Jun-Hee Na

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.

Physical mechanism for flat-to-lenticular lens conversion in homogeneous liquid crystal cell with periodically undulated electrode

A convertible lenticular liquid crystal (LC) lens architecture is demonstrated using an index-matched planarization layer on a periodically undulated electrode for the homogeneous alignment of an LC. It is found that the in-plane component of the electric field by the undulated electrode plays a primary role in the flat-to-lens effect while the out-of-plane component contributes to the anchoring enhancement of the LC molecules in the surface layer. Our LC device having an index-matched planarization layer on the undulated electrode is capable of achieving the electrical tunability from the flat surface to the lenticular lens suitable for 2D/3D convertible displays.

Liquid crystal-based square lens array with tunable focal length

We demonstrate a liquid crystal (LC)-based square lens array with two focusing modes according to the polarization state of the input light. The homogeneously aligned LC layer is placed on an array of static square lenses fabricated using a photo-curable polymer whose refractive index is matched with the refractive index of the LC. For the input beam polarized parallel to the easy axis of the LC, the focal length is varied with the applied voltage from a few meters to 21 mm which corresponds to the focal length of the static lens. For the perpendicularly polarized input beam, the focal length is independent of the applied voltage and remains constant. The two focusing effects with high optical performance over fully activated areas are useful for polarization-dependent imaging systems and three-dimensional displays in projection and integral imaging.

Self-Organized Anisotropic Wrinkling of Molecularly Aligned Liquid Crystalline Polymer

Anisotropic wrinkling which utilizes the anisotropic nature of liquid crystalline polymer (LCP) is demonstrated as a means of physical self-assembly to produce periodic microstructures. Through the plasma treatment on the molecularly aligned LCP film surface, one-dimensionally ordered wrinkle pattern was spontaneously formed on glass substrates without employing external thin-film deposition or prestrain control of the system. Experimental results indicate that the directionality of the wrinkle pattern can be tailored by the structural ordering of LCP molecules in the bilayer system of a hard skin layer on a soft substrate. Studies on process variables, such as the plasma treatment time and the film thickness, were conducted to figure out the effect on the wrinkling morphology. Due to its spatial periodicity over a large area and undemanding requirement of the process, this approach can be a candidate for the microfabrication in various applications.

Fully continuous liquid crystal diffraction grating with alternating semi-circular alignment by imprinting

We demonstrate a fully continuous liquid crystal (LC) grating device with the alternating semi-circular alignment which exhibits the switching effect between the diffraction orders independent of the thickness of the LC cell. The continuous phase modulation in the LC grating with the rotational symmetry was achieved on a micro-imprinted surface where the semi-circular alignment of the LC was spontaneously produced. Our LC grating device in the hybrid geometry exhibited the perfect continuity of the phase retardation and the switchable diffraction with the diffraction efficiency of 44% at ±1st orders as a function of an applied voltage. It was found that the symmetry of the input polarization direction with respect to the grating patterns results in the interchange between two symmetric grating configurations.

Realizing the concept of a scalable artificial iris with self-regulating capability by reversible photoreaction of spiropyran dyes.

We report a scalable artificial iris with self-regulating spoke patterns of a liquid crystalline polymer that resemble the iris frills of the human eye. The underlying concept relies primarily on the reversible photo-induced conformation change of spiropyran dyes embedded in a transparent polymer. The spoke patterns around the scalable pupil, defined by selective wetting inscription, were produced using reactive liquid crystalline mesogens on a flexible substrate through polymerization. The self-regulation capability of our artificial iris, not requiring driving circuits and peripheral sensors, plays a central role in developing a new class of biomedical and photonic devices in a monolithic architecture.

Tunable binary retarder using self-aligned liquid crystal on anisotropic polymer film by photo-assisted imprinting

We demonstrate an electrically tunable binary retarder (ETBR) with a self-aligned liquid crystal (LC) on an anisotropic polymer film produced by photo-assisted imprinting. The ETBR has two parts: a tunable optical layer of an LC and a static optical layer of an imprinted anisotropic polymer film possessing two different in-plane optic axes. The anisotropic polymer film was produced using reactive mesogens spontaneously aligned along the topographic microgrooves by imprinting under the exposure of ultraviolet light. An electrically tunable hybrid wave plate, whose phase retardation varies from a quarter to a half-wave, is constructed using the self-aligned LC layer on the imprinted polymer film that behaves as a quarter wave plate with two alternating optic axes. This approach can be used to design a new class of tunable optical devices with multiple in-plane optic axes.

Symmetric-viewing liquid crystal display with alternating alignment layers in an inverse-twisted-nematic configuration

A symmetric-viewing inverse-twisted-nematic (ITN) liquid crystal display (LCD) with alternating alignment layers was developed using a stamping-assisted rubbing (SAR) technique. A patterned layer of a fluorinated acrylate polymer was transferred onto the first rubbed vertical-alignment layer prepared on a substrate by stamping. The fluorinated acrylate polymer provided a protective layer covering the first rubbed alignment layer during the second rubbing process, which promoted the vertical alignment of the LC molecules. The LC cell in the ITN geometry with two orthogonally rubbed alignment layers showed symmetric-viewing characteristics with fourfold symmetry. The SAR technique was shown to be a mask-free alignment method of producing multidomains for symmetric-viewing LCDs.

A Mean-Field Photoreaction Model for the Pretilt Generation of a Liquid Crystal on Photopolymer Layers upon Ultraviolet Exposure

We present a mean-field photoreaction model for the pretilt generation of a nematic liquid crystal (NLC) on the surfaces of photopolymers upon the exposure of ultraviolet (UV) light. The angular distribution function of photopolymer side chains, calculated in the photoreaction model, is used for determining the surface anchoring energy within the Rapini–Papoular approximation. The pretilt angle transition from the homeotropic alignment to the planar alignment of the NLC is demonstrated in two photopolymers with different alkyl chain lengths as a function of the UV exposure time. The main features of the experimental results agree well with theoretical predictions.

Stamping-assisted Fabrication Technique of the Bidirectional Alignment Layer for Wide-Viewing Twisted-Nematic Liquid Crystal Displays

Abstract A stamping‐assisted rubbing technique for generating bidirectional alignment in the fabrication of wide‐viewing twistednematic (TN) liquid crystal displays (LCDs) was developed. A patterned layer of a fluorinated acrylate polymer was transferred onto the first rubbed alignment layer prepared on a substrate by stamping. The fluorinated acrylate polymer provides a protective layer that covers the first alignment layer during the second rubbing process to facilitate the bidirectional alignment of the LC molecules. The LC cell in the twisted geometry with two bidirectional‐alignment layers showed stable electro‐optic properties and wide‐viewing characteristics. The stamping‐assisted rubbing technique serves as a mask‐free alignment method of producing multidomains for wide‐viewing LCDs.