Ji-Sub Park

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.

Efficient exciton generation in atomic passivated CdSe/ZnS quantum dots light-emitting devices

We demonstrate the first-ever surface modification of green CdSe/ZnS quantum dots (QDs) using bromide anions (Br-) in cetyl trimethylammonium bromide (CTAB). The Br- ions reduced the interparticle spacing between the QDs and induced an effective charge balance in QD light-emitting devices (QLEDs). The fabricated QLEDs exhibited efficient charge injection because of the reduced emission quenching effect and their enhanced thin film morphology. As a result, they exhibited a maximum luminance of 71,000 cd/m2 and an external current efficiency of 6.4 cd/A, both significantly better than those of their counterparts with oleic acid surface ligands. In addition, the lifetime of the Br- treated QD based QLEDs is significantly improved due to ionic passivation at the QDs surface.

Multi-spatial-frequency and phase-shifting profilometry using a liquid crystal phase modulator

Optical profilometry is widely applied for measuring the morphology of objects by projecting predetermined patterns on them. In this technique, the compact size is one of the interesting issues for practical applications. The generation of pattern by the interference of coherent light sources has a potential to reduce the dimension of the illumination part. Moreover, this method can make fine patterns without projection optics, and the illumination part is free of restriction from the numerical aperture of the projection optics. In this paper, a phase-shifting profilometry is implemented by using a single liquid crystal (LC) cell. The LC phase modulator is designed to generate the interference patterns with several different spatial frequencies by changing selection of the spacing between the micro-pinholes. We manufactured the LC phase modulator and calibrated it by measuring the phase modulation amount depending on an applied voltage. Our optical profilometry using the single LC cell can generate multi-spatial frequency patterns as well as four steps of the phase-shifted patterns. This method can be implemented compactly, and the reconstructed depth profile is obtained without a phase-unwrapping algorithm.

Design and Fabrication of Multi-Focusing Microlens Array with Different Numerical Apertures by using Thermal Reflow Method

We present design and fabrication of a multi-focusing microlens array (MLA) using a thermal reflow method. To obtain multi-focusing properties with different numerical apertures at the elemental lens of the MLA, double-cylinder photoresist (PR) structures with different diameters were made within the guiding pattern with both photolithographic and partial developing processes. Due to the base PR layer supporting the thermal reflow process and the guiding structure, the thermally reflowed PR structure had different radii of curvatures with lens shapes that could be precisely modeled by the initial volume of the double-cylinder PR structures. Using the PR template, the hexagonally packed multi-focusing MLA was made via the replica molding method, which showed four different focal lengths of 0.9 mm, 1.1 mm, 1.6 mm, and 2.5 mm, and four different numerical apertures of 0.1799, 0.2783, 0.3973, and 0.4775.

Enhanced Adhesion and Transmittance Uniformity in Laminated Polymer-Dispersed Liquid Crystal Films

We propose a two-step UV irradiation procedure to fabricate polymer-dispersed liquid crystal (PDLC) films by lamination. During the first UV treatment, before lamination, the UV-curable monomers coated on one film substrate are solidified through photo-polymerization as the phase separation between the liquid crystals and the monomers. Introducing an adhesion-enhancement layer on the other plastic substrate and controlling the UV irradiation conditions ensure that UV-induced cross-linkable functional groups remain on the surfaces of the photo-polymerized layers. Thereby, the adhesion stability between the top and bottom films is much improved during a second (post-lamination) UV treatment by further UV-induced cross-linking at the interface. Because the adhesion-enhancement and PDLC layers prepared by the bar-coating process are solidified before lamination, the PDLC droplet distribution and the cell gap between the two plastic substrates remain uniform under the lamination pressure. This ensures that the voltage-controlled light transmittance is uniform across the entire sample.

Pixel Design Optimization of Fringe-Field Switching Mode for Applying Negative Liquid Crystals in High-Resolution Mobile Displays

We demonstrate an optimized fringe-field switching pixel structure to apply negative liquid crystals (nLCs) for high-resolution mobile displays. Pixel transmittance can be improved by applying nLCs, but the driving voltage increases because Δε of the nLC is lower than that of the positive LC (pLC). In spite of applying an nLC, a similar driving voltage can be achieved by changing the pixel structure. It is easy to obtain a low driving voltage in a strong electric field by applying a thinner passivation layer using photo acryl (PAC). In addition, while the edge electrodes of the conventional Vcom-on-top (VOT) structure are operated with in-plane switching, all electrodes of the pixel-on-top (POT) structure are operated with fringe-field switching. Therefore, the driving voltage can be reduced by applying a POT structure with PAC. Furthermore, crosstalk can be improved because the capacitance (Cdp) between data lines and pixel electrodes is reduced by applying a POT structure. As a result, applying the proposed structure with nLC has two advantages. First, transmittance increases by more than 20% based on 4.5-in-high definition pixels. Second, crosstalk in the POT structure is more stable than that in the VOT structure, despite variations in low gate voltage (VGL). Consequently, because of higher transmittance and stable crosstalk, the proposed POT structure with nLC can be used for mobile displays.

Mixed-field-switching liquid crystal mode using in-plane and fringe fields self-adjusted by bottom floating electrode for transmittance enhancement

We demonstrate a liquid crystal (LC) mode switched by mixed electric fields of in-plane and fringe fields, which are self-adjusted by adopting a bottom floating electrode for enhanced electro-optical properties. In our LC mode structure, conventional in-plane switching (IPS) electrodes are formed as pixel electrodes and common electrodes on an insulating layer and floating electrodes that are patterned per the sub-pixels. When the areas of the pixel and common electrodes are identical, the voltage of the bottom floating electrode is spontaneously determined to be half the value of the pixel voltage, which ideally generates symmetric fringe fields with both pixel and common electrodes. Due to the in-plane fields additionally generated between the pixel and common electrodes, the proposed LC structure operates by mixed-field switching (MFS), which shows higher transmittance than fringe-field switching (FFS) and IPS LC modes. Transmittance of the conventional FFS and IPS LC modes is highly sensitive to the in-plane electrode’s width (w) and spacing (l) condition, but the proposed MFS LC mode shows good transmittance without degradation with large variations of the in-plane electrode’s spacing-to-width ratio (l/w).

Liquid crystal anchoring utilizing surface topological effects of self-structured dual-groove patterns

We demonstrate topologically patterned dual-groove surfaces for liquid crystal (LC) surface anchoring, where the dual-groove structure is made by a replica-moulding method from a self-structured poly(dimethylsiloxane) (PDMS) surface. In our method, the micro-groove structure with a periodicity on the order of micrometres is self-structured on the oxidized PDMS mould surface having a photolithographically defined macro-groove pattern owing to a thermally induced anisotropic stress effect. The direction of the micro-groove is determined to be perpendicular to the macro-groove direction and to form a mutually orthogonal dual-groove structure. With the presented method, the relative azimuth anchoring strength ratio (g) between the macro-groove and the micro-groove can be controlled precisely and easily. We investigated the LC anchoring effect created by the self-structured dual-groove pattern based on g, where the monostable LC anchoring surface is provided by the dual-groove surface with a g value close to 0 or infinity, and bistable LC anchoring is promoted as g approaches 1.

Vertical alignment of liquid crystals by ordering effect of selfassembled monolayers on the Ion‐beam‐irradiated anisotropic surface

Abstract In this paper, vertically aligned (VA) liquid crystal (LC) modes were investigated using the alkyl chain ordering effect of self‐assembled monolayers (SAMs) prepared on the anisotropic inorganic surface. On the anisotropic surface prepared through oblique ion beam irradiations, the SAM molecules are adsorbed, producing macroscopic alkyl chain ordering, which can determine the pretilt direction of the vertically aligned LC molecules through the intermolecular interactions on the surface.

Optical measurement of flexoelectric polarisation change in liquid crystals doped with bent-core molecules using hybrid-aligned structure

ABSTRACT We proposed an optical measurement method for determination of flexoelectric polarisation change in liquid crystals (LCs), which can be induced in highly distorted LC geometries. A hybrid-aligned nematic LC (NLC) mode was introduced to evaluate the direction and magnitude of the flexoelectric polarisation. We measured the DC offset amounts for equivalent brightness levels between forward and reverse bias vertical electric fields to estimate the sign and magnitude of es−eb of flexoelectric coefficients. Additionally, the optical incident angle (αmax) for the maximum effective birefringence was investigated to predict the depth distribution of the LC director affecting the magnitude of the flexoelectric polarisation. The relationship between the variations of the DC offset and αmax by the flexoelectric polarisation changes was examined using the NLC mixtures doped with three selected bent-core LCs. Graphical Abstract