Charles D. Hoke

OPTICAL COMPENSATION OF LIQUID CRYSTAL MATERIALS USING NEGATIVE BIREFRINGENCE COMPENSATION FILMS

A unique optical compensation film, based on a negative birefringence material, is shown to significantly improve the viewing angle characteristics of liquid crystal display (LCD) devices. A super-twist nematic (STN) LCD was used as basis for our numerical simulations. The performance of the negative birefringence compensation film for the STN LCD was modeled using a 4×4 matrix calculation and then compared with the ideal positive birefringence compensator of the double layer STN (D-STN).

Multidimensional alignment structure for the liquid crystal director field

An alignment technique for liquid crystal devices is presented that consists of polymer walls made from monomers that are oriented by an electric field while cured with a spatially patterned UV light source. The technique provides defined boundary conditions of the liquid crystal director field in three dimensions.

Design and performance of a reconfigurable liquid-crystal-based optical add/drop multiplexer

The authors describe a reconfigurable optical add-drop multiplexer designed for 50-GHz channel spacing over either the C- or L-bands. The system may also function as a variable optical attenuator. The design features a reflective liquid-crystal modulator, a compact free-space spectrometer, polarization diversity, and fine-scale attenuation control.

High Speed Addressing of a 0??360? Bistable Twisted Nematic Liquid Crystal Display

In this letter we report the results of an investigation on the dynamics and optics of a twist bistable liquid crystal display. For the first time, the parameters required for high speed addressing of a 0°–360° bistable device are presented. The display consists of a chiral-doped nematic liquid crystal layer with a nominal surface pretilt. The characteristics of the drive waveform allows selection between two optically distinct twist states of 0° and 360°. Optimizing the ratio of cell thickness to inherent pitch and waveform parameters leads to line selection times, for a multiplexed display, of <100 µs; in contrast with typical nematic bistable displays that require times of ~10 ms. Full optical response can be achieved in times on the order of 15 ms.

Long Term Bistable Twisted Nematic Liquid Crystal Display and Its Computer Simulations

The multidimensional alignment (MDA) technique realized by the polymer wall has been previously proposed as a means to stabilize the liquid crystal configurations for twist angles of 0 and 2π so that very low power and real bistable twisted nematic (BTN) liquid crystal display (LCD) can be obtained. In this study, a two-dimensional liquid crystal director simulation software was employed to explore the stability of BTN devices. The simulation results showed that the homeotropic aligned polymer wall will stabilize the BTN device when the ratio of d/p is close to the switching d/p ratio, but when the d/p ratio is higher than 0.9, the director field will convert the configuration from a high-energy 0 twist state to low-energy 2π twist state automatically, therefore the BTN bistability will be destroyed.

Light propagation in variable-refractive-index materials with liquid-crystal-infiltrated microcavities

A liquid-crystal-infiltrated microcavity structure is proposed as a variable-refractive-index material. It has the advantages over previously considered nanostructured materials of having a larger phase-angle change and lower driving voltage. Two-dimensional liquid-crystal director and finite-difference time-domain optical simulations are used to select liquid crystal material parameters and optimize the dimension of the microcavity structured material.

Studies of the Bistability of Highly Twisted Nematics

The bistable electro-optic effect of highly twisted nematic liquid crystals was studied. A numerical relaxation technique was used to minimize the free energy density of a liquid crystal medium. The director configurations at an applied field, especially the mid-cell director, may be significantly different depending on the initial conditions. The elastic energy of the liquid crystal medium was used to describe its bistability. The effects of cell twist angle, elastic constants, thickness to pitch ratio, dielectric parameter, and surface pretilt angle were studied.

The Effect of Polymer Networks on the Electrodistortional Characteristics of Nematic Liquid Crystals

Abstract The effect of polymer networks is studied for highly twisted nematic liquid crystals. A numerical relaxation technique was used to minimize the free energy density of a liquid crystal medium. Assuming the networks are distributed uniformly, an polymer network contribution to the system free energy is added in the form of − 1/2Ka (n · N)2, where Ka, n and N are polymer network coefficient, liquid crystal director and polymer network unit vector, respectively. Ignoring the polymer networks elasticity, the liquid crystal director configuration and elastic energy under the influence of an external electric field are calculated numerically. The obtained electro-distortional characteristics are found to depend on the value of Ka and N. The results are compared to the electro-optic threshold and hysteresis behaviors of a set of highly twisted nematic cells with different monomer concentrations. The elastic energy of the liquid crystal medium is used to describe its bistability.

Polarized infrared study of a polymer network deformation in a nematic liquid crystal host

Using polarized infrared (IR) spectroscopy we have observed deformation of a polymer network in a liquid crystal host during the reorientation of the liquid crystal by an external electric field. In the system studied, containing 2% BMBB-6 polymerized at zero applied field in the host nematic liquid crystal 6CB, the observed deformation angle was between 20o and 40o 10o. These experimental results provide some of the first conclusive experimental evidence that the polymer network elastically deforms as a direct result of the reorientation of the liquid crystal host.

The High-Field Cured Polymer Networks in Nematic Liquid Crystals

The polymer networks formed in nematic liquid crystals at high-field condition are studied. Assuming the networks are distributed uniformly, an polymer network contribution to the system free energy is added in the form of -½K a(nN)2, where K a, n and N are polymer network coefficient, liquid crystal director and polymer network unit vector, respectively. Ignoring the polymer networks elasticity, the liquid crystal director configuration and elastic energy under the influence of an external electric field are calculated numerically. The obtained electro-distortional characteristics are found to depend on the value of K a and N. The results are compared to the electro-optic threshold and hysteresis behaviors of a set of highly twisted nematic cells with different monomer concentrations. A good agreement between the monomer concentration and polymer network coefficient is presented.