Seungbin Yang

Improvement of the relaxation time and the order parameter of nematic liquid crystal using a hybrid alignment mixture of carbon nanotube and polyimide

We examined the electrooptical properties of a nematic liquid crystal (LC) sample whose substrates were coated with a mixture of carbon nanotube (CNT) and polyimide (PI). The relaxation time of the sample coated with 1.5 wt. % CNT mixture was about 35% reduced compared to the pure polyimide sample. The elastic constant and the order parameter of the CNT-mixture sample were increased and the fast relaxation of LC could be approximated to the mean-field theory. We found the CNT-mixed polyimide formed more smooth surface than the pure PI from atomic force microscopy images, indicating the increased order parameter is related to the smooth surface topology of the CNT-polyimide mixture.

Multifunctional Optical Thin Films Fabricated by the Photopolymerization of Uniaxially Oriented Lyotropic Liquid Crystal Monomers for Electro-Optical Devices

A multifunctional optical thin film (MOTF) is fabricated by coating the newly synthesized perylene-based reactive mesogen (PBRM) and stabilized by the subsequent photopolymerization. Based on the spectroscopic results combined with morphological observations, it is found that nematic liquid crystal (NLC) is aligned parallel to the molecular long axis of PBRM not only due to the long-range physical anchoring effect but also due to the short-range molecular physical interactions between alignment layer and NLC molecules. From the electro-optical properties of LC test cells fabricated with the PBRM MOTF, it is clearly demonstrated that the PBRM MOTF can work as the planar LC alignment layer as well as the in-cell coatable polarizer. The coatable PBRM MOTF from lyotropic chromonic reactive mesogens can pave a new way for the flexible optoelectronic devices.

Single layer retarder with negative dispersion of birefringence and wide field-of-view

A single layer retarder possessing negative dispersion (ND) of birefringence as well as wide field-of-view (FOV) was long-term objective in optical science. We synthesized new guest reactive monomers with x-shape and mixed them with the host smectic reactive mesogen. The host-guest molecules formed two dimensionally self-organized nanostructure and showed both the ND of birefringence and wide FOV properties. We simulated the antireflection property of a circular polarizer using the optical properties of the retarder. The average reflectance of the retarder was 0.52% which was much smaller than that of the commercial single layer ND retarder 1.83%.

Negative dispersion retarder with a wide viewing angle made by stacking reactive mesogen on a polymethylmethacrylate film

Abstract. We experimentally demonstrated a retarder with a wide field-of-view as well as a negative dispersion (ND) of birefringence. We stacked reactive mesogen, which has a positive birefringence, and polymethylmethacrylate, which has a negative birefringence. The out-of-plane retardation (Rth) of the stacked film was 6 nm. This value is about a tenth of that of a general retarder and demonstrates a small change in the retardation (Re) at an oblique viewing angle. The stacked film showed an ND of the birefringence, where Re(450  nm)/Re(550  nm)=0.818 and Re(650  nm)/Re(550  nm)=1.110. The dispersion properties were close to the ideal dispersion of an achromatic retarder and showed a small Re-dependence on the wavelength of light.

Negative dispersion retarder using two negative birefringence films

The achromatic response and wide viewing angle for varying wavelength of incident light are of long waiting research to be utilized it for the display devices. Such response can be obtained by employing the retarder that exhibits negative birefringence and negative dispersion. In this paper, negative dispersion half-wave retarder and negative dispersion quarter-wave retarder have been demonstrated by optimizing the retardation and the angle between the extraordinary axes of polystyrene and poly-methylmethacrylate films. The optimum angles for half and quarter-wave retarders were found to be 40° and 70°, respectively for different retardation values of polystyrene and poly-methylmethacrylate films.

Electro-optical properties of a nematic liquid crystal aligned with a mixture of nanofibres and polyimide

We examined the electro-optical properties of a nematic liquid crystal sample whose substrates were coated with a mixture of nanofibres (NFs) and polyimide (PI). The samples coated with a NF–PI mixture showed a faster turn-off time than the sample coated with pure PI. We found that the elastic constant and the order parameter of the NF–PI samples were greater than those of the pure PI sample, thus inducing the reduction in turn-off time. The NF–PI mixture-coated surface showed a more homogeneous striped surface topology along the rubbing direction compared to the pure PI surface. The uniform topological anisotropy of the surface seems to be related to the increase in the elastic constant and the order parameter of the NF–PI mixture sample.

Negative dispersion of birefringence of smectic liquid crystal-polymer composite: dependence on the constituent molecules and temperature

We investigated the dependence of the negative dispersion of birefringence of smectic liquid crystal-polymer composites on the constituent molecules and temperature. The dispersion of birefringence was significantly varied from positive dispersion to negative dispersion by the change of the relative fraction of the constituent monomers. For the temperature dependence of the dispersion, a composite with more fraction of monomers located at the inter-layer space showed a wider temperature range of the negative dispersion of birefringence.

Viewing angle compensation of vertical alignment liquid crystal display using a triphenylene-based discotic reactive mesogen

We synthesised a triphenylene-based discotic reactive mesogen 2,3,6,7,10,11-hexakis(but-3-enyloxy)triphenylene (HABET) and investigated its performance as the compensation film for a vertical alignment-liquid crystal display (VA-LCD). The HABET molecule has a negative birefringence with an extraordinary refractive index (ne) that is smaller than the ordinary refractive index (no). The optic axis of the HABET is parallel to the ne direction and was vertically aligned to function as a negative C-plate. We measured the refractive indices and birefringence of the HABET layer in the visible wavelength range. Using the experimental data, we simulated the viewing angle property of the VA-LCD with the HABET negative C-plate and a positive A-plate. The viewing angle property of the VA-LCD with the HABET compensation film was wider than those without the compensation film and those using triacetylcellulose films.

Optical Simulation of Viewing Angle Property of Biaxial Nematic Bent-Core Liquid Crystal

The conventional liquid crystal displays (LCDs) have been using optically uniaxial liquid crystal (LC) medium and this often causes a significant image change at oblique viewing angles. We simulated the viewing angle properties of the biaxial nematic (Nb) phase of a bent-core liquid crystal (BLC) and compared the results with the vertically aligned (VA)- and the in-plane switching (IPS)-LCDs. The Nb phase of the BLC showed a smaller transmittance at the dark state as well as a greater contrast ratio at wide viewing angle. The viewing angle property of the Nb-mode without any compensation film was slightly superior to the IPS-mode with the compensation films eliminating the decross of the polarizers at oblique viewing angle.