Showing papers by "Hoi Sing Kwok published in 2001"

Room-temperature ultraviolet emission from an organic light-emitting diode

Abstract: Ultraviolet emission was obtained from N,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine. Gallium nitride was used as a hole-blocking layer to contain the holes. A peak emission wavelength of 400 nm was measured. The external quantum efficiency is about 0.35%.

Enhanced drilling using a dual-pulse Nd:YAG laser

Abstract: A new method for improving the efficiency of laser drilling has been developed. Two synchronized free-running laser pulses from a tandem-head Nd:YAG laser are capable of drilling through 1/8-in-thick stainless-steel targets at a stand-off distance of 1 m without gas-assist. The combination of a high-energy laser pulse for melting with a properly tailored high-intensity laser pulse for liquid expulsion results in the efficient drilling of metal targets. We argue that the improvement in drilling is due to the recoil pressure generated by rapid evaporation of the molten material by the second laser pulse.

Dependence of the current and power efficiencies of organic light-emitting diode on the thickness of the constituent organic layers

Abstract: The dependence of the current and power efficiencies of bi-layer organic light-emitting diodes (OLEDs) on the thickness of the constituent organic layers is reported. The thickness of the electron and hole transport layers was simultaneously varied to determine the optimal configuration for power efficiency. It was verified that the inclusion of a suitable electrode buffer layer reduced the effective energy barrier against carrier injection, thus improved the injection efficiency. An optimal thickness for the electrode buffer layer was also identified and explained.

Transmissive liquid crystal cell parameters measurement by spectroscopic ellipsometry

Abstract: A powerful method for liquid crystal (LC) cell twist angle and retardation measurement is presented. This method is based on the spectroscopic ellipsometry. By varying the polarizer and analyzer angles only, a transmission spectrum containing null transmission at some particular wavelengths can be obtained. Analytical equations derived bases on a polarization analysis of twisted birefringent layers are then solved to give all the LC cell parameters. In general, the twist angle, twist sense, retardation, and the rubbing direction can all be determined. Measurement results on commercial liquid crystal display panels are presented.

3 × 3 Matrix for unitary optical systems

Abstract: We introduce a 3×3 matrix for the study of unitary optical systems. This 3×3 matrix is a submatrix of the 4×4 Mueller matrix. The elements of this 3×3 matrix are real, and thus complex-number calculations can be avoided. The 3×3 matrix is useful for illustrating the polarization state of an optical system. One can also use it to derive the conditions for linear and circular polarization output for a general optical system. New characterization methods for unitary optical systems are introduced. It is shown that the trajectory of the Stokes vector on a Poincare sphere is either a circle or an ellipse as the optical system or input polarizer is rotated. One can use this characteristic circle or ellipse to measure the equivalent optical retardation and rotation of any lossless optical system.

Method of manufacturing photo-alignment layer

Abstract: A photo-alignment layer having excellent long-term stability to light and heat is manufactured by coating a material for the photo-alignment layer, which contains a dichroic dye having two or more polymerizable groups per molecule, on a substrate, and exposing the coating layer to polarized light, thereby imparting a photo-alignment function, and polymerizing the polymerizable groups by heating or light exposure.

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

Abstract: 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.

Mueller calculus and perfect polarization conversion modes in liquid crystal displays

Abstract: We introduce a 4×4 Mueller matrix for describing the polarization states of a liquid crystal cell. Using this Mueller matrix, it is possible to derive conditions whereby a linearly polarized input can be converted to either a perfectly linearly or perfectly circularly polarized output. These are called the perfect polarization conversion (PPC) conditions. These PPC conditions can be used in many different ways. For example, based on these PPC conditions, polarization mode switching schemes can be derived for the analysis of most liquid crystal displays.

Optically optimized permanently bistable twisted nematic liquid crystal displays

Abstract: The structure and construction of a bistable twisted nematic liquid crystal display with very long lifetime are disclosed. The two stable twist states in this display when no voltage is applied have almost infinite lifetimes and do not decay. These two stable twist states differ by a twist angle of 180°. New design conditions of this bistable twist structure are disclosed such that the contrast and the brightness are both optimized. This display can be operated in transmittive mode with two polarizes or in the reflective mode with one polarizer.

Photo-induced dichroic polarizers and fabrication methods thereof

Abstract: The present invention provides a method of forming a polarizing material comprising exposing a layer of dichroic material to activating light illumination to provide an ordered structure with a distinguished absorption axis and thus photo-induce polarization, and fixing the induced polarization by polymerisation of the dichroic layer. The present invention also provides novel polarizing materials formed thereby. By selectively exposing regions of the dichroic material to differing activating radiation, different regions with different polarization axes can be created. The polarizing material can also be provided with a coating or coatings to alter the spectral response, and a stack formed of a plurality if dichroic layers may be provided.

Helix unwinding of doped cholesteric liquid crystals

Abstract: 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.

The Novel 8x8 Transfer Matrix Method of the LC Optics Simulations: Application to Reflective LCDs

Abstract: A new effective matrix method for the calculation of optical characteristics of Reflective Liquid Crystal Displays (RLCDs) with consideration for multiple reflection is proposed.

Dynamic Flow, Broken Surface Anchoring, and Switching Bistability in Three-terminal Twisted Nematic Liquid-crystal Displays

Abstract: We investigate the switching bistability between the Φ and Φ+π twist states in liquid-crystal cells with a three-terminal electrode structure. Based on general nematohydrodynamic equations and a modeling potential of surface anchoring, numerical calculation shows that dynamic shear flow and electric-field-induced breaking of surface azimuthal anchoring are essential to the realization of the switching bistability observed experimentally.

Characterization and minimization of flicker in silicon light valves

Abstract: We have performed systematic characterizations of flicker in silicon light valves. It was found that there were four conduction mechanisms accounting for the flicker. These four mechanisms were residual dc charge on the silicon surface, voltage holding capability of the liquid crystal cell, voltage holding capability of the silicon panel, and parasitic capacitor coupling of the pixel. Major causes of these four mechanisms were identified. Solutions of flicker minimization were obtained for each mechanism. Among these solutions, offset of common voltage was found very useful to compensate for residual dc charge and parasitic capacitor coupling. Frame rate multiplication was found very useful for the minimization of flicker due to low voltage holding capabilities of the liquid crystal cell and silicon panel.

Pulse Sequence Addressing for Gray-Scale Bistable Cholesteric Displays

Abstract: 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.

Method of manufacturing a photo-alignment layer

Abstract: A photo-alignment layer having excellent long-term stability to light and heat is manufactured by coating a material for the photo-alignment layer, which contains a dichroic dye having two or more polymerizable groups per molecule, on a substrate, and exposing the coating layer to polarized light, thereby imparting a photo-alignment function, and polymerizing the polymerizable groups by heating or light exposure

Application of re‐crystallized metal‐induced unilaterally crystallized polycrystalline‐silicon thin‐film‐transistor technology to reflective liquid‐crystaldisplay

Abstract: — Single-polarizer reflective twisted-nematic (RTN) liquid-crystal modes offer larger viewing angles, higher contrast ratios and lower power dissipation compared to regular double-polarizer transmissive- reflective liquid-crystal implementations. The application of re-crystallized metal-induced unilaterally crystallized polycrystalline-silicon thin-film-transistor (TFT) technology to realize active matrices and peripheral circuit components for hand-held direct-view RTN-mode video displays is reported.

Bistable twisted nematic liquid crystals display with permanent grayscales and fast switching

Abstract: Permanent grayscales in a π bistable twisted nematic liquid crystal display have been demonstrated. This display can be switched between –22.5° and 157.5° twist states by means of a combination of strong in-plane electric fields and vertical electric field for creating both the planar anchoring breaking and an electrohydrodynamic flow. The switching of the twist states during the Frederiks transition is sufficiently inhomogeneous such that domains of –22.5° twisted and 157.5° twisted states are formed. The ratio of these domains can be controlled by the vertical electric field. Fast switching of 1 ms was achieved for gray scales. These grayscales have very long lifetime.

Liquid crystalline light-emitting thermally stable readily processable substituted polyacetylenes

Abstract: Mesomorphic and luminescent poly(propiolates) with different skeleton structures (-{(R)CequalsC[CO 2 (CH 2 ) 6 OCO-Biph- OC 7 H 15 ]} n -; R equals H (1), CH 3 (2), C 6 H 5 (3), Biph equals biphenylyl} are synthesized. The backbone absorption in 2 and 3 are weak but upon photoexcitation, the polymers emit strong violet light of 369 nm, whose intensities are higher than that from poly(l-phenyl-l-octyne), a well-known highly-fluorescent polyacetylene. The main chain of 3 absorbs strongly at 380 nm, and the polymer is completely nonluminescent. All the polymers are thermally stable and form enantiotropic monolayer SmA phase, with 1 adopting well-ordered packing arrangements.