Ji-Ho Baek

Fast response time of fringe-field switching liquid crystal mode devices with reactive mesogens in a planar alignment layer

We report the response time of fringe-field switching (FFS) liquid crystal (LC) mode devices using an ultraviolet-curable reactive mesogen (RM) mixed in a planar alignment layer. The RM polymers within the alignment layer increase the surface azimuthal anchoring energy and the order parameter of the LC molecules. As a result, the falling time and the rising time in the FFS mode are improved by the enhanced azimuthal anchoring energy and the increased dielectric torque induced by higher order parameter of LCs.

Polarizer-free liquid crystal display with electrically switchable microlens array

We propose a polarizer-free liquid crystal display (LCD) with an electrically switchable microlens array. The incident lights are controlled to focused or defocused states by index matching of the lens polymer and LC layer. By adopting two light-blocking masks that have a circular stop pattern and the complementary open pattern, the LCD was able to realize the entire gray scale. Additionally, to achieve fast response time characteristics, we introduce polymerized RMs within the alignment layers.

Enhanced surface anchoring energy for the photo-alignment layer with reactive mesogens for fast response time of liquid crystal displays

We propose a method for enhancing the surface anchoring energy in photo-alignment (PA) through a two-step ultraviolet (UV) exposure process using reactive mesogens (RM) for fast response time characteristics of liquid crystal displays. We establish the azimuthal alignment direction through the first linearly polarized UV exposure process, as the conventional PA method does. We then achieve the stable alignment and strong surface anchoring energies of our method through the second unpolarized UV exposure process, which polymerizes the RM monomers within the vertical alignment layer. As a result, the fast response time characteristics are remarkably improved, and thermal stability is obtained.

Transient flickering behavior in fringe-field switching liquid crystal mode analyzed by positional asymmetric flexoelectric dynamics

We analyzed a transient blinking phenomenon in a fringe-field switching liquid crystal (LC) mode that occurred at the moment of frame change even in the optimized DC offset condition for minimum image flicker. Based on the positional dynamic behaviors of LCs by using a high-speed camera, we found that the transient blink is highly related to the asymmetric responses of the splay-bend transitions caused by the flexoelectric (FE) effect. To remove the transient blink, the elastic property adjustment of LCs was an effective solution because the FE switching dynamics between the splay-enhanced and bend-enhanced deformations are highly dependent on the elastic constants of LCs, which is the cause of momentary brightness drop.

Effects of surface modification with reactive mesogen on the anchoring strength of liquid crystals

We studied the effects of polymer chain ordering in the alignment layer and resulting molecular interactions on the surface anchoring energy by introducing a reactive mesogen (RM) coating to the alignment layer. Directionally polymerized RMs on the rubbed alignment layer decrease the steric repulsion and increase the electronic interaction with liquid crystal molecules, and, as a result, the surface anchoring energy is enhanced in both the out-of plane and in-plane directions. We also demonstrated that this enhanced surface anchoring energy can be used to improve the response time characteristics of liquid crystal displays.

Highly polarized emission of the liquid crystalline conjugated polymer by controlling the surface anchoring energy

We demonstrated a highly polarized organic light-emitting diode (OLED) through the enhancement of the orientational ordering of the emitting polymer with a nematic liquid crystalline (LC) phase. The highly ordered state of the conjugate polymer was obtained by thermal annealing at the nematic temperature and strong azimuthal anchoring energy of the underlying polyimide. The order parameter of the conjugate polymer was analyzed using a second-harmonic generation model and the dichroic ratio was measured to be 22 : 1. Also, we applied our optimized OLED with high optical polarizability to an effective light source for a twisted nematic LC display.

Effects of pretilt angle and anchoring energy on alignment of uniformly lying helix mode

ABSTRACT We investigated the effects of pretilt angle and anchoring energy on the formation of a uniformly lying helix (ULH) texture of cholesteric liquid crystals (CLCs). Pretilt angle was controlled by the thickness of a vertical alignment layer coated onto a planar alignment layer. In the given pretilt angle, the anchoring energy was enhanced by introducing reactive mesogen to the vertical alignment material. To characterise quantitatively the formation of the ULH texture we introduced reflectance, governed by areas of the ULH region and the planar-aligned CLC region. We found that the ULH texture was more widely formed under the condition of higher pretilt angle and weaker anchoring energy. Also, a more uniform alignment of the ULH texture was achieved with the higher pretilt angle even under the same anchoring energy condition. GRAPHICAL ABSTRACT

Low Cell Gap Polymeric Liquid Crystal Lens for 2-D/3-D Switchable Auto-Stereoscopic Display

We propose a planar polymeric liquid crystal (LC) lens with a low cell gap (~ 4.6 μm) for a 2-D/3-D switchable auto-stereoscopic display. The proposed lens consists of two LC layers, one is a photopolymerized LC lens layer with a non-uniform refractive index with parabolic curve distribution, and the other is a half-wave LC switching layer of the vertical alignment type, which performs the 2-D/3-D image switching. The optimized refractive index of the LC lens layer was simply realized by polymerizing the LC molecules using a reactive mesogen. The proposed LC lens allow fast 2-D/3-D switching with low voltage and a simple fabrication process because of a low cell gap of the switching layer. We verified the electro-optical characteristics of the proposed LC lens by fabricating each layer after optimizing the cell structure. The measured focal length as a function of the applied voltage in the 3-D mode was compared with the calculated focal length and we confirmed that the measured results agreed with the calculated results.

Optically isotropic switchable microlens arrays based on liquid crystal

We present an optically isotropic switchable microlens array (MLA) based on liquid crystals (LCs) using the Joule heating electrode structure. The LC molecules were initially aligned vertically on the lens and electrode surfaces. By applying voltage to the transparent electrodes, the temperature of the LC layer could be changed. Above the clearing point temperature of LCs, the LC layer shows an averaged refractive index that differs from the nematic state refractive index. The MLA could have switching characteristics by index matching between the LC layer and polymer lens structure. The proposed switchable MLA shows high light efficiency with truly optically isotropic properties.

Dependence of planar alignment layer upon enhancement of azimuthal anchoring energy by reactive mesogens

Reactive mesogens (RMs) can enhance the azimuthal anchoring energy of planar alignment layers used in liquid crystal (LC) devices; herein, we studied the interactions between the RMs and the planar alignment material that determine whether this enhancement can occur. Two alignment-layer materials were studied: polyamic acid (PA) and polyimide (PI). The addition of RMs to the PI-type alignment layer was effective in enhancing the azimuthal anchoring energy, whereas the addition of RMs to the PA-type alignment layer had little effect. Surface analysis revealed that the RMs adhered well to the PI-type alignment surface only; in the resulting cell, the presence of the RMs enhanced both the rise and decay times in fringe field switching (FFS)-mode operation.