Dong Han Song

Fast switching of long-pitch cholesteric liquid crystal device

We propose a long-pitch cholesteric liquid crystal (ChLC) device capable of operation in both the dynamic mode and the memory mode. Fast switching between the homeotropic state and the focal conic state allows the display of moving pictures at a low operating voltage. In addition, we can write text messages on the proposed ChLC device by applying an external pressure locally to switch it from the focal conic state to the planar state.

Switching of liquid-crystal devices between reflective and transmissive modes using long-pitch cholesteric liquid crystals

We propose liquid-crystal (LC) devices capable of switching between reflective and transmissive modes using the scattering and transparent states of long-pitch cholesteric LCs (CLCs). Two different device configurations can be realized by changing the location of a CLC layer. Low-power operation without the parallax problem can be achieved using the bistable switching of CLCs. We believe that the proposed devices are potential candidates for highly efficient transflective displays.

Ultrafast switching of randomly-aligned nematic liquid crystals.

We propose an ultrafast nematic liquid crystal (LC) device without alignment layers, where both the dark and bright states can be realized by applying an electric field. A vertical electric field is applied to vertically align the LCs for the dark state, whereas an in-plane electric field is applied to homogeneously align the LCs for the bright state. We achieved a total response time of less than 3 ms in the proposed device. This device may contribute, not only to a significant improvement of the switching speed in liquid crystal devices, but also to the simplification of the device fabrication by the omission of the alignment layer coating and the rubbing process.

Formation of Polymer Networks for Fast In-Plane Switching of Liquid Crystals at Low Temperatures

We formed a polymer structure to enable fast in-plane switching of liquid crystals at low temperatures. The problem of the inevitable slow response at low temperatures was reduced by the formation of in-cell polymer networks in in-plane switching (IPS) cells. The electro-optic characteristics of polymer-networked IPS cells were measured at temperatures ranging from -10 to 20 °C. The turn-on and turn-off times of an IPS cell were reduced by 44.5 and 47.2% at -10 °C by the formation of polymer networks. We believe that the proposed technology can be applied to emerging display devices such as mobile phones and automotive displays that may be used at low temperatures.

Pixel electrode structure for high transmittance in a multi-domain vertical alignment liquid crystal display device

We propose a pixel electrode structure for the multi-domain vertical alignment liquid crystal mode with high optical efficiency. We form a micro-slit electrode structure on the top substrate to reduce the width of disclination lines at domain boundaries. We insert additional electrodes on the bottom substrate to improve the transmittance at the edges of each pixel. We confirm that using both structures, the transmittance can be increased from 22.9% to 27.3% with no change in the operating voltage.

Bistable switching of twist direction in a twisted-nematic liquid crystal cell

We propose a bistable liquid crystal mode based on switching of the twist direction in a conventional π/2-twisted nematic cell. Switching between the −π/2 and +π/2 twist states can be performed by applying vertical and/or in-plane electric fields. The proposed bistable mode has an infinite memory time because the two stable twist states have the same elastic free energy.

High-transmittance Multi-domain Vertical Alignment Liquid Crystal Device with Protrusion Structure

We propose a high-transmittance multi-domain vertical alignment liquid crystal device with a protrusion structure. Disclination lines, which inevitably appear at the boundaries of domains in a multi-domain structure, can be reduced by adding a protrusion structure on the top substrate. The transmittance was improved by 11% using the proposed structure with no change of either the dark state or the operating voltage.

Fast Fringe-field-switching Liquid Crystal Cell with a Protrusion Structure

We propose a cell structure for the fast switching of liquid crystals by the fringe field. By forming protrusion between patterned electrodes, we can obtain enhancement of response time. The protrusion reduces the effective cell gap, with which a fast response time can be realized. There is little decrease in the transmittance because the protrusion is located between patterned electrodes. We confirmed that the total response time can be decreased by 33% with little loss of the transmittance by optimizing the shape of the protrusion structure.

Multi-Dimensional Liquid Crystal Alignment Effect of Polymer Wall on Vertically Aligned Liquid Crystal Cell

This paper investigates the multi-dimensional liquid crystal (LC) alignment effect of a polymer wall. The polymer wall is formed on the pixel boundary by an anisotropic phase separation method, which affects the nearby LC orientation. Vertically aligned (VA) LC cells are fabricated in order to show the effect of the polymer wall on LC displays. The electro-optic characteristics of the LC cell are affected by the shape and orientation of the molecules that form the polymer wall. The VA LC cell that has a polymer wall formed by vertically-aligned anisotropic-molecules on the pixel boundary shows much faster turn-on and turn-off times than the conventional LC cell without polymer wall, as well as no loss of transmittance within its pixel area.

Liquid Crystal Alignment on Ion-Beam-Treated Polyimide with a Long Alkyl Side Chain: Near Edge X-ray Absorption Fine Structure Spectroscopy Analysis

Liquid crystal alignment on ion-beam-treated polyimides with a long alkyl side chain was investigated using near edge X-ray absorption fine structure (NEXAFS) spectroscopy. The long alkyl side chains and the asymmetric distribution and orientational order of the pi-bonds of the polyimide surface can be determined by analyzing the angular dependent resonance intensities of the NEXAFS measurements. Herein, we demonstrate that the pretilt angle of the LC cell made by our method decreases as more long alkyl side chains are destroyed. Additionally, the tilt direction of the LC molecules can be determined from the asymmetric distribution of pi-bonds of the polyimide created by the ion beam irradiation.