| dc.description.abstract | The research topic in this thesis is divided into three main parts. The first topic focuses on the reflection spectra and structural changes of homogeneous alignment of oblique helicoidal state of chiral nematics at different temperatures, as well as the variations in the operating voltage. In this study, the concentration of the adopted materials, liquid crystal (LC) cell gap, and alignment films are kept invariant, while only the temperature parameter is varied. The temperature has a significant impact on the characteristics of the oblique helicoidal state of chiral nematics. The formation of an oblique helicoidal state requires the addition of LC dimers, which reduces the overall bend elastic constant (K33) of the LC mixture. The bend elastic constant (K33) increases with the temperature, making the characteristics of the oblique helicoidal state less prominent. The Berreman 4×4 matrix is used to fit the measured reflection spectra, and the pitch length and cone angle are obtained through the fitting process. The second topic involves the measurements of the slanted angle, diffraction angle, diffraction efficiency, and Stokes parameter (S3) of the obtained polarization volume gratings (PVGs). It explores the variations of the slanted angle by adjusting the pitch length of chiral nematics with the same period along the x-directional, which in turn affects the diffraction angle. Finally, the Q parameter′s impact on Bragg volume gratings′ polarization selectivity is examined. The third topic involves the combination of the first two topics, aiming to fabricate Oblique Helicoidal State of Polarization Volume Gratings (OHPVGs). The goal is to achieve electrically controllable modulation of the grating′s diffraction angle by manipulating the pitch length through the use of oblique helicoidal state of chiral nematics. However, the experimental results did not show this phenomenon. The thesis speculates that the small Q value prevents the diffraction angle from being related to the pitch length and slanted angle, and instead, it depends on the wavelength and the grating′s periodicity and refractive index of LCs. In addition, this study also found that OHPVGs can address the issue of a short-wavelength increase in the reflection spectrum of homogeneous alignment of the oblique helicoidal state of chiral nematics under an applied electric field. Chapter 6 of this thesis proposes some solutions and outlines potential research directions in the future. | en_US |