modes are used to explore the director profile in a twisted nematicliquid crystalcell built using a pair of high-index glass plates. This allows access to the pseudocritical angles corresponding to the liquid-crystal indices. Optical data is recorded over a range of angles of incidence with the cell subject to ac voltages. At higher-applied fields, the director in the cell center aligns almost homeotropically. Then the two surface regions become elastically decoupled, the directors at the surfaces realigning along the original alignment axes. Beyond the pseudo-critical angle corresponding to the low index of the liquid-crystal two minima of the polarization conserving transmission signal, for transverse electric polarization, (over a particular range of incidence angles) are found as the cell rotation angle is changed. These two minima give the original alignment directions of the director, the easy axes, at the two substrates, information vital to the measurement of the torsional anchoring energies at the interfaces of the cells.
92(2002); http://dx.doi.org/10.1063/1.1492863View Description Hide Description
We discuss the formation of pretilt by oblique irradiation of photosensitive polymers that homogeneously align liquid crystals in the direction corresponding to the maximum density of unreacted polymer. We show that a tilted polymer distribution is formed by oblique illumination with one beam of unpolarized or partially polarized light. When unpolarized radiation is used, a small azimuthal anisotropy is generated and the polymer tilt angle increases with incident angle. Pretilted liquid crystal alignment is possible up to a threshold angle at which the azimuthal alignment direction changes. Irradiation with partially polarized light gives a higher azimuthal anisotropy but smaller polymer tilt angles. We obtain liquid crystal pretilt angles up to 4° by oblique irradiation of polyimide with light from an excimer laser operating below the threshold for ablation. A simple two step process is used to obtain pretilted alignment for liquid crystals using surface gratings: a grating etched into polyimide by laser ablation is subsequently irradiated at oblique incidence to give planar liquid crystal alignment with a pretilt angle of 3°.