| Summary: | 博士 === 國立中山大學 === 物理學系研究所 === 106 === This dissertation proposes two tunable devices based on liquid crystals: a switchable two-dimensional phase grating and an electrically and optically tunable Fresnel lens. The following two topics are discussed.
Topic I demonstrates a switchable two-dimensional phase grating in blue phase liquid crystal (BPLC), which is fabricated by sawtooth in-plane-switch (IPS) electrodes. They are used to generate the horizontal electric field on a single indium-tin-oxide (ITO) glass substrate and, as a result, the 1-D and 2-D phase gratings can be mutual switched via different polarizations of incident light with an applied voltage. The first-order diffraction efficiency is up to 20 % and 10 % for the 1-D and 2-D phase grating at V = 150 V, respectively. Moreover, the rise and decay time is 0.9 and 1.1 ms, respectively, which is suitable for wide applications of high-speed optical manipulations.
Topic II investigates an electrically and optically tunable Fresnel lens in a liquid crystal (LC) cell with an erasable and rewritable photoconductive layer. By using a Sagnac interferometer, a Fresnel-like pattern can be induced on the photoconductive polymer layer which results in conductive and nonconductive structures in bright and dark zones. This effect causes the mismatch of the LC refractive index between adjacent zones with external voltage, generating a LC Fresnel lens. The focal length of the proposed Fresnel lens can be easily tuned by varying the Fresnel pattern size, and the focusing efficiency can be optically and electrically controlled.
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