Gabriela Iacobescu

Controlling the orientation of microgrooves and the depth of the ripple structure in dye-doped liquid crystal cells

The permanent structure induced by an Ar+ linearly polarised laser beam on an asymmetric dye-doped nematic liquid crystal cell was investigated in a pump-probe experiment. The polarisation direction of the probe beam was parallel to the easy axis and two configurations of pump beam were used: polarisation directions perpendicular and parallel to the easy axis. The transmitted intensity of the probe beam was recorded during irradiation and it was observed that it depended both on the power and polarisation direction of the pump beam. We explained the evolution of the transmitted intensity and evaluated the start time of formation of the ripple structure. The induced permanent structure in the irradiated zones and the laser-induced surface morphology was studied using a polarising optical microscope and an atomic force microscope, respectively. The surface morphology in the irradiated zones was also dependent on both the power and polarisation of the Ar+ laser beam. The orientation of the microgrooves in the ripple structure was parallel to the polarisation direction of the pump laser beam in both configurations. For a given pump power, the depth of the ripple structure was greater in the case of an Ar+ beam polarised parallel to the easy axis. The induced azimuthal anchoring energy provided by the ripple structure was evaluated.

MAGNETO-OPTIC EFFECTS IN NEMATIC LIQUID CRYSTAL DOPED WITH AZO-DYES

Nematic liquid crystal cells were subjected to different DC magnetic fields and the optical transmission of a He-Ne laser beam was recorded for different field values. It was found that a small addition of Methyl Orange (MO) induced optical activity into the nematic sample. When the cells were subjected to increasing magnetic field, the rotatory power (rotation angle) varied periodically; the same occurred for the light transmission and ellipticity. We found that the light emerging from the liquid crystal cell is elliptically polarized and in some cases circularly polarized. After UV light irradiation (365 nm) the rotatory power of the sample increases. The position of rotation angle maxima and minima is slightly affected by the irradiation.

Surface morphology of doped nematic liquid crystals: role of dye concentration

We report on laser-induced ripple structures in dye-doped nematic liquid crystals for different dye concentrations varying between 0.5% and 2% wt. The surface morphology was studied by atomic force microscopy. For dye concentrations smaller than 0.56% the ripple structure was not obtained. The depth of the ripple structure depends on both the laser power and the dye concentration. Using the Berreman formula for periodic structure, the azimuthal anchoring energy was evaluated. An increase of the anchoring energy with the dye concentration and with the pump power was noted.

The influence of the easy axis on laser-induced ripple structures in dye-doped liquid crystal films

We have investigated the role of the easy axis in the photoalignment effect induced by an Ar+ linearly polarized laser beam on a dye-doped nematic liquid crystal cell in a pump–probe experiment. The induced permanent structure in the irradiated zones was studied using a polarizing optical microscope, and the laser-induced surface morphology was studied by atomic force microscopy. The depth of the ripple structure depends on both the pump power and the irradiated surfaces. The azimuthal anchoring energy provided by the ripple structure was evaluated and it was concluded that it varies with the cells symmetry.

THERMALLY STIMULATED DEPOLARIZATION CURRENTS IN A NEW CHOLESTERIC LIQUID CRYSTAL

The phase transitions in cholesteryl-beta-chloropropionate were studied by using thermally stimulated depolarization current (TSDC) methods. Optical transmission studies were performed over the whole temperature range. From the experimental data, useful information with reference to the conduction mechanism was obtained. The TSDC spectra displayed a peak at the solid-cholesteric phase transition temperature. Using the plots obtained when different depolarization fields were applied, the activation energies and the relaxation time were determined.

ORDER PARAMETER FROM (p-ETHOXYBENZYLIDENE-)-p-n-BUTYLANILINE CHANNELED SPECTRA

The dispersion of refractive indices and birefringence of EBBA were determined at different temperatures and the band contributions to these dependencies were computed. An improved Talbot–Rayleigh experimental setup was used and, in order to increase the accuracy and minimize the computation time, a new data processing method was proposed. The temperature dependence of the order parameter and of the correction factor g, originating from the anisotropy of the local field, were computed.