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1.3 PHYSICAL SCIENCES
Liquid crystals have become natural candidates for use in electro-optic devices for their ability to change the orientation of the director with the application of an electric field, and exhibiting large range of refractive index. The aim of the work presented in this thesis is to fabricate liquid crystal optoelectronic devices such as electrically switchable liquid crystal diffraction gratings and polarization rotators by exploiting the holographic surface relief effect in photopolymer and by developing novel polymer dispersed liquid crystals (PDLCs). Alignment of liquid crystals is commercially achieved by creating grooves on a conducting layer such as polyimide or indium tin oxide (ITO) by rubbing. This process has disadvantages such as creation of static electricity and dust which are undesirable. An attractive alternative technique to rubbing is investigated. A photopolymer layer coated on a conducting ITO layer on a glass plate has the grooves inscribed holographically in it. An acrylamide based dry photopolymer developed in the Centre for Industrial and Engineering Optics, Technological University Dublin is used in this study. The dependence of photoinduced surface relief on the holographic recording parameters, chemical composition of photopolymer and on physical parameters of the photopolymer layer were studied. A model explaining the mechanism of surface relief grating formation is proposed. Electrically switchable diffraction gratings and polarization rotators were fabricated by filling these grooves with liquid crystals using the capillary filling technique. In the second approach, holographic switchable diffraction gratings were fabricated using a novel PDLC, which was also developed in the Centre for Industrial and Engineering Optics. PDLCs consist of microscopic liquid crystalline droplets embedded in a polymer matrix. Preliminary results for the recording parameters and the physical parameters of the PDLC layer needed to fabricate gratings are presented. The redistribution of LCs was observed by using techniques such as phase contrast microscopy and Raman spectroscopy. The electrically switchable diffraction gratings were characterized using linearly polarized light by measuring the dependence of the intensity in the first diffracted order on the applied electric field. The polarization rotator was characterized by studying the influence of the applied electric field on the twist angle and the variation of intensity in the zero and the first orders of diffraction. The capabilities of the photoinduced surface relief effect in the photopolymer and of a newly developed PDLC material for the fabrication of liquid crystal devices are demonstrated.
Pavani, Kotakonda. (2009). Holographic liquid crystal devices. Technological University Dublin. doi:10.21427/D7BP4Q