Sang-Hyeok Kim

Optical and electrical switching of cholesteric liquid crystals containing azo dye

We propose an optically and electrically switchable cholesteric liquid crystal (ChLC) cell doped with a push–pull azo dye. When the proposed ChLC cell is exposed to UV light, it is switched from the focal-conic to isotropic state by a cholesteric-isotropic phase transition through a trans–cis photo-isomerization of push–pull azo dye molecules. With removal of UV light, the ChLC cell will rapidly relax back to its initial state without exposure to a second light source that has a longer wavelength. We confirmed that the proposed optically and electrically switchable ChLC cell can be used as a smart window that can be switched on by applying sunlight or an electric field.

Fabrication of an initially-focal-conic cholesteric liquid crystal cell without polymer stabilization

Abstract A dye-doped cholesteric liquid crystal (LC) cell with the focal-conic initial state is demonstrated. Although dye-doped LC/polymer composites have been widely used for LC light shutters, dye-doped LC cells with polymer structures suffer from the degradation of dichroic dyes during the UV curing process. To avoid this problem, we propose a dye-doped cholesteric LC cell using vertical alignment layers instead of polymer structures. We have shown that the focal-conic initial state can be obtained by homeotropic anchoring without the polymer structure. The proposed device not only provides the black color but can also hide objects behind a display panel in the focal-conic initial state without the degradation of dichroic dyes.

Transmittance control of a liquid crystal device using a dye mixture

Transmittance-control devices, such as a suspended particle device, electrochromic device, and dye-doped liquid crystal (LC) device, have been studied for a smart window, eyewear, and automotive applications. These devices require a high transmittance difference between the transparent and opaque states. Among the dye-doped LC devices, a dye-doped chiral-nematic LC (CNLC) cell has been widely used for transmittance-control devices. However, the colors of cells are different between the homogeneously aligned and CNLC cell. In this study, we demonstrated a systematic approach to find optimum dye concentrations for black color in a dye-doped CNLC cell. We took its transmission spectrum into account in the numerical calculation to realize the black color in a dye-doped CNLC cell. Through the iterative method, we could optimize the concentration of each single dye for realizing the black color. We confirmed that a dye-doped CNLC cell designed by considering transmission spectrum of it could provide the black color in the CIE 1931 color space.

Thermal control of transmission property by phase transition in cholesteric liquid crystals

Cholesteric liquid crystals (LCs) responsive to external stimuli, such as temperature or light, can be used for color switching or tuning through the change in the cholesteric pitch. This paper presents the thermal control of the transmission property via the phase transition of cholesteric LCs without any change in the cholesteric pitch. We used the chiral smectic A, chiral nematic, and isotropic phases as the transparent, high-haze opaque, and haze-free opaque states of a dye-doped cholesteric LC cell. We showed that the proposed device could be used as a temperature sensor or self-shading smart window for energy saving.

Smart window using a thermally and optically switchable liquid crystal cell

Light shutter technologies that can control optical transparency have been studied extensively for developing curtain-free smart windows. We introduce thermally and optically switchable light shutters using LCs doped with push-pull azobenzene, which is known to speed up thermal relaxation. The liquid crystal light shutter can be switched between translucent and transparent states or transparent and opaque states by phase transition through changing temperature or photo-isomerization of doped azobenzene. The liquid crystal light shutter can be used for privacy windows with an initial translucent state or energy-saving windows with an initial transparent state.

Cholesteric liquid crystal cell with the focal-conic initial state

Dye-doped liquid crystal (LC) light shutters with polymer structures suffer from the degradation of dichroic dyes during the UV curing process. To avoid this problem, we propose a dye-doped cholesteric LC light shutter using vertical alignment layers instead of polymer structures for the focal-conic initial state.

Smart windows using polymer-networked liquid crystals doped with push-pull azobenzene

We propose a smart window using polymer-networked liquid crystals doped with push-pull azobenzene. Azobenzene is used to provide phase transition from the nematic to isotropic state through the trans-cis isomerization of azobenzene. When exposed to sunlight, the device switches from the opaque nematic phase to the transparent isotropic phase. Switching from the transparent to opaque state can be obtained through rapid cis-trans isomerization of push-pull azobenzene without sunlight exposure. The proposed device can reduce the transmittance of the incident sunlight during daytime, whereas it can scatter the incident light during the night for privacy.