Thomas X. Wu

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.

Correlations between liquid crystal director reorientation and optical response time of a homeotropic cell

Correlations between the director reorientation time and its consequent optical response time (both decay and rise) of a homeotropic liquid crystal (LC) cell under crossed polarizers are derived theoretically based on small angle approximation. Results indicate that the optical response time is linearly proportional to the LC director reorientation time and is weakly dependent on the initial bias voltage. To validate the derived correlations, transient phase and transmittance responses at various bias voltages are analyzed numerically by solving the Erickson–Leslie equation. Pretilt angle is found to make an important contribution to the optical response time. Gray scale switching of the homeotropic cell is also investigated.

Extraordinarily high-contrast and wide-view liquid-crystal displays

A computer simulation model based on oblique-angle Jones matrix and Poincare sphere is developed for optimizing the design of film-compensated multidomain vertical-alignment liquid crystal display (VA-LCD). According to this design, a contrast ratio higher than 10 000:1 is predicted over the entire ±85° viewing cone for the four-domain VA-LCD. Potential application for liquid-crystal display television is emphasized.

Reflective liquid-crystal displays with asymmetric incident and exit angles.

The 2×2 matrix methods are extended to calculate the optical behaviors of reflective liquid-crystal displays with asymmetric incident and exit angles. Both the unfolding method and the backward-eigenwave method are employed to derive the 2×2 matrix representations. The simulation results for symmetric incident and exit angles from these two methods are identical and agree well with those obtained from the 4×4 matrix method when the air–panel surface reflections are neglected. Further, the derived 2×2 matrix methods are applied to the asymmetric cases with different incident and exit angles. The simulated results on the normally black vertical alignment and normally white mixed-mode twisted nematic reflective displays show reasonably good agreement with the reported experimental data. In addition, a rubbing effect related to contrast values is observed and analyzed in asymmetric reflective cases. We also find that this effect has a significant influence on the contrast ratios once the difference between the incident and exit angles becomes large.

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.

Ultrawide-view liquid crystal displays

The wide viewing angle technologies for liquid crystal displays (LCDs) are reviewed. The most promising liquid crystal modes for wide view technologies, such as in-plane switching, multidomain vertical alignment, patterned vertical alignment, and advanced-super-view are compared. By optimizing the phase-compensation films and their device configurations, the ultrawide-view LCDs with a contrast ratio higher than 100:1 at /spl plusmn/85/spl deg/ viewing cone are demonstrated.

Soliton switching and multi-frequency generation in a nonlinear photonic crystal fiber coupler.

Soliton switching in nonlinear directional couplers implemented in photonic crystal fibers (PCF) examined here. A vector finite element method (FEM) has been developed to precisely calculate the dispersion along with coupling length of the guided modes. The PCF coupler geometry was carefully designed so that it can support soliton pulses. Soliton switching is demonstrated numerically at 1.55 microm for 100 femto-second (fs) pulses. Our theoretical results explain some of the key spectral features previously observed in the experiment.

MOSFET failure modes in the zero-voltage-switched full-bridge switching mode power supply applications

Phase-shifted zero-voltage-switching (ZVS) full bridge topologies are gaining popularity due to their extremely low switching losses in the power devices even at higher switching frequency. However the intrinsic body diode is required to conduct in order to create the ZVS turn-on rendition for the power MOSFET. Due to the extremely low reverse voltage, the reverse recovery charges might not be swept out before turning off the MOSFET. Therefore, the body diode might be subjected to the dv/dt stress when it is not yet capable of blocking reverse voltage. Also, not able to maintain the ZVS operation at low load will force the on-state MOSFET to turn off at hard-switching condition. Like in the hard-switched full bridge topology, the cdv/dt shoot-through current might produce a voltage spike at the gate of the off-state MOSFET on the same leg and cause devices failure. Several silicon technologies are presented to resolve the above-mentioned failure modes in the ZVS topology. Fast reverse recovery time and better dv/dt ruggedness make this new MOSFET technology suitable for higher frequency ZVS full-bridge applications. Inherent with extremely high silicon density and low gate charge, these new MOSFETs can reduce the component count with the same or better performance and will enable much higher power density for the next generation telecom/server SMPS designs.

Design Optimization of Reflective Polarizers for LCD Backlight Recycling

We systematically study and optimize the design of multilayer birefringent reflective polarizers for recycling the backlight of liquid crystal displays. Factors affecting the Bragg reflection are analyzed in detail, including number of layers for establishing Bragg reflection, refractive index difference, effective refractive index, and thickness ratio. Different methods for achieving broadband reflection are investigated, so that the reflective polarizer could cover the entire visible wavelengths and a large incident angle. In addition, the effects of material dispersion on the device design are analyzed.

Optical wave propagation in a cholesteric liquid crystal using the finite element method

Bragg reflections of cholesteric liquid crystals at normal and oblique incidences were investigated using the finite element method (FEM). Detailed FEM derivations together with the consideration of boundary conditions are given. Two methods for achieving broadband Bragg reflection are analysed: one is to use high birefringence liquid crystal in the uniform pitch structure, the other is to use the gradient pitch structure. In each case, the number of cholesteric pitches required for establishing the Bragg reflection was simulated.