10.1117/2.1201012.003411 Optically isotropic liquid crystals for next-generation displays Shin-Tson Wu and Jin Yan Optically isotropic liquid crystals have seen recent progress toward reducing operating voltage and suppressing hysteresis. LCDs have become indispensable in our daily lives. They are used everywhere, such as in cell phones, computers, television sets, and data projectors. Based on continuous innovation, their overall display quality has reached a satisfactory level. Viewing- angle restrictions are no longer an issue following implementa- tion of multidomain structures and phase-compensation films, their contrast ratio has exceeded one million to one through local LED-backlight dimming, and sunlight washout has been over- come by employing transflective displays. LCD technology is, thus, relatively mature. However, some challenges still remain, including their relatively slow response time. This causes image blur for fast-moving objects and holds back adoption of color- sequential displays, which promise to triple light efficiency and resolution density. Much effort has been devoted to improving LCD response times. With continuously improving low-viscosity liquid-crystal (LC) properties, device configuration, and driving methods, the response time has been reduced to 2–5ms. 1 However, it is still not fast enough to eliminate the color breakup observed in color- sequential displays. Suppressing this phenomenon requires LC response times of less than 1ms. To achieve this goal, optically isotropic LC is emerging as a potential next-generation display technology. A conventional, nematic LC turns isotropic when the temper- ature exceeds the clearing point. Within a few degrees above the nematic-isotropic transition temperature, LCs can still be controlled by an electric field and exhibit 100ns response time. 2 However, the remaining phase is too small and the required operating voltage too high for display applications. By introduc- ing a certain amount of chiral dopant and polymers into a LC host, polymer-stabilized LC composites can be made optically isotropic over a reasonably wide temperature range. 3 Compared to conventional, nematic LCDs, this new optically isotropic LC exhibits several advantages. First, the system can be Figure 1. Optically isotropic liquid crystals (LCs) in an in-plane- switching electrode cell. Left/right: Voltage-off/on. P: Polarizer. A: Analyzer. E: Electric field. optimized to achieve submillisecond response time. Second, because the dark state is optically isotropic, its viewing angle is intrinsically wide and symmetric. In addition, the alignment layer for conventional LCDs is no longer needed, which simpli- fies the fabrication process and reduces cost, while the mater- ial’s performance is insensitive to the LC-layer thickness. This cell-gap insensitivity is particularly desirable for fabrication of large-panel LCDs for which cell-gap uniformity is a significant concern. However, several technical challenges (e.g., high operating voltage, hysteresis, and the relatively narrow characteristic temperature range) remain to be addressed before widespread commercialization can be realized. Our research team is work- ing on material optimization and new device structures to lower the operating voltage and optimize display performance. Figure 1 illustrates the working principle of optically isotropic LCDs. We employ an in-plane-switching electrode to generate a hor- izontal electric field. In the voltage-off state, the LC composite appears optically isotropic. This results in blocking of the inci- dent light by the top analyzer and, therefore, a dark state. Upon application of a lateral electric field, the refractive-index profile becomes anisotropic because of the Kerr effect. 4 Consequently, phase retardation occurs and the incident light is transmitted. Continued on next page