Dudok T. Lasing in an imperfectly aligned cholesterics

The dissertation is devoted to the experimental study of laser generation in imperfectly aligned layers of cholesteric liquid crystals doped with fluorescent dyes. The work presents the review of current literature on the mechanisms of mirrorless feedback at optically pumped lasing, namely: random laser generation in scattering media, lasing in photonic crystals and microresonators. The optical properties of a mixture of cholesteric liquid crystal doped with the dye 4 - ((E) -2- (N- (ethylhexyl) -1,8 - naphthalimide - 4-yl) vinyl ) phenyl) benzenamine (DG56) as a material for laser generation were investigated. For this material spectral dependences of the birefringence, absorption and dichroism coefficients, excitation and fluorescence emission spectra, as well as the values of the scalar orientation order parameter for the dye molecules in the liquid crystal matrix were obtained. Laser generation in a mixture of cholesteric liquid crystal doped with dye DG56 is obtained at optical pumping. Generation of radiation is observed at the long-wave edge of the cholesteric photonic bandgap (PhBG) . It is shown that lasing is possible not only in relatively thin cells with high-quality planar orientation of cholesteric layers, but also in thicker cells with considerably imperfect alignment of the cholesteric layers. It is established that the imperfections of orientation of the liquid crystal structure are manifested in the laser spectra of cholesteric. Obtained results suggest that the line, which is detected inside the PhBG in imperfectly aligned cholesteric, should be classified as random lasing. Basing on the results of the experiments we conclude that the processes of line generation at the PhBG edges and inside the PhBG are coupled. It is theoretically shown and experimentally proven that the cell thickness is an additional parameter, which together with the scalar parameter of orientational order of the dye molecules in the cholesteric matrix governs, which of the two laser lines (at short or long wavelength edges of the PhBG) has lower excitation energy threshold, and, consequently, higher laser intensity for a given cell thickness. It is experimentally shown that laser generation is possible not only in a dye doped cholesteric with planar layers, but also in a cell with a hybrid alignment. In particular, optically pumped laser generation was detected at the long-wavelength edge of the PhBG of a cholesteric liquid crystal doped with the Nile Red dye It is shown that the lasing is possible not only in a droplet of dye-doped cholesteric, freely suspended in an isotropic liquid, but also for a droplet of the same dye doped isotropic liquid freely suspended in the cholesteric. In particular, laser generation was detected by optical pumping in the emulsion of droplets of glycerol doped with the dye Rodamin 6G freely suspended in the cholesteric liquid crystal matrix. Generation of radiation is observed at the shortwave edge of the PhBG. Comparison with laser generation in droplets of the same cholesteric liquid crystal matrix, doped with dye, freely suspended in glycerol. A matrix of laser point sources was fabricated, the capsules of which were filled with the mixture of the cholesteric liquid crystal doped with the Nile Red dye. It is found that the optically pumped laser spectra of the capsule contain spectral bands similar to those observed in an imperfectly aligned cholesteric.