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

Abstract

The permanent structure induced by an Ar þ 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 þ 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 þ 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

“…A pump power larger than 30 mW was not used because from our previous studies [13,14] it has been shown that beyond this value a rough and inhomogeneous surface appears due to a rapid and random aggregation of adsorbed cis MR molecules.…”

“…These values are characteristic for a high-intensity regime. The direction of rubbing (easy axis) was perpendicular to the direction of polarisation of the Ar + laser beam; a smaller value of the azimuthal anchoring energy is obtained for this configuration [13]. The induced permanent structure in the samples is twisted nematic, as shown in our previous studies [13,14].…”

“…*Corresponding author. Email: dascaluc@yahoo.com Recently, Palarie et al [13,14] reported on laserinduced ripple structures in dye-doped asymmetric samples of LC films in the high-intensity regime. The control surface (namely, that with unrubbed polyvinyl alcohol film) was the first impinged upon by the Ar + laser beam (pumping laser), the polarisation directions of which were parallel or perpendicular to the easy axis, respectively.…”

Electrical Freedericksz transitions in nematic liquid crystals containing ferroelectric nanoparticles

“…The cantilever had a nominal length of 125 mm, a nominal force constant of 40 N/m, and oscillation frequencies in the range of 275373 kHz. We used horizontal line by line flattening as planarization method [20]. The sample was prepared by evaporation on silicon substrate of a dilute solution of BaTiO 3 in water.…”

Measurement of magnetic anisotropy of multiwalled carbon nanotubes in nematic host

“…There have been developed many devices based on NLC with inserted dyes, micro-particles or nano-particles. These mixtures can be used for the study of particle's physical properties as well as in engineering to improve existing devices or to develop new ones [10][11][12][13][14][15][16][17][18][19]. The liquid crystals, mainly used in LCD's, combine the properties of crystalline structure of solid state with those of liquid phase, leading to an organized movement of inserted to CNT.…”