W.C. Yip

Efficient polarization converter for projection displays

In the waveguiding limit, a twisted nematic liquid crystal cell behaves as an achromatic polarization rotator. We propose and demonstrate the application of such a polarization rotator to convert unpolarized light into linearly polarized light with almost 100% efficiency. This polarization converter has a 2:1 aspect ratio, which is close to the 16:9 ratio for modern televisions. It can be used therefore in a projection display with polarization-dependent light valves such as a liquid crystal light valve. Both transmittive and reflective light valves can be used. The temperature dependence of the achromatic polarization rotator is also studied.

New photo-aligning and photo-patterning technology: superthin internal polarizers, retarders, and aligning layers

The photo-aligning materials based on azodye layers are proposed. The azodye aligning layers enable (i) high order parameter; (ii) excellent alignment quality of LCD with a high contrast ratio; (iii) temperature stability, suitable for LCD manufacturing; (iv) perfect adhesion and high voltage holding ratio due to the specific molecular groups (v) pretilt angle generation. The azodye layers can be used to fabricate thin internal patterned (pixelated) polarizers with different local orientations of the absorption axis and/or absorption colors. Our new methods allow to produce defect-free highly uniform alignment of lyotropic LC or iodine-doped azodye layers themselves with a fine resolution of the polarization pattern. The photo-aligned internal polarizers are cost-effective and enable new LCDs with excellent electro-optical response, including good viewing angles and high brightness. We prepared an internal phase retarder using UV-cured photo-polymerized material. 4-(6- acryloyloxyhexyloxy) benzoic acid had been synthesized and the synthesis procedure was modified for a better yield. We had shown that by applying an electric or magnetic field, the director deformation of the liquid crystalline monomer could be in-situ UV-cured for the optimal phase compensation generation.

Helix Unwinding of Doped Cholesteric Liquid Crystals

By introducing benzoyl–benzene into a cholesteric liquid crystal, the helix unwinding voltage was reduced. This reduction was roughly proportional to the concentration of the dopant and was present for driving frequencies across the audio spectrum. It is believed that this voltage reduction is primarily due to a perturbation of the intermolecular coupling in the liquid crystal mixture. It was found that so long as the long-range order was not destroyed, the helix unwinding voltage could be reduced by as much as 24% at 60 Hz.

45.3: Azo Dye Materials for the Alignment of Liquid Crystal

In this paper, the photo-induced alignment of liquid crystal on an azo dye film is discussed for the liquid crystal display (LCD) applications. This is a non-contact technique to align the liquid crystals so that the dust particles and static charges generated by the rubbing process can be eliminated. To prepare for the photo-alignment layer, the azo dye film is exposed to a linearly polarized or non-polarized light. As a result of the dipole absorption, the photo-induced molecular distribution is non-uniform and has an angular dependence. Consequently, since the dispersion forces between the dye and the liquid crystal molecules dominate, the induced orientation of the liquid crystal molecules is possible. The oblique incidence of the light exposure will also favour a non-zero pretilt angle. We find that the order parameter associated with the aligned liquid crystal medium is high, and very good contrast has been measured in the Twist Nematic (TN) LCD.

Pulse Sequence Addressing for Gray-Scale Bistable Cholesteric Displays

In the study of the transition dynamics, we found that the reflectance of bistable cholesteric displays depended on the addressing sequence of the bipolar pulses. Two different pulse amplitudes corresponding to the high and low voltages about the quiescent point were chosen. By a proper permutation of such pulses, the final reflectance of even difference could be obtained for gray-scale display applications. In contrast with the root-mean-square (rms) modulation techniques, this scheme proposed for the first time does not primarily depend on the rms voltage. In this paper, we study the electro-optics and the domain characteristics, and propose a phenomenological proposition to describe this effect. We shall also extend our previous driving scheme to demonstrate the gray-scale control at 5 ms/line addressing.

Achromatic wave retarder by phase subtraction

We analyze the method of phase subtraction in two identical optical structures to build an achromatic phase retarder. The two structures are made of right-angle prisms and are aligned orthogonal to each other. They are also made of materials of different refractive indices so that dispersion compensation can be taken advantage of. Essentially the phase retardation between the s and p waves in the first structure is subtracted from the phase retardation in the second structure. This can be done by reversing the roles of the s and p waves. By choosing the materials of the prisms properly, the phase retardation can be made to be constant over a broad spectral range. Indeed, calculations made with commercial optical glasses show that phase errors in the visible and near-infrared regions can be rather small. For example, for a 90° phase retarder (quarter-wave plate), a phase error of 0.35° can be obtained from 0.35 to 0.81 μm and from 0.59 to 1.26 μm.

Cost-effective Driving Scheme for Bistable Cholesteric Displays

For VLSI implementation, low operation voltage and switching current are the major concerns. Yet, none of the driving schemes published so far has addressed this combined issue. We studied the electro-optic characteristics of bistable cholesteric displays at the low transition voltage in this paper, and the effect of slew-rate on the maximum output current and average output power in a forthcoming paper. The dynamic responses were measured and an efficient scheme was devised accordingly. The most cost-effective addressing time was 4 ms/line using the low-end twist nematic (TN) or super twist nematic (STN) mixtures.

Effect of Waveform Slew-rate on Bistable Cholesteric Displays

Since the typical peak-to-peak voltage needed to drive the bistable cholesteric display is two to three times higher than that for super twist nematic (STN) displays, the transient current can be excessively large and detrimental to most semiconductor drivers. To reduce the maximum switching current, the voltage transition time should be restricted in addition to reducing the peak voltage. In this paper, we studied the effect of slew-rate on the maximum output current, average output power and the dynamic optical responses. Based on circuit theory, we obtained a good agreement between experimental and calculated results. The optimal settings can be incorporated into our driving scheme so that low voltage and current requirements can be met for very large-scale integration (VLSI) implementation.