Chien-Hui Wen

Refractive indices of liquid crystals for display applications

This paper reviews the extended Cauchy model and the four-parameter model for describing the wavelength and temperature effects of liquid crystal (LC) refractive indices. The refractive indices of nine commercial LCs, MLC-9200-000, MLC-9200-100, MLC-6608, MLC-6241-000, 5PCH, 5CB, TL-216, E7, and E44 are measured by the Multi-wavelength Abbe Refractometer. These experimental data are used to validate the theoretical models. Excellent agreement between experiment and theory is obtained.

High Birefringence Isothiocyanato Tolane Liquid Crystals

The phase transition temperatures, birefringence and visco-elastic coefficient of several high birefringence isothiocyanato tolane compounds were evaluated. The polarizability of these compounds was calculated by the Austin Model 1 (AM1) and Modified Neglect of Diatomic Overlap (MNDO) methods. Using these compounds, we have formulated a eutectic mixture exhibiting a wide nematic range and high figure of merit.

Super High Birefringence Isothiocyanato Biphenyl-Bistolane Liquid Crystals

We have designed, synthesized, and evaluated the physical properties of some high birefringence (Δn) isothiocyanato biphenyl-bistolane liquid crystals. These compounds exhibit Δn~0.7–0.8 at room temperature and wavelength λ=633 nm. Laterally substituted short alkyl chains and fluorine atom eliminate smectic phase and lower the melting temperature. The moderate melting temperature and very high clearing temperature make those compounds attractive for eutectic mixture formulation. A eutectic mixture based on those compounds was formulated and its physical properties evaluated. The simulated absorption spectra agree reasonably well with the measured results.

Dielectric heating effects of dual-frequency liquid crystals

A noncontact birefringence probing method is developed to monitor the temperature rise of dual-frequency liquid crystals (DFLCs) due to the dielectric heating effect. This method allows us to determine the temperature change accurately without using a thermocouple. The dielectric heating effects of three DFLC mixtures are investigated quantitatively. By properly choosing the molecular structures, the dielectric heating effect can be minimized while keeping other desirable physical properties uncompromised.

Polarization-independent and submillisecond response phase modulators using a 90° twisted dual-frequency liquid crystal

A polarization-independent phase modulator using a 90° twisted dual-frequency liquid crystal (DFLC) is demonstrated. In addition to being polarization independent, such a phase modulator exhibits many other advantages such as being scattering-free and having large phase change, low operating voltage, and submillisecond response time. Using a 15μm transmissive DFLC cell, the phase shift achieves 1π at λ=633nm and the applied voltage is lower than 25Vrms. Potential applications of such a phase modulator for laser beam steering, tunable-focus lenses, and switchable two-dimensional/three-dimensional liquid crystal displays are foreseeable.

Ultraviolet stability of liquid crystals containing cyano and isothiocyanato terminal groups

The ultraviolet (UV) stabilities of liquid crystal compounds containing cyano (CN) and isothiocyanato (NCS) terminal groups are compared. UV exposure is found to degrade the clearing temperature, birefringence, dielectric constants and visco-elastic coefficient of the liquid crystal compounds. The measured data show that the NCS molecular structure exhibits a better UV stability than do the corresponding CN structures. Lewis resonance structures are used to explain this phenomenon.

UV Stable High Birefringence Liquid Crystals

High birefringence and low viscosity nematic liquid crystal single compounds and mixtures were developed and their properties evaluated. The UV stability of the single isothiocyanianes liquid crystal compounds was studied by different methods. Excellent UV durability of some of the single high birefringence isothiocyanianes was discovered. The new eutectic liquid crystal mixtures based on these compounds were tested in comparison to commercial high birefringence Mercks E44. Our new mixtures exhibit similar birefringence, ~4 times lower visco-elastic coefficient and significantly better UV stability than E44 mixture.

Photostability of liquid crystals and alignment layers

— Photostability of liquid-crystal (LC) materials and surface alignment layers was evaluated using a UV lamp and a blue laser beam. Both organic polyimide (PI) and inorganic silicon-dioxide (SiO2) alignment layers were studied under nitrogen environment. Two commercial TFT-grade LC mixtures (low-birefringence MLC-9200-000 and high-birefringence TL-216) were used for comparisons. Results indicate that SiO2 alignment layers are much more robust than PI layers, and low birefringence LCs are more stable than the high-birefringence ones. At the He-Cd laser wavelength (λ = 442 nm), both LC mixtures and SiO2 alignment layers are hardly damaged. To lengthen the lifetime of an LCD projector, inorganic SiO2 alignment layers, high-optical-density UV filter, long cutoff-wavelength blue filter, and short-conjugation (low birefringence) LC materials should be considered.

High figure‐of‐merit nematic mixtures based on totally unsaturated isothiocyanate liquid crystals

High birefringence and low viscosity isothiocyanate liquid crystal single compounds, and eutectic mixtures based solely on unsaturated rigid core structures, are reported. Extraordinarily high values of figure‐of‐merit were observed at room temperature for the formulated nematic mixtures. Potential applications of such mixtures for laser beam steering at λ = 1.55 µm using optical phased arrays are emphasized.

Polarization-independent phase modulation of a homeotropic liquid crystal gel

A homeotropic liquid crystal (LC) gel with submicron domain size is fabricated and its phase-only modulation property evaluated. The LC gel is highly transparent in the voltage-off state and exhibits pure phase modulation before light scattering occurs. Compared to a nanosized polymer-dispersed liquid crystal (nano-PDLC), our gel possesses a larger phase change but at a lower operating voltage because of a higher LC concentration. Similar to a nano-PDLC, our gel also exhibit submillisecond response time, hysteresis-free, and polarization-independent phase change.