I. Palarie

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.

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.

A Percolation Model to Evaluate the Correlation Length of Dye-Nematic Liquid Crystal Interaction

ABSTRACT The orientation effect of methyl-red (MR) molecules on pentylcyanobiphenyl (5CB) nematic liquid crystal has been investigated in a pump-probe experiment at various concentrations of azo-dye and for different power values of an exciting Ar+ laser beam. Excited dye molecules reorient the LC molecules within their correlation spheres. There is a percolation probability that the whole LC bulk be reoriented. Analyzing the threshold energy necessary for this percolation in terms of dye concentration, the model allows evaluating the lowest concentration of dye molecules (critical concentration), the correlation length and the quantum efficiency of the reorientation process.

Computer simulation and experimental verification of the diaphragms theory

Both experimental and computer simulated results are obtained on the following problems: Calculation of the aperture diaphragm and its related pupils, the field-of-view diaphragm, and its related windows, the view field for a centered optical system and a given object point. The influence of object position and of the optical system characteristics on all these variables are studied using a computer simulation program and comparing the results to the experimental ones.