Haiqing Xianyu

Electro-optics of polymer-stabilized blue phase liquid crystal displays

Electro-optics of polymer-stabilized blue phase liquid crystal displays (BP LCDs) is analyzed and validated experimentally. A numerical model for characterizing and optimizing the electro-optical and display properties of BP LCDs in in-plane switching and fringe field switching cells is developed. The simulated voltage-dependent transmittance curves agree well with the measured results. To lower the operating voltage while keeping a high transmittance, both electrode width and gap, and large Kerr constant make important contributions. A wide-view BP LCD using a single biaxial compensation film is simulated.

Submillisecond Gray-Level Response Time of a Polymer-Stabilized Blue-Phase Liquid Crystal

The gray-to-gray response time of a polymer-stabilized blue-phase liquid crystal is investigated. As the voltage increases, rise time decreases gradually but decay time remains unchanged. A set of analytical equations is used to fit the measured results, and an acceptable agreement is obtained.

Hysteresis Effects in Blue-Phase Liquid Crystals

Hysteresis effects of two blue-phase (BP) liquid crystals BP-I and BP-II, and polymer-stabilized BP-I and BP-II are investigated. BP-I exhibits a relatively slow response time and a large hysteresis. After polymer stabilization, its operating voltage is increased slightly, but its response time is dramatically reduced and hysteresis narrowed. In contrast, BP-II exhibits almost no hysteresis and submillisecond response time, but after polymer stabilization its hysteresis is amplified.

Anchoring energy and cell gap effects on liquid crystal response time

The anchoring energy and cell gap effects on liquid crystal response time (τ0) is analyzed theoretically and validated experimentally. Analytical expressions are derived using two different approaches: effective cell gap and surface dynamic equation methods. Consistent results are deduced from these two approaches. A simplified equation τ0∼dx also fits the experimental data well, where d is the liquid crystal cell gap and x is the exponent. Under two extreme (strong and weak) anchoring limits, the exponent x approaches 2 and 1, respectively. This information is helpful for optimizing liquid crystal devices for display applications.

Adaptive dielectric liquid lens

A tunable-focus liquid lens using dielectrophoretic effect is demonstrated. When a voltage is applied to a dielectric liquid droplet, the generated electric field inside the droplet is inhomogeneous. As a result, the liquid bears a dielectric force and its surface profile can be reshaped which causes the focal length to change. Adaptive lenses with different apertures are fabricated and their performances evaluated. In comparison to the patterned-electrode liquid lenses, our lens uses continuous electrode which is much simpler for fabrication.

Dual frequency liquid crystals: a review

We review the recent progress in developing high performance dual frequency liquid crystal (DFLC) materials. A new figure of merit is derived for evaluating the DFLC performance. The dielectric relaxation and electro-optical properties of several high performance compounds, which possess a positive dielectric anisotropy at low frequencies and a relaxation frequency in the kilohertz region, are presented. Potential applications of these compounds are discussed. Compounds with a negative dielectric anisotropy were also compared and discussed. Two experimental DFLC mixtures were demonstrated with one intended for room temperature operation and another for elevated temperature operation.

In-plane switching of cholesteric liquid crystals for visible and near-infrared applications

We have investigated the in-plane switching of cholesteric liquid crystals for reflective wavelength shifters for visible and near-infrared applications. These devices are based on the elongation of the cholesteric pitch by an electric field perpendicular to the helical axis. The transmission notch-reflection peak position can be tuned continuously to a longer wavelength (redshift) by application of an in-plane electric field. The helix is completely unwound when the electric field is higher than the cholesteric-to-nematic transition field, and the sample is transformed to a transparent state. We have investigated the electro-optic performance of in-plane switching of cholesteric samples and developed a simple phenomenological model to describe the underlying electro-optic phenomena.

Pretilt Angle Effects on Liquid Crystal Response Time

Pretilt angle effect on liquid crystal dynamics is analyzed theoretically. Analytical expressions are derived to describe liquid crystal response time under nonzero pretilt angle conditions. The theoretical analysis is confirmed experimentally using vertically aligned liquid crystal cells. This finding quantitatively correlates pretilt angles with liquid crystal response time. This study improves the understanding of the liquid crystal dynamic process which is helpful for optimizing liquid crystal response time.

Rollable multicolor display using electrically induced blueshift of a cholesteric reactive mesogen mixture

Electrically controllable blueshift of the reflection band in a planar cholesteric reactive mesogen cell is observed. The responsible mechanism is electric-field-induced Helfrich deformation [J. Chem. Phys. 55, 839 (1971)]. The modified director configuration can be solidified by photopolymerizing the reactive mesogens when a voltage is applied. The correlation between the director configuration and optical properties is validated by the scanning electron microscope photos and the transmission spectra of a planar and an undulated cholesteric film. With masked curing at different voltages, a rollable multicolor display is demonstrated.

Low absorption liquid crystals for mid-wave infrared applications

A partially fluorinated terphenyl liquid crystal with low absorption in both mid-wave infrared (MWIR) and near IR regions is developed and its properties evaluated. This compound exhibits a nematic phase (although only about 2 °C), reasonably high birefringence (Δn~0.2), low visco-elastic coefficient, and modest dielectric anisotropy (Δε = -2.7). This compound serves as an important first example for future development of low-loss MWIR liquid crystals and devices.