||Nowadays, development of technology is rapidly getting better and better. The applications of liquid crystals on display technology are considerably matured. At the same time, many scientists have also been paying much attention to the relative applications. Additionally, the characteristics of polarization optics, used in many electro-optical products, such as liquid crystal displays, 3D technology, etc., therefore, play the very important roles. Hence, it is the modulation of light polarization that has become a significant technique in electro-optical field. Regarding the rotation of polarization direction of linearly polarized light, the currently developed electro-optical products, such as half-wave plates, twisted nematic liquid crystal (TN-LC), and others, for modulating the polarization direction of linearly polarized light possess their different pros and cons. It can be understood that the performance of a half-wave plate is wavelength dependent, and its optical axis should be mechanically rotated to change the polarization direction of the incident linearly polarized light. Moreover, TN-LC can be able to rotate the polarization direction of the incident linearly polarized light to a specific angle within the Mauguin limit so that it cannot be used to simply change the polarization direction of the incident linearly polarized light. Hence, the development of novel optical devices to rotate the polarization direction of the incident linearly polarized light is a significantly important trend.|
This study presents a novel approach to rotate the polarization direction of linearly polarized light from one angle to others via the exposure of UV light based on chiral azobenzene-doped cholesteric liquid crystals (CLCs). Because the adopted chiral azobenzene is a kind of chiral dopants with optically tunable helical twisting power, the polarization direction of linearly polarized light can be rotated optically due to the optical tuning of the CLCs pitch length. According to the experimental results, if the CLCs with planar textures and with the selective Bragg reflection of wavelength longer than infrared, the polarization direction of the incident linearly polarized light with relative shorter wavelength (visible wavelength and near infrared) can be rotated. Notably, the rotated angle is dependent on the wavelength of incident light, the pitch length of CLCs, the pitch numbers, and others. In this thesis, the following three topics will be reported and discussed, including (1) various factors described above to affect the rotation of the polarization direction of linearly polarized light; (2) demonstration of an optically controllable linear-polarization rotator using chiral azobenzene-doped CLCs and (3) verification and comparison of experimental and simulated results.
 B. Bahoadur, Liquid crystals-applications and uses, (World Scientific Press, 1990).
 F. Reinizer, “Beitrage zur kenntiniss des cholesterins,” Monatsh. Chem. 9, 421 (1888).
 O. Lehmam, “On flowing crystals,” Z. Phys. Chem. 4, 462 (1889).
 陳言愈，電控及光控膽固醇液晶光柵之研究 (國立成功大學，碩士論文，民國100年).
 E. G. Virga, Variational theories for liquid crystals, (Chapman & Hall London, 1994).
 I. C. Khoo and S. T. Wu, Optics and nonlinear optics of liquid crystals, (World Scientific, 1993).
 O. Francescangeli, S. Slussarenko, and F. Simoni, “Light-induced surface sliding of the nematic director in liquid crystals,” Phys. Rev. Lett. 82, 1855 (1999).
 M. Marinelli and F. Mercuri, “Effects of fluctuations in the orientational order parameter in the cyanobiphenyl (nCB) homologous series,” Phys. Rev. E 61, 1616 (2000).
 松本正一，角田市良，液晶之基礎與運用 (國立編譯館, 1996).
 H. Keller, “History of liquid crystals,” Mol. Cryst. Liq. Cryst. 21, 1 (1973).
 G. W. Gray, Thermotropic liquid crystals, (the Society of Chemical Industry 1987).
 W. H. de Jeu, Physical properties of liquid crystalline materials, (Gordon & Breach, 1980).
 H. S. Kitzerrow and C. Bahr, Chirality in Liquid Crystals, (Springer, New York, 2001).
 P. J. Collings and Michael Hird, Introduction to liquid crystals chemistry and physics, (Taylor ＆ Francis Ltd, 1997).
 A. Yariv, Optical Electronics in Modern Communications, (Oxford University Press, New York, 1997).
 A. Yariv, Quantum Electronics, (Wiley, New York, 1988).
 P. Yeh and C. Gu, Optics of liquid crystal displays, (John Wiley ＆ Sons, Inc., 2006).
 G. R. Fowles, Introduction to modern optics, 2nd ed., (University of Utah, 1975).
 M. Hara, H. Takezoe, and A. Fukuda, “Forced Rayleigh scattering in nCB′s (n=5-9) with methyl red and binary mass diffusion constants,” Jpn. J. Appl. Phys. 25, 1756 (1986).
 P. G. de Gennes, and J. Prost, The physics of liquid crystals, (Oxford University Press, 1993).
 L. M. Blinov and V. G. Chigrinov, Electrooptic effects in liquid crystal materials, (Springer-Verlag Publishing Co., 1994).
 V. Fréedericksz and A. Repiewa, “Theoretisches und experimentelles zur frage nach der natur der anisotropen flüssigkeiten,” Zeitschrift fur Physik 42, 532 (1927).
 S. T. Wu and D. K. Yang, Reflective liquid crystal displays, (John Wiley ＆ Sons Ltd, 2001).
 P. G. de Gennes, “CALCUL DE LA DISTORSION D’UNE STRUCTURE CHOLESTERIQUE PAR UN CHAMP MAGNETIQUE,” Sol. State Commun. 6, 163 (1968).
 R. B. Meyer, “Effects of electric and magnetic fields on the structure of cholesteric liquid crystals,” Appl. Phys. Lett. 12, 281 (1968).
 T. V. Galstyan, V. E. Drnoyan, and S. M. Arakelian, “Self-induced oscillations and asymmetry of the light angular spectrum in a dye doped nematic,” Phys. Lett. A 217, 52 (1996).
 Y. C. Liu, K. T. Cheng, H. F. Chen and A. Y. G. Fuh, “Photo- and electro-isomerization of azobenzenes based on polymer-dispersed liquid crystals doped with azobenzenes and their applications,” Opt. Express 22, 4404 (2014).
 Eugene Hecht, Optics (4th edition), (Addison Wesley, 2001).
 A. Yariv and P. Yeh, Optical waves in crystals: propagation and control of laser radiation, (John Wiley & Sons Inc, 2002).
 丁啟倫，運用二維時域有限差分法分析液晶元件光學性質 (國立成功大學，碩士論文，民國94年).
 林宗賢，液晶光子晶體雷射現象與其光控制研究 (國立成功大學，碩士論文，民國92年).
 K. T. Cheng, P. Y. Lee, M. M. Qasim, C. K. Liu, W. F. Cheng, and T. D. Wilkinson, “Electrically switchable and permanently stable light scattering modes by dynamic fingerprint chiral textures,” ACS Appl. Mater. Interfaces 8, 10483 (2016).
 Q. Li, L. Green, N. Venkataraman, I. Shiyanovskaya, A. Khan, A. Urbas, and J. W. Doane, “Reversible photoswitchable axially chiral dopants with high helical twisting power,” J. Am. Chem. Soc. 129, 43 (2007).
 許維婷，液晶盒厚度量測方法的研究 (國立成功大學，碩士論文，民國93年).
 S. W. Ko, S. H. Huang, A. Y. G. Fuh, and T. H. Lin, “Measurement of helical twisting power based on axially symmetrical photo-aligned dye-doped liquid crystal film,” Opt. Express 17, 15927(2009).