Optical Studies of Liquid Crystal Films: Structures, Surface Ordering, and Dynamics.

Optical techniques can be very effective probes for the investigations of liquid crystals, since these materials have rather unique optical properties in contrast to those of conventional liquids or solids. This thesis presents several experiments employing newly developed linear or nonlinear optical techniques in studying the surface ordering, structures, or fluid dynamics of liquid crystal films. We have developed an ellipsometric technique to study the interface of the liquid crystal 4'-n-pentyl-4 -cyanobiphenyl (5CB) and a glass substrate coated with surfactant molecules n,n-dimethyl-n-octadecyl-3-aminopropyl-trimethoxysilyl chloride (DMOAP). An evanescent optical wave was used to probe the optical birefringence in the interface region, which is a manifestation of the nematic order induced in 5CB by the wall-anchoring force. We found that, at temperatures above the isotropic-nematic transition point T(,c), the interfacial layer possessed a weak nematic order, with the layer thickness exhibiting a critical divergence as T (--->) T(,c). The results are at variance with predictions from a Landau-de Gennes theory as well as the wetting theories for the liquid crystal-wall system. Optical second-harmonic generation (SHG) was used to probe the structure of freely-suspended films of 4' -n-octyl-4-cyanobiphenyl (8CB) in the smectic-A phase. The intensity of the SHG from ultra-thin films with thickness varying from 2 to 10 molecular layers appeared quantized, but its behavior suggested no modifications of molecular arrangement in the layers in the presence of the two "free" surfaces. We have observed transient molecular reorientation induced in nematic 5CB films by a giant laser pulse. Even though the response time of molecular reorientation was several orders of magnitude longer than the laser pulsewidth, the effect on the refractive indices was still detectable by a Mach-Zehnder interferometer. Transient laser heating of the medium also affected the refractive indices, but with proper geometric and polarization arrangement, it could be decoupled from molecular reorientation and measured separately. Theoretical calculations using the Ericksen -Leslie theory for molecular reorientation in liquid crystals and the heat diffusion theory for the laser heating effect were able to explain the experimental observations quantitatively.