| Summary: | 博士 === 國立成功大學 === 機械工程學系碩博士班 === 93 === The properties of photonic band gaps in chiral photonic crystals are discussed in the thesis. The research is devoted to the photonic defect modes in the chiral photonic structures with gradient pitch length and the introduction of the different kinds of defects. In addition, various optical applications with respect to the defect modes are also demonstrated. Self-organized cholesteric liquid crystals (ChLCs) with spatially varying pitch are the examples. The finite element method and the 4×4 transfer matrix method are carried out for the study of the optical propagation simulations in the ChLC films. First, the optics of the cholesteric medium with constant pitch is talked over, including dispersion relations, the group velocity, the group delays, the electromagnetic density of modes, transmission and reflection spectra, the properties of the defect modes, etc. Then, the optics of the cholesterics with spatially varying pitch is focused on the photonic defect modes created by the following defects: twist defect (phase jump), gradient jump, and pitch jump.
The defect mode due to the introduction of a twist defect is considered in the chiral structures with spatially varying pitch. An unusual crossover behavior in reflection at the defect resonance wavelength of a single circularly polarized mode appears when the structure thickness increases over a specific value. Two different resonance wavelengths can be created by a twist defect in the identical ChLC composite film with linearly varying pitch. The behavior constitutes the operational mechanism for a passive optical diode.
The introduction of photonic defect modes in a ChLC can be achieved by a global deformation of the helix with a chiral anti-symmetry relative to the middle of the ChLC configuration. The defect modes can be excited only when the circularly polarized incident wave has the same handedness as the ChLC. The transmittance of the defect modes is greatly reduced when the structure thickness is large far away from a critical value, and even some defect modes will be completely suppressed. A wider forbidden band for polarization-selective reflection can be obtained. Such a ChLC film can be useful for the production of the ultra-wide-band reflective polarizer.
For normal and near-normal incidence of circularly polarized light with the same handedness as structure, the defect caused by a pitch jump in the cholesteric helix results in discrete peaks within a forbidden band in the transmission. The particular spectrum is similar to the feature of a Fabry-Perot interferometer. By introducing an additional phase jump, linear blue shifts of the defect modes in transmission spectra are correlated with an increase in the twist angle. Polarization-dependent or polarization-independent optical filters based on the special transmission properties of multiple photonic defect modes are described.
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