| Summary: | 博士 === 國立交通大學 === 光電工程系所 === 92 === The texture transitions in cholesteric liquid crystals are very interesting and are of importance for both fundamental science and applications. It is essential to understand the mechanisms of texture transitions in ordr to design driving schemes for cholesteric devices.
In this thesis, we investigated the mechanisms of three types of texture transitions in planar-aligned cholesteric liquid crystal cells: homeotropic-planar texture transition, homogeneous-planar texture transition and planar-focal conic texture transition. In addition, the mechanism of homeotropic-fingerprint texture transition was discussed in homeotropic-aligned cholesteric liquid crystal cells. In order to understand the dynamics of texture transitions, a computer program for three-dimensional simulation based on the finite element method was developed.
When the applied electric field was turned off abruptly, the dynamics of homeotropic-planar texture transition was numerically analyzed and experimentally confirmed in a planar-aligned cholesteric liquid crystal cell. Furthermore, the effect of bias waveform on the homeotropic-planar relaxation process was also studied. On the basis of this knowledge, a bias waveform was designed to reduce the long relaxation time. Similarly, we also numerically investigated the dynamics of the homogeneous-planar texture transition when the unwound electric field was removed abruptly. The simulation results agreed well with the previous experimental observations. Moreover, we found that the Helfrich deformation can be induced not only by an electric field but also by an elastic force. Therefore, we compared the elastic-induced Helfrich deformation during the homeotropic-planar and homogeneous-planar texture transitions with the electric-induced Helfrich deformation during the planar-focal conic texture transition. On the other hand, we observed the modulated textures during the planar-focal conic texture transition by changing the applied electric field. It was found that the modulated textures exhibit either an ordered striped texture or an ordered hexagonal texture depending on the applied electric-field strength. Of these textures, the ordered hexagonal texture was experimentally observed for the first time.
Finally, we observed the stripe formation during the homeotropic-fingerprint texture transition when the applied electric field was turned off abruptly in homeotropic-aligned cholesteric liquid crystal cells with patterned electrode configurations. The striped direction depended not only on the thickness-to-pitch ratio, but also on the applied electric field. In this work, the cholesteric liquid crystal phase grating with the field-controllable grating orientation and grating period was realized and the operational mechanism of this device was presented.
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