We show that introducing anisotropy into periodic dielectric structures leads to new optical phenomena as well as to a new approach to a variety of applications. One-dimensional anisotropic structures allow a new type of chiral twist defect resulting in a localized photonic mode with unusual properties. Unlike isotropic layers of alternating index of refraction, where the periodicity can be destroyed only by changing the refractive index or thickness of a layer, a defect can be created in anisotropic media by introducing an additional rotation between consecutive layers. Computer simulations show that introducing an additional rotation in the middle of a sample with cholesteric ordering produces a localized state whose frequency can be tuned from one edge of the photonic stop band to the other by varying the angle of rotation from 0 to 180 degrees. Most of the energy of this mode exists as a circularly polarized standing wave with the same handedness as the structure, independent of the polarization of the exciting wave. This localized mode gives rise to a crossover in the nature of propagation. Below a crossover thickness, the localized mode is excited only by a wave with the same handedness as the structure and exhibits a peak in transmission at the defect frequency. Above the crossover, however, the defect mode can be excited only by the oppositely polarized wave and a resonant peak appears in reflection. Simulations for lengths below the crossover are in agreement with measurements of microwave transmission through stacks of overhead transparencies, ordered in the same way as the molecular layers of a cholesteric liquid crystal. Three types of defect are introduced: (1) an additional 90 degrees rotation, (2) an additional 45 degrees rotation, and (3) a combination of a 45 degrees rotation and a quarter-wavelength separation.
The synthetic pathways of preparation of new optically active 1,1-binaphthyl (BN) derivatives containing various radicals including mesogenic ones were suggested and realized. No mesophase was found, but having atropisomeric chirality origin the novel substances proved to be effective chiral dopants inducing the highly twisted supramolecular structure both in low molecular mass and in polymer liquid crystals (LC). The helical twisting power (HTP) in two different standard nematic mixtures and in the copolymers based on the nematogenic methoxy-phenyl benzoate acrylic monomer was investigated. The systematic growth of the HTP in the low molecular mass nematics was found when the longer or mesogenic radicals were attached to the binaphthyl core. In the LC-copolymers the HTP of the BN-monomer and mesophase stability were analyzed depending on temperature and the molar fraction of the BN-fragments. The observed HTP value was found to be about thrice as much as those reported for the cholesterol and binaphthyl chiral monomers.
The helical twisting power (HTP) of a homologies series of novel series of novel chiral binarphthol (BN) derivatives in open and bridged forms has been studied int ow different nematic solvents. The direct correlation between molecular structure and HTP was revealed: increasing the length of the substituents in 6,6' positions leads to the increasing of helical twisting power. Open-chain compounds display a significant decrease of twisting power on heating. The decrease of HTP is explained in terms of changing of the molecular conformation. The conformational properties of BN derivatives dissolved in a pentylcyanobiphenyl liquid crystalline (LC) matrix were determined by molecular dynamics simulation. Due to the influence of intermolecular interactions between the molecules of the LC matrix and the BN molecules, the conformations of the BN molecules in LC cell are found to change from transoid to cisoid on heating that results in decrease of observed HTP.
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