This thesis is devoted to the study of mechanisms for controlling topological traps in liquid crystals. The methods that can be used to control the position of topological defects in liquid crystals were studied.
Controlling the orientation, macrostructure, and optical properties of liquid crystals is a hot topic today, both from a fundamental and applied point of view, as liquid crystals are actively used in various fields of science and technology, ranging from modern electronics to medicine. liquid crystal displays, thermal imagers and thermographs, guided lenses and optical shutters, liquid crystal lasers, and decorative cosmetics are just a few of the applications of liquid crystals. Even Kevlar, which is now most commonly used for body armor, is made of a liquid crystal polymer.
In the course of the studies presented in this paper, the formation of light-induced patterns in the form of concentric rings visible only between polarizers in a planar liquid crystal (LC) cell with a photosensitive film of chalcogenide As20Se80 was observed for the first time. An unusual distribution of reorientation of the liquid crystal molecules, which appears when the chalcogenide surface is irradiated with a Gaussian laser beam through the LC layer, was observed. The experimental results are explained in terms of heat transfer from the chalcogenide film to the LC after light absorption in the chalcogenide layer. Light-induced heating causes a change in the birefringence of the LC and, as a result, a spatially modulated polarization state in the plane of the chalcogenide layer. Since the chalcogenide film is a photo-alighning material, the spatial modulation of light polarization leads to the formation of a spatially modulated axis of easy orientation and an ring director structure in the cell.
The formation of a light-induced orientation modulation on a light-sensitive surface in combination with a quarter-wave plate can form a phase delay whose value corresponds to the director reorientation angle. We also propose a model of a liquid crystal Gaussian prism. We believe that this effect can be applied to the development of various optical and electro-optical elements, such as beamformers, microlenses, zone plates, etc.
To study the structure of a liquid crystal, we use combined cells consisting of transparent substrates with a predefined orientation of the liquid crystal on the surface. This method allows us to obtain a predetermined orientation of the liquid crystal molecules in the entire volume of the cell. One of the interesting features of liquid crystals is the ability to generate various patterns in polarized light, which they form due to the birefringence of liquid crystals. These patterns are almost entirely due to the structure that arises in the distant molecular order of the liquid. At the same time, topological defects can occur in the ordering of the structure of liquid crystals. A topological defect in the plane of a rotary cell filled with liquid crystals appears when the torsion angle of the director becomes uncertain.
The position of the defect in the liquid crystal is completely determined by the orientation of the liquid crystal on the orienting surfaces of the substrates and the structure of the liquid crystal. This allows, by controlling the orientation on the surface, to change the orientation in the volume and control the position of the defect.
Visualization of the position of a topological defect is an important component for the construction of various devices based on measuring the position of a topological defect, which is why the possibility of capturing various particles to make the topological defect visible to the naked eye was investigated in this work.
Recently, methods of photo-alighnment of liquid crystal molecules using photo-isomerization of a layer of molecules deposited on the surface of substrates by polarized radiation have been actively developed. The use of photosensitive substrates is relevant both from the fundamental point of view and for practical applications (for the construction of the latest types of liquid crystal displays, radiation dosimeters, devices for controlled capture and movement of particles, optical devices with controlled characteristics based on liquid crystals, etc.)
In this paper, we present a number of studies aimed at controlling the position of a topological defect in a liquid crystal, as well as visualizing its position by placing microscopic particles in it.
Keywords: Liquid crystal, photo-alignment, chalcogenide films, topological defects, disclinations, nematic, cholesterol.
