Anna M. Lackner

Optimal operation temperature of liquid crystal modulators

This paper describes analyses and confirming experiments on the optimum temperature for fast response in nematic liquid crystal (LC) modulators. It is demonstrated that the LCs or LC mixtures with higher nematic-isotropic phase transition temperatures have higher optimum temperatures and greater potential for improving the figure of merit. Also discussed is the performance of the LC mixture exhibiting optimum temperature at around room temperature.

Liquid crystal millimeter wave electronic phase shifter

The birefringence of two common nematic liquid crystals were measured in the millimeter wave range, and were found to be in the range of 0.19–0.22 at room temperature. Using liquid crystal electro‐ and magneto‐optical effects, we present the first experimental data of a novel liquid crystal millimeter wave electronic phase shifter. Our experiments show that liquid crystals could play an important role in future millimeter wave device technologies.

Droplet Size Control In Polymer Dispersed Liquid Crystal Films

We have established correlations between the liquid crystal (LC) droplet size and selected characteristics of polymer dispersed liquid crystal (PDLC) films, formed by photopolymerization of LC dissolved in a monomer. The LC droplet size was controlled by means of the ultraviolet (UV) exposure intenstiy, LC concentration, and concentration of polymerization initiator. Correlations were noted between the LC droplet size and the resulting PDLC threshold and operating voltages for optical transmission, contrast ratio, and electro-optical response times. Good contrast PDLC films were obtained which were operable with less than 20 V, rms of applied signal. Special types of films, including in situ gratings were obtained by varying the conditions of polymerization across the film to produce a periodic spatial variation in the LC droplet size.

Liquid crystal birefringence for millimeter wave radar

Abstract Many liquid crystals are found to have relatively high birefringence (Δn) values in the microwave and millimeter wave regions, as calculated from the phase shift induced by their reorientation by magnetic or electric fields. At 30 GHz, Δn values were obtained in the range of 0.08 to 0.18 for eleven liquid crystal mixtures of various types. The most favourable liquid crystal structures for high millimeter wave birefringence are highly conjugated rod-like molecules containing biphenyl, terphenyl, phenylpyrimidine, biphenylpyrimidine, and tolane groups in nematics of positive dielectric anisotropy (Δe). However, other liquid crystal structures including Schiffs base, azoxybenzene, and aromatic ester groups also have substantial birefringence, including nematics with negative and crossover Δe, as well as cholesteric nematics. The Δn varied only slightly at different frequencies of microwave millimeter wave in the 15–94 GHz range. Studies on magnetic and electrical field liquid crystal orientation in ...

Mylar‐film‐compensated π and parallel‐aligned liquid crystal cells for direct‐view and projection displays

Optical properties of stretched Mylar films [poly(ethylene terephthalate)] were evaluated. Similar birefringence dispersion to liquid crystals over a wide spectral range makes Mylar an attractive phase retardation plate for display applications. Two display modes employing an antiparallel (or π) and a parallel‐aligned cell with a Mylar phase compensator were demonstrated. The film‐compensated π cell shows an 80:1 black‐to‐white contrast ratio, 60 Hz frame rate and ±45° viewing angle, and is attractive for direct view display. The film‐compensated parallel cell shows an 85:1 black‐to‐white contrast ratio, 180 Hz frame rate, and ±10° viewing angle, and is suitable for projection display.

Near Ultraviolet Photostability of Liquid Crystal Mixtures

Abstract Liquid crystal material photostability was investigated for application in high intensity projectors operating with both near ultraviolet (UV) and visible light. Commercially available mixtures were studied in transparent test cells and their lifetimes were correlated with their chemical structures. Substantial improvements were observed in the photostability of hybrid field effect cells by using pure nematic mixtures whose major components had one or two cyclohexyl groups in place of phenyl groups. However. overriding the importance of these structural effects, polar impurities present in some of the nematic mixtures with cyclohexane groups caused a large decrease in photostability. Results were obtained by examining the photostability of eight different commercial liquid crystal mixtures, with highly positive dielectric anisotropy, whose components include a variety of structural types. Although the resistivity of each liquid crystal mixture decreased steadily with exposure, the reported end of...

Variable Grating Mode Liquid Crystal Device for Optical Processing Computing

Abstract Certain nematic liquid crystal mixtures are observed to form a “variable grating mode” (VGM) for appropriate choices of cell design and applied voltage. In this mode of operation, a phase grating in the plane of the cell arises from a periodic variation in the orientation of the liquid crystal director. The grating spatial frequency is observed to vary linearly as a function of the applied voltage above the formation threshold. Liquid crystal and device parameters characteristic of the observed variable grating mode are presented. Utilization of the VGM effect in a photo-conductively-addressed device is shown to provide an intensity-to-spatial frequency conversion. Applications of this unique type of optical transducer to arbitrary nonlinear optical processing problems are described. Results of level slicing experiments and implementation of optical logic functions are presented.

Effects of Molecular Length on Nematic Mixtures

The properties of nematic mixtures of 4-alkoxyphenyl 4-alkylbenzoates are studied as a function of their average molecular length (L). The Ls of the mixtures vary between 20.46 and 27.14 A, but they all have clearpoints in the 51 to 58° C range. The flow viscosity increases linearly with L. The dielectric constant, refractive index, birefringence, and density decrease linearly with L, and the dielectric anisotropy becomes more negative. With tetrabutylammonium tetraphenylboride added as a salt dopant, the conductivity anisotropy (σ∥/στ) decreases with increasing L, and the dynamic-scattering (DS) threshold voltage increases correspondingly. The effect of L on the DS decay time (τD) is highly dependent on the surface alignment. In surface ⊥ cells, τD increases strongly with L, while in surface ∥ cells, τD decreases slightly with increasing L. The temperature dependence of σ∥/στ indicates that the longer L mixtures, with about ten or more total alkyl carbons from both end groups, have cybotactic n...

Addressing factors for polymer-dispersed liquid-crystal displays

Suitable addressing methods for polymer dispersed liquid crystal (PDLC) displays are dependent upon both the type of display and the PDLC properties. Video displays require fast frame rates, high resolution, and gray scale. The addressing method depends upon PDLC properties such as threshold voltage, operating voltages, response times, resistivity, and hysteresis effects. the properties of PDLC films which are formed by ultraviolet (UV) polymerization of LC/monomer solutions depend upon their formation conditions. The threshold and operating voltages, contrast ratio and response times are affected by formation factors including the UV intensity, temperature, LC concentration, LC composition, field pre- alignment conditions, cell thickness, and droplet shape. The PDLC resistivity and hysteresis properties are more difficult to control, and can strongly affect the light throughput, response times, and gray scale of video displays. Apparent limitations from these properties can be overcome by utilizing pulse mode addressing factors pertinent for active matrix and photoactivated light valve PDLC displays.

Effects of Molecular Length on Nematic Mixtures: II Anisotropic and Dynamic Scattering Properties of 4-Alkoxyphenyl 4-Alkylcyclohexanecarboxylate Mixtures

The properties of nematic liquid-crystal mixtures of 4-alkoxyphenyl trans-4-alkylcyclohexane-carboxylates are studied as a function of temperature and their average molecular length (L). Mixtures are prepared with clearpoints near 72° C (±3° C) and with [Lbar] varying between 21.20 and 26.15 A. At 25° C, their flow viscosity increases exponentially (from 16.3 to 50.6 cP) as [Lbar] increases. The conductivity anisotropy (σ∥/στ) also varies tremendously, decreasing from 1.62 to 0.49 as L increases, when compared at 25° C with tetrabutylammonium tetraphenylboride as dopant. The low values of σ∥/στ and its temperature dependence indicate that cybotatic nematic characteristics occur when the average total number of alkyl carbons from both end groups is 8.5 or more. Short range smectic effects are dominant in the longer mixtures. All of the mixtures have a negative dielectric anisotropy, which linearly becomes less negative with increasing L. At 25° C, dynamic scattering (DS) is observed only in the sh...Abstract The properties of nematic liquid-crystal mixtures of 4-alkoxyphenyl trans-4-alkylcyclohexane-carboxylates are studied as a function of temperature and their average molecular length (L). Mixtures are prepared with clearpoints near 72° C (±3° C) and with [Lbar] varying between 21.20 and 26.15 A. At 25° C, their flow viscosity increases exponentially (from 16.3 to 50.6 cP) as [Lbar] increases. The conductivity anisotropy (σ∥/στ) also varies tremendously, decreasing from 1.62 to 0.49 as L increases, when compared at 25° C with tetrabutylammonium tetraphenylboride as dopant. The low values of σ∥/στ and its temperature dependence indicate that cybotatic nematic characteristics occur when the average total number of alkyl carbons from both end groups is 8.5 or more. Short range smectic effects are dominant in the longer mixtures. All of the mixtures have a negative dielectric anisotropy, which linearly becomes less negative with increasing L. At 25° C, dynamic scattering (DS) is observed only in the sh...