Chang-Jae Yu

Surface-controlled patterned vertical alignment mode with reactive mesogen

We proposed a patterned vertical alignment (PVA) mode controlled by a modified surface with ultraviolet (UV) curable reactive mesogen (RM) mixed with vertical alignment material for a liquid crystal display (LCD) with fast response time. In the surface-controlled PVA (SC-PVA) mode, the RM monomers in the alignment layer are polymerized along the LC directors by UV exposure under an applied voltage. The polymerized RMs produce a pretilt against the substrate normal depending on the applied field direction in the patterned electrode configuration. In such SC-PVA mode, fast response time was achieved at whole grey levels with the predetermined rotational preference of the LC directors governed by the pretilt direction.

Concept of a liquid-crystal polarization beamsplitter based on binary phase gratings

We propose a concept of fabricating a liquid-crystal (LC) polarization beamsplitter based on binary phase gratings. The binary grating phenomenon in the LC layer physically originates from two different interfacial interactions at periodically alternating domain boundaries. The periodic LC domains are produced by employing a single-step photoalignment technique which precisely controls the surface orientation of the LC molecules in an alternating homeotropic and hybrid aligned geometry. In this binary configuration, the polarization-separating phase modulation of an input beam is achieved in a wide range of the wavelengths under an applied voltage. The experimental data are consistent with our linearly graded binary model.

Polarization-insensitive liquid crystal Fresnel lens of dynamic focusing in an orthogonal binary configuration

We report on a polarization-insensitive liquid crystal (LC) Fresnel lens with the centrosymmetry of dynamic focusing in an orthogonally alternating hybrid configuration. The polarization-insensitive Fresnel lens having such successive orthogonal hybrid alignment in two adjacent zones was fabricated using a single-masking process through two-step exposure of a linearly polarized ultraviolet light. It was found that the dynamic focusing property of the LC Fresnel lens is electrically controllable and entirely centrosymmetrical irrespective of the polarization state of an incident light.

Multimode transflective liquid crystal display with a single cell gap using a self-masking process of photoalignment

We demonstrate a transflective liquid crystal display (LCD), doped with a chiral agent to produce a low helical twisting power, in a multimode configuration consisting of the homogeneous alignment and the hybrid alignment. The multimode transflective LCD was fabricated by a single-step exposure of the UV light through an array of metal reflectors used as an amplitude photomask which gives an alternating homogeneous and homeotropic LC geometry. This single-step UV exposure produces no cell gap variations. In our configuration, the electro-optical disparity between the transmissive region and the reflective region was found to be significantly reduced by the low helical twisting power of the chiral dopant.

Mechanical stability of a flexible ferroelectric liquid crystal display with a periodic array of columnar spacers

We report on a high-speed flexible display based on a deformed helix ferroelectric liquid crystal (DHFLC) in a vertically aligned configuration. The mechanical stability of the flexible DHFLC display was achieved using a periodic array of columnar spacers formed directly on the top side of in-plane electrodes by the photolithography technique. Several unique features of display performances such as flexibility, uniform alignment, fast response, and gray scale capability were obtained. The switching time was found to be a few hundred microseconds.

Polarization-invariant grating based on a photoaligned liquid crystal in an oppositely twisted binary configuration

We demonstrate a polarization-invariant liquid-crystal (LC) grating in an oppositely twisted and mutually orthogonal configuration. The polarization-invariant LC grating with high diffraction efficiency is fabricated using a single-masking process with two-step exposure of a linearly polarized ultraviolet light. It is found that the zeroth- and the first-order diffracted beams are linearly polarized and perpendicular to each other irrespective of the incident polarization for the phase retardation of m pi (m=integer).

Investigations of the polymer alignment, the nonradiative resonant energy transfer, and the photovoltaic response of poly(3-hexylthiophene)/TiO2 hybrid solar cells

We report the effects of annealing on the performance of hybrid photovoltaic (PV) cells containing poly(3-hexylthiophene) (P3HT) coated onto TiO2/Sn doped In2O3 (ITO) and ITO substrates. In the optimized device, which exhibits a higher efficiency, the backbone axes of the P3HT chains were found to lie within the substrate plane, their conjugated planes are slightly tilted, and their side chains are substantially tilted. The carboxylate group is attached via bidentate or bridging coordination to the TiO2 surface and enables photoinduced charge transfer between TiO2 and P3HT. The observed large quenching (with excitation at 488 nm) and enhanced emission (with excitation at 325 nm) indicates that efficient Forster resonance energy transfer occurs between TiO2 and P3HT. Thus, the main influences on the high efficiency of the hybrid PV cells are the photon-mediated electronic transition and the photoinduced charge transfer.

Fast vertical alignment mode with continuous multi-domains for a liquid crystal display

We proposed a fast liquid crystal display (LCD) in a vertical alignment (VA) mode with continuous multi-domains for wide-viewing characteristics. The fast VA LCD was fabricated by the mixed vertical alignment layer with UV curable reactive mesogen (RM) polymer memorizing the switching directions of the LC molecules. The wide-viewing and fast response characteristics were obtained by axially symmetric switching directions and the memorization of them without any specific electrode patterns or surface structures.

A Novel Technique for Optical Imaging of Liquid Crystal Alignment Layers

Abstract We have employed a phase modulation method using a photoelastic modulator to determine the uniformity and quality of the alignment layer of liquid crystals (LCs). It is demonstrated that a two-dimensional map of the phase retardation, associated with irregular rubbing and/or chemical contaminants, provides invaluable information about an early detection of defects involved for aligning the LC molecules on a substrate.

Design of binary diffraction gratings of liquid crystals in a linearly graded phase model

We report on a linearly graded phase model for the design of binary diffraction gratings of liquid crystals (LCs) associated with the periodic interfacial effect. The binary nature of the LC grating is produced by use of periodic striped domains in an alternating homeotropic and hybrid geometry. In our graded phase model the diffraction patterns and the diffracted intensities of the LC binary grating is primarily governed by three length scales: the cell thickness and two distortion parameters scaled by the grating period at two domain boundaries. The experimental data agree well with theoretical predictions made in our linearly graded phase model as well as the elastic continuum theory.