Study On The Bragg Reflection Properties Of Electrically Controlled Helix Distortion In Cholesteric Liquid Crystals

The liquid crystals material acquires a thorough research and the extensive application on various photoelectron devices because it has unique electro-optical characteristics. Entering the 21th century, information technology penetrates into all areas of society, in various spheres,the liquid crystal display devices are playing the important rule of the information display.Cholesteric liquid crystals can be considered to a kind of spiral nematic liquid crystals, because of their unique molecular structure and electro-optic effect, cholesteric liquid crystal display has a very wide application. Currently people are familiar with the optical properties of cholesteric liquid crystals in planar texture, which many applications are just based on their nature of Bragg reflection and polarimetry. However, in previous studies, people were not very clear with the optical properties of Bragg reflection properties induced by electrically controlled helix distortion, so some of the conclusions now appear to be one-sided or not accurate.In this paper we studied the optical properties and the change of Bragg reflection of the cholesteric liquid crystal in planar texture when the applied electric field is perpendicular to the direction of the helical axis. Using LCD matrix optics, Jones matrix method, and Matlab matrix calculation software, reflectance spectra characteristics were analyzed and numerically calculated. According to the numerical and experimental results, the central wavelength of reflected light does not change with various voltages and the Bragg reflected light intensity decreases with the increasing voltage. We proposed one model of field induced helix distortion to give the reasonable explanation on the experimental and numerical results. We also found that the theory for designing the LCD with color change using electrically controlled helix distortion is not feasible. Nevertheless, in many fields people can take the use of this thoery for designing some liquid crystal devices and display devices that their given color can vary between disappearance and appearance. For example, the cholesteric liquid crystal filter can filtrate red light, after the electric field is applied to it, it wound lose the function of filtrating red light, etc.