ABSTRACT
The cholesteric phase of liquid crystals (LCs) presents a macroscopic helical structure, which was first observed by Reinitzer in 1888.1 Since then, the cholesteric phase, also known as the chiral nematic phase, can be introduced by external chiral molecules in the nematic phase rather than cholesterol. As shown in Figure 1.1, the spatial orientation of the rod-shaped molecules (or mesogens) spontaneously rotates by a constant angle along the direction of helical axis in cholesteric liquid crystals (CLCs). The helical sense of the director is nonsuperimposable on its mirror image; thus, it is determined by the configuration of the chiral group within the molecule and it can be expressed as the positive sign (+) for right-handed (RH) helix and the negative sign (−) for left-handed (LH) helix, respectively. The helical pitch, p, is the distance over which the LC molecules rotate 2π twist. This periodic helical arrangement of the molecules causes unique selective reflection of light according to the Bragg regime.
