Investigations of Optical Properties and Photo-Alignment in Bistable Nematic Liquid Crystal Displays
Sammanfattning: In recent years portable electronic devices, such as mobile phones and personal digital assistants, have increased the demand for high performance displays with low power consumption. An interesting candidate with the potential of fulfilling these demands is the reflective single-polarizer surface controlled bistable twisted nematic liquid crystal display. The main focus of this work involves the optical properties of displays based on these bistable structures. In the investigations, the display is considered as an integrated optical system, containing not only the liquid crystal cell, but also components such as polarizers and retardation films. The specific aim of the thesis was to derive new optical modes of the reflective single-polarizer bistable twisted nematic device using the Jones matrix method to study the interaction between the polarization of light and the optically anisotropic media. The electro-optical properties of the derived modes have been studied and evaluated both theoretically and experimentally. The modes possess excellent brightness and high contrast ratio. By introducing a quarter-wave retardation film into the optical configuration, the contrast ratio can be significantly increased by preventing spectral leakage of light in the dark state. To experimentally realize the derived optical modes, special layers for the alignment of the liquid crystal molecules on the cell substrate surface have been proved needed. Therefore, also the photo-alignment technology of nematic liquid crystals on novel alignment materials has been studied with the aim to control liquid crystal cell parameters such as pretilt angle and anchoring energy, both critical for the bistable switching. The results of this thesis will increase the understanding of the optical properties of the reflective single-polarizer bistable twisted nematic liquid crystal display and will be valuable when considering this type of device for practical applications.
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