Blue Phase Liquid Crystal And Its Applications In Optical Elements

As a kind of new liquid crystal material, blue phase liquid crystals (BPLCs) have attracted many research attentions in recently years. In comparison with normal nematic LCs, they show significant advantages such as fast response and quasi-isotropic optical properties.In this thesis, the properties of BPLC materials are studied. Some optical components are developed based on the polymer stabilized BPLC. A blue phase state is induced when we add a few bent-shaped monomers in the chiral nematic liquid crystal mixture. Furthermore, the blue phase temperature range is broadened after UV exposure. We build an optical setup to characterize the performance of related devices. The performances of the devices are optimized through theoretical analysis, numerical simulation and optical design. We also develop two fast-response polarization independent one-dimensional/two-dimensional gratings and an optical attenuator with fast-response, which may have promising applications in telecommunication band. The content of the thesis is mainly divided into these three sections: 1. A blue phase is induced by bent-shaped monomer with allylic end bonds when doped into chiral nematic liquid crystal. The mechanism of blue phase induction and stabilization is investigated. Its temperature range is further widened to10.2K by UV irradiation with a slowly cooling process. The widening principle is distinguishable from traditional polymer stabilization mechanism. This study provides some useful insights into the molecular design of suitable bent-shaped dopants towards wide range blue phase liquid crystals.2. A BPLC grating is proposed by applying a vertical electric field with lateral periodic distribution. Simulation on electric-field distribution is also carried out, the results of which suggest the alternation of isotropic and ordinary refractive indices in the lateral direction. Through the electrode configuration design, both1D and2D gratings are demonstrated with high transmittance of ca.85%. The diffraction efficiency of the first order reaches up to38.7%and17.8%for the1D and2D cases, respectively. The field-induced fast phase modulation permits a rapid switching of diffraction orders down to the sub-millisecond scale.3. A BPLC based variable optical attenuator (VOA) is demonstrated with a polarization independent design. The device shows normally-off feature when no field is applied. Response time down to sub-millisecond scale is achieved in switching between two arbitrary attenuation states. The attenuation range is also measured from1480to1550nm, which covers the whole telecomm S-band and part of the C-band. The overall performances reach the requirements for practical use; while still have room for further improvement. Through this experiment, the applicability of BPLC in fiber optic devices is presented, which may impel the development of many other photonic applications from infrared to even microwave regions.