Tae-Hoon Yoon

Nontwist quarter-wave liquid-crystal cell for a high-contrast reflective display

We propose an optical configuration of a nontwist quarter-wave liquid-crystal cell for a high-contrast reflective display that can be applied to most nontwist display modes. By fabricating a homogeneous cell with the proposed configuration, we have demonstrated a high contrast ratio of 50:1 without using a wideband retardation film.

A Multimode-Type Transflective Liquid Crystal Display Using the Hybrid-Aligned Nematic and Parallel-Rubbed Vertically Aligned Modes

We propose a transflective liquid crystal display (LCD) of the multimode type, which is composed of the hybrid-aligned nematic (HAN) mode in the reflective region and the parallel rubbed vertically aligned (PRVA) mode in the transmissive region. An optimized optical configuration is also proposed. In order to make the reflective and transmissive regions in one pixel independent, an Ar+ ion-beam alignment method was used. We confirmed that the transflective LCD of the proposed multimode type had high reflectance and transmittance, wide-viewing-angle property, and high contrast ratio.

Vertical alignment of liquid crystal on a-SiOx thin film using the ion beam exposure

In this letter we demonstrate the vertical alignment of liquid crystal on inorganic thin film surfaces using the ion beam exposure. Nematic liquid crystal can be aligned vertically by the rotational oblique evaporation of a-SiOx thin films. However, the electro-optic switching behavior of liquid crystal along random directions results in disclination lines. By using the ion beam exposure, we can achieve highly uniform alignment without disclination lines. We found from x-ray diffraction and x-ray photoemission spectroscopy data that the vertical alignment can be achieved when x approaches 1.5 at the a-SiOx film surface. We have shown that the pretilt angle can be controlled by changing ion beam parameters, such as the ion beam energy, the angle of incidence, and the exposure time. We also have shown that a liquid crystal cell aligned vertically by the ion beam exposure exhibits the voltage-transmittance curve similar to that of a rubbed polyimide cell.

Fast In-Plane Switching of a Liquid Crystal Cell Triggered by a Vertical Electric Field

We propose a fast two-step switching method of a homogeneous-aligned liquid crystal (LC) cell, where a trigger pulse is applied to align LCs vertically for a moment before they are in-plane switched to show a bright state. Because LCs aligned vertically by a trigger pulse are in a transient state rather than the stable state, the turn-on becomes faster. The turn-off also becomes faster because the pulse forcibly aligns the LC vertically to show the dark state. Experimental results show that significant acceleration of the grey-to-grey response as well as on-off switching can be achieved by the proposed switching method.

Long-pitch cholesteric liquid crystal cell for switchable achromatic reflection.

We propose a switchable achromatic reflector using a long-pitch cholesteric liquid crystal (CLC) whose Bragg reflection wavelength is chosen to be infrared by controlling the pitch of the CLC so that the planar texture is transparent over the entire visible wavelength. By using the light scattering of the focal conic texture, achromatic reflection can be achieved. Both textures are stable at zero electric field and the operating voltage of the proposed CLC device is much lower than that of conventional CLC devices. The proposed switchable reflector, which can be operated at a low voltage with low power, can be applied to reflective displays and to light shutters. By coupling with a reflective polarizer the efficiency of light scattering at the focal conic texture can be enhanced.

Bistable chiral-splay nematic liquid crystal device using horizontal switching

The bistability in a chiral-splay nematic liquid crystal cell, which is obtained by adding a chiral additive to a splay cell, is proposed. In this letter, we describe a horizontal way of switching between the two states of the bistable chiral-splay cell, one state being characterized by a nontwisted director field with splay, the second state showing a director-field with 180° twist. The optimization of the various parameters of the device is described.

Alignment of liquid crystal on a polyimide surface exposed to an Ar ion beam

In this paper properties of nematic liquid crystals aligned on polyimide surfaces exposed to a low-energy Ar ion beam are investigated. We studied how the pretilt angle of a liquid crystal cell is affected by ion-beam conditions, such as the energy of the incident ions, the angle of incidence, and the exposure time. X-ray photoelectron spectroscopy data indicate that ion-beam exposure changes the chemical bonding states of the polyimide surface, which may be one of the main causes of anisotropic liquid crystal alignment. We also found that twisted-nematic cells fabricated by the ion-beam alignment method have the voltage holding time enough for application to thin-film-transistor liquid crystal displays.

Light shutter using dichroic-dye-doped long-pitch cholesteric liquid crystals

We propose a light shutter device using dichroic-dye-doped liquid crystals (LCs) whose Bragg reflection wavelength is set to be infrared by controlling the pitch of cholesteric liquid crystals (ChLCs). A dye-doped long-pitch ChLC cell is switchable between the dark planar state and the transparent homeotropic state. It has the advantages of high transmittance, low operation voltage, and an easy fabrication process relative to previous LC light shutter devices. The proposed light shutter device is expected to achieve high visibility for transparent organic light-emitting diode displays and emerging smart windows, which can be used in airplanes, cars, and other similar applications.

Tristate liquid crystal display with memory and dynamic operating modes

So far, monostable and bistable liquid crystal display (LCD) modes have independently been researched and developed. The authors propose a tristate LCD that can be operated in memory mode as well as dynamic mode. The proposed device has a unique texture of splay, π twist, and bend states with applied voltages and is operated in memory mode or dynamic mode by selective switching of two states among them. They demonstrate switching characteristics of the tristate LCD. This tristate LCD has a long term memory time for memory mode as well as a fast response time for dynamic mode.

A Novel Transflective Liquid Crystal Display with a Periodically Patterned Electrode

We propose a novel transflective liquid crystal display (LCD) with a periodically patterned electrode on a substrate. By optimizing the pattern size of the electrodes, the retardations which are required in reflective and transmissive modes are periodically achieved by the same voltage. The region with a small retardation is used as reflective mode and the region with a large retardation as transmissive mode. In the proposed transflective LCD, we can expect a very simple manufacturing process and a high optical performance as complex process such as a multi-cell gap process and multi driving method are not applied.