| dc.description.abstract | The trans-cis (cis-trans) transitions of azobenzene materials adopted in this thesis occur when they are illuminated with an ultraviolet (visible) light. Such a reversible reaction is called the photoisomerization effect. The physical properties of trans-isomers are usually different from those of cis-isomers, so azobenzenes can be widely applied to the applications of photosensitive optical devices.
In this thesis, the mixture of nematic liquid crystals (LCs) with negative dielectric anisotropy (HNG30400-200), right-handed chiral dopant (R1011), and left-handed chiral azobenzene (ChAD-2-S) was adopted. The disparities of the adopted chiral azobenzenes between trans-isomers and cis-isomers, such as the tunable helical twisting power (HTP), play the key to shift the reflection band of the cholesteric LCs (CLCs) with the illumination of different wavelength light sources. The topics in the thesis include three parts. The first one is to study the tunable reflection bands of the CLCs by simultaneously illuminating with green and purple light onto the CLCs. With the illumination of green light having a constant intensity, the CLC reflection band will be blue-shifted. The range of shifted wavelength increases with the increase of the intensity of purple light within a specific range. We also observed that the reflection band can be broadened by proper increasing the intensities of both lights. Additionally, the shifts of the reflection bands, resulting from the simultaneous illumination of green and purple lights from the same side and the opposite sides of the CLC cell were investigated. Second, binary planar textures with two different pitch lengths, generated by illumination of light through a suitable mask, in a single CLC cell are also proposed to spatially broaden the reflection band of the CLCs. The difference between the reflection band, resulting from the illumination of a single purple light, and that obtained by the simultaneous illumination of purple and green lights, was examined. As described in the first part, the simultaneous illumination of green and purple lights from the same side and the opposite sides of the CLC cell is worth to be discussed. In the last part, the approach to obtain a light source having a narrow bandwidth was proposed. With the illumination of purple light onto the part of the CLC cell, two regions, reflecting lights with different wavelength ranges, can be obtained. A wide-band light source becomes a narrow-band one if the incident light passes through the two regions in the CLC cell twice. As a result, the undesirable component of light can be filtered out, and a narrow band unpolarized light source can be retained. | en_US |