Mykytiuk T. Lasing spectra of chiral liquid crystals with the photo-induced change of the helical pitch

In the dissertation, the influence of the processes of phototransformation of molecules of cholesteric liquid crystals (CLCs) under the action of light on the lasing spectra and the dependence of the lasing parameters on the orientation of the layers of molecules on substrates are studied. The processes of phototuning of the lasing frequency are investigated with the use of various CLC-materials. It is shown that the inverse tuning can be realized in the presence of azo- or azoxy-groups of molecules. In such molecules, the mutually spaced absorption bands of their trans- and cis-isomers must be revealed. We study the conditions, under which the lasing frequency phototuning ranges in a CLC-laser, are leveled. The optical quality of a planar structure is of great significance in this process. For the first time, a method of fast phototuning of the lasing frequency in CLCs, which has no analogs in the world, is developed. Using a highly photosensitive twisting azo-admixture, the record phototuning duration equal to 148 ms in the internal of wave lengths at 26 nm is attained. The dependences of the lasing characteristics in steroid CLCs on the quality of a planar texture and the orientation of surfaces layers of molecules on substrates are studied. It is shown that the minimum lasing threshold and the ring structure of a laser beam are observed at the parallel orientation of the molecular surface layers of CLCs deposited on substrates. The lasing spectrum is positioned at the center of the selective reflection band in agreement with the model of coupled waves. At the orthogonal arrangement of molecular surface layers on orienting substrates, the strong selection of longitudinal modes is observed. In this case, a single-mode lasing operation arises, and the ring structure of a beam vanishes. An increase in the lasing threshold, which was registered in this process, testifies a possibility of the defect formation in this spiral structure. T