Mark S. Spector

Dynamics of the acousto‐optic effect in a nematic liquid crystal

In a nematic liquid crystal cell, the application of an ultrasonic wave induces a rotation of the director, leading to a change in the optical transmission through the cell. In this study, we investigate the dynamic response of the optical intensity after the ultrasonic wave is switched on or off. Our experiments show that the optical intensity follows a double‐exponential function of time, indicating that the system has two relaxation modes with widely different time scales. The fast mode has an amplitude and time scale qualitatively consistent with the dynamics of the Fréedericksz transition, but the slow mode shows novel behaviour associated with the acousto‐optic effect.

Chiral twisting of a smectic-A liquid crystal

Chiral twisting of the molecular orientation within the layer of a smectic-A liquid crystal has been investigated using circular dichroism spectroscopy. The results indicate that a rotation of the layers away from the alignment direction is induced by the surface electroclinic effect. This leads to an interfacial region where the molecular director twists from the alignment direction until it reaches the layer normal direction. A theory is presented to explain the observed field and temperature dependence of the circular dichroism.

Acousto-optic response of nematic liquid crystals

The dependence of the acousto-optic response of two nematic liquid crystals on several possible future device parameters is investigated. Several paths for possible optimization are identified. A large enhancement of the acousto-optic response is obtained through optimization of the incident angle of the acoustic wave. The power required for a given cell to reach the maximum optical intensity is also reduced by adjusting the optical angle. This reduction comes at the expense of higher zero-field intensity, and therefore lowers the overall contrast. Finally, the acousto-optic response shows a significant dependence on liquid crystal layer thickness, scaling with the theoretically predicted fifth power of the cell thickness.

Surface Electroclinic Effect in a Smectic-A Liquid Crystal

Polar interactions between liquid crystal molecules and cell surfaces give rise to the surface electroclinic effect. This results in rotation of the layer normal away from the alignment direction near the surface and an interfacial region where the molecular director twists from the alignment direction until it reaches the layer normal direction. We have investigated the molecular orientation within the smectic layer using linear and circular dichroism spectroscopy. Contrary to theoretical predictions, the magnitude of the surface electroclinic tilt is relatively temperature independent, while the bulk electroclinic coefficient strongly depends on temperature.

Electroclinic liquid crystals with large tilt angles for grayscale applications

Smectic-A liquid crystals exhibiting a large electroclinic effect are important for applications in view of their analog gray scale capability. In most of these materials, large electroclinic tilt angles are accompanied by buckling effects due to layer compression. This layer buckling is easily observed in an optical microscope as periodic stripes and drastically reduces the high contrast ratio necessary for optical devices. We have performed optical and x-ray scattering studies on a chiral, organosiloxane smectic-A liquid crystal. It is found that while the optical tilt angle exhibits a large dependence on the field, reaching values of about 31 degrees (for 5 V/micrometer applied field), the layer spacing shows only a very weak field-dependence, suggesting that the molecules have a nonzero tilt even with no applied field, and that the primary effect of the field is to induce long range order in the direction of the molecular tilt. This important result -- large field-induced optical tilt without a layer shrinkage -- has led to the development of materials with 256 gray -- large field-induced optical tilt without a layer shrinkage -- has led to the development of materials with 256 gray levels.