Yeongyu Choi

Fast fringe-field switching of a liquid crystal cell by two-dimensional confinement with virtual walls

We report a simple method for reducing the response time of a fringe-field switching liquid crystal cell by using two-dimensional confinement of the liquid crystals. Through both numerical calculations and experiments, we show that the switching speed can be increased by several fold in a fringe-field switching cell by simply using a rubbing angle of zero, which causes virtual walls to be built when an electric field is applied between the interdigitated electrodes and the common electrode, without requiring additional fabrication steps or complicated drive schemes. Furthermore, the devices fabricated with this method exhibit a reduced color shift and excellent dynamic stability, even with a high applied voltage and under external pressure.

Interdigitated pixel electrodes with alternating tilts for fast fringe-field switching of liquid crystals

We propose an interdigitated pixel electrode structure with alternating tilts for fast fringe-field switching of liquid crystals (LCs). In contrast to an LC cell, where the pixel electrodes are parallel to the LC alignment direction, this device does not require a non-zero pretilt angle, owing to an obliquely applied electric field; thus, it can retain a much wider viewing angle by aligning the LCs without a pretilt. In addition to a short response time and wide viewing angle, the proposed device allows a much larger deviation of the LC alignment direction, which is essential for mass production. Moreover, LCs with negative dielectric anisotropy can be used to minimize the transmittance decrease.

Electro-optical characteristics of an in-plane-switching liquid crystal cell with zero rubbing angle: dependence on the electrode structure.

When an electric field is applied to in-plane switching (IPS) and fringe-field switching (FFS) cells with zero rubbing angle, virtual walls are built such that the switching speed can be increased several-fold. In this study, we investigate the dependence on the interdigitated electrode structure of the electro-optical characteristics of IPS and FFS cells with zero rubbing angle. We found that when the rubbing angle is zero, the single-layered IPS electrode structure provides a higher transmittance than the double-layered FFS electrode structure because of the reduced width of dead zones at domain boundaries between interdigitated electrodes. Single-layered IPS electrodes not only minimize the transmittance decrease but also provide a shorter response time than double-layered FFS electrodes, although the operating voltage is higher and fabrication requires a more precise rubbing process. The transmittance decrease due to the zero rubbing angle in an IPS cell can be minimized using optimization of the electrode structure while retaining a short response time.

Effect of two-dimensional confinement on switching of vertically aligned liquid crystals by an in-plane electric field

We investigated the two-dimensional (2-D) confinement effect of liquid crystals (LCs) on the switching of vertically aligned LCs by an in-plane electric field. When an in-plane field is applied to a vertical alignment (VA) cell, virtual walls are built at the center of the interdigitated electrodes and at the middle of the gaps between them. The LC molecules are confined not only by the two substrates but also by the virtual walls so that the turn-off time of a VA cell driven by an in-plane field is dependent on the pitch of the interdigitated electrodes as well as the cell gap. Therefore, the turn-off time of a VA cell driven by an in-plane field can be reduced simply by decreasing the pitch of the interdigitated electrodes as a result of the enhanced anchoring provided by the virtual walls. The experimental results showed good agreement with a simple model based on the 2-D confinement effect of LCs.

Study on Arrangement of Self-Resonant Coils in Wireless Power Transfer System Based on Magnetic Resonance

본 논문에서는 자기 공명 방식의 무선 전력 전송 시스템에서 공진 코일의 배열에 따른 특성을 제시한다. 공진 코일에는 헬리컬 구조를 사용하였다. 공진 코일을 설계하기 위해 헬리컬 코일의 인덕턴스와 커패시턴스를 구하였다. 유한 요소법을 이용하여 설계된 공진 코일을 시뮬레이션 하였고, 송수전 공진 코일 간의 다양...In this paper, characteristics on arrangement of coils in the wireless power transfer system based on magnetic resonance is presented. A helical structure is used for a self-resonant coil. To design a proper self-resonant helical coil, its inductance and capacitance are obtained. Using the finite element method, the self-resonant coil designed is simulated and characteristics of wireless power transfer with various arrangement between Tx and Rx resonant coils is analyzed. For verification, a prototype of a wireless power transfer system based on magnetic resonance is fabricated and efficiency of different arrangement such as both vertical and parallel arrangements is measured. From the measurement, transmission efficiency of 50 % for parallel arrangement is obtained within twice the diameter of the coil while for the vertical arrangement it is measured within one and a half diameter of the coil. Maximum efficiency of 84.25 % is observed at the distance 40 cm from the resonant coil in the case of parallel arrangement.

Cell Gap Effects on Electro-Optic Performance of a Polymer-Stabilized Liquid Crystal Cell

We report cell gap effects on the electro-optic characteristics of a polymer-stabilized liquid crystal (LC) cell. We found that the transmittance of a polymer-stabilized LC cell can be increased by increasing the cell gap with little increase in response time. To better understand the cell gap effect on response time of the polymer-stabilized LC cell, we employed a simple method to evaluate and predict the response time by extracting the average distance between two neighboring bundles through experiment. The experimental results related to the response time of a polymer-stabilized LC cell matche well with the calculated results. We experimentally obtained turn-off switching that was three times faster than that of a conventional fringe-field switching cell at room temperature. Even at -20 °C, we achieved a relatively fast turn-off time of 18 ms.

Fast Control of Haze Value Using Electrically Switchable Diffraction in a Fringe-Field Switching Liquid Crystal Device

In this paper, we present a diffractive liquid crystal (LC) device capable of rapid switching between the transparent and translucent states for window display applications. In contrast to previously reported LC light shutters based on light scattering, the proposed LC device relies on diffraction of white incident light by an electric field-induced periodic continuous LC profile. It can be switched between the transparent and translucent states without a complicated driving scheme or a polymer structure. This device exhibits outstanding features for window display applications, such as a high transparency and a wide viewing angle in the transparent state, a low operating voltage, and a short response time.

Fabrication of flexible light shutter using liquid crystals with polymer structure

ABSTRACT We fabricated a light shutter using plastic substrates for high visibility of a flexible see-through display. To achieve a flexible light shutter using liquid crystals (LC), it is essential to maintain the cell gap when the light shutter is bent. We studied methods to fabricate flexible LC light shutters using plastic substrates. We demonstrated light shutters that are initially transparent and flexible with or without polymer walls. We have elucidated that polymer walls and networks provide mechanical stability against the bending of an LC light shutter without any degradation in the electro-optic characteristics. We predict that a flexible light shutter provides not only high visibility but also mechanical stability to a flexible see-through display by positioning it at the back of a flexible see-through display panel. Graphical Abstract

Fast switching of vertically aligned nematic liquid crystals by two-dimensional confinement with virtual walls

We introduce a simple method for fast switching of vertically-aligned nematic liquid crystals (LCs). When an electric field is applied to a patterned vertical alignment (PVA) LC cell, virtual walls are formed in the middle of the gaps between and at the center of the patterned electrodes. These virtual walls formed in a PVA cell results in the turn-off time being dependent on the pitch of the patterned electrodes as well as the cell gap. Therefore, a short response time can be achieved by fine patterning of pixel electrodes without requiring additional fabrication steps or complicated drive schemes. A similar behavior has been observed in switching of vertically-aligned LCs with positive dielectric anisotropy by an in-plane electric field.

Fast in-plane switching of nematic liquid crystals by two-dimensional confinement with virtual walls

We introduce a method for achieving a short response time in homogeneously aligned liquid crystal cells by twodimensional confinement of LCs with virtual walls. When an electric field is applied to in-plane switching (IPS) and fringe-field switching (FFS) cells with interdigitated electrodes parallel to the LC alignment direction, virtual walls are built so that the switching speed can be increased several-fold. We also introduce an interdigitated pixel electrode structure with alternating tilts for a much wider viewing angle by aligning the LCs without a pretilt. In addition to a short response time and wide viewing angle, this device allows a much larger deviation of the LC alignment direction which is essential for mass production. Moreover, LCs with negative dielectric anisotropy can be used to minimize the transmittance decrease.