On the Optics and Surface Physics of Liquid Crystals

Doctoral thesis, 1999

Good knowledge about optics and surface physics of liquid crystals is indispensable when utilising them for applications, such as liquid crystal displays (LCDs). However, they are also two central subjects in fundamental research on liquid crystals. We have studied the alignment of nematic liquid crystals on obliquely evaporated silicon oxide, as a function of evaporation angle, temperature and applied electric fields. As the evapora-tion angle runs from approximately 67° to 75° (defined as the angle from the substrate normal), the average orientation of the liquid crystal molecules (n) changes continuously from a planar orientation perpendicular to the evaporation plane to a tilted orientation within the plane. Since the evaporated surface exhibits mirror symmetry, there exist two equally probable molecular orientations for each evaporation angle, one on each side of the evaporation plane (.theta. = .theta.0, .phi.1,2 = +-|.phi.0| ). We show that this two-fold degenerate alignment is strongly affected by changing the temperature or applying an electric field, with induced transitions in the orientation of more than 80° and 70°, respectively. In an effort to improve our understanding of these phenomena, we have developed theoretical models based on Landau-deGennes expansions and elastic theory. An important conclusion is that there is a strong coupling between the polar and azimuthal angles of n, which may be exploited for realising optical devices, based on the effects described here. The optical part of our investigations mainly concerns the properties of short-pitch (p<<.lambda.) helical liquid crystal phases. A homogeneous model for chiral nematic and chiral smectic C liquid crystals is presented, in which the optical properties of a periodic liquid crystal are simulated by a non-periodic macroscopic medium. In analysing this model's validity, we compare its optical properties to the ones of the true medium, which have been calculated by numerical integrations of Maxwell's equations (the Berreman formalism). Special attention is paid to the optical activity, which for these materials, contrary to p>=.lambda. - substances, is very small for light propagating along the helix axis and large in directions perpendicular to it. We present results, both theoretical and experimental, which show that the macroscopic description breaks down for samples in the homeo-tropic helix geometry, where the orientation of the molecules close to the boundary, with respect to the incidence plane of the light, becomes important. Large rotation angles due to the optical activity are predicted to occur only for chiral smectic C samples where the tilt angle approaches a critical value making the medium optically isotropic. Finally, we treat electrically deformed chiral smectics with respect to structure and optical properties, and in particular we analyse the selective reflection from them.