Electro-optical properties of cholesteric liquid crystal devices and applications of dual frequency cholesteric liquid crystals.

摘要

The helical structure of cholesteric liquid crystals originates the optical property of Bragg reflection, employed in numerous devices. A typical cholesteric device consists of glass plates, electrodes, alignment layers and a cholesteric liquid crystal layer. We systematically studied the reflection from individual interfaces by measuring the reflection spectra under various polarization conditions and simulating the spectra using Berreman's 4 x 4 matrix method. The results demonstrate that device structures have significant effects on optical performance. We studied the optical behavior of various cholesteric textures. We report for the first time an anomalous reflection of the left circularly polarized light from imperfect planar textures with right-handed twist under normal incidence. We modeled the imperfect planar texture as a multi-domain structure and were able to simulate the reflection of the multi-domain planar texture. We also studied the optical properties of the gray scale states of the cholesteric reflective display. The results are very useful in the implementation of full color displays.; We developed two types of devices utilizing dual frequency cholesteric liquid crystals, the dual frequency cholesteric light shutters and reflective displays. The advantages of these light shutters are their high on-state transmittance and low off-state transmittance. By designing and optimizing a 3-phased dual frequency drive scheme, we reduced the response time of the device from over 10 seconds to less than 50ms. The device is a good candidate for applications such as laser protection goggles. Unlike the conventional reflective display, the dual frequency one does not need a homeotropic state as an intermediate switching state. Thus it has the potential of reducing the drive voltage. We designed a dual frequency drive scheme to drive the display between various gray scale states directly, promising an easier implementation of gray scales.