Investigation of surface-induced alignment of liquid-crystal molecules by optical second-harmonic generation

We apply the technique of optical second-harmonic generation to study homogeneously aligned liquid-crystal cells. The surface dipole sensitivity of the technique made it possible to study the monolayer in the absence and presence of a bulk of liquid crystal. By comparing the monolayer orientational distribution functions of three surface treatments (rubbed polymer-coated substrates, rubbed surfactant-coated substrates, and substrates made from oblique evaporation of ${\mathrm{SiO}}_{\mathit{x}}$ film), we find that two different surface-originated mechanisms are effective in aligning liquid-crystal films. For rubbed polymer samples, it is shown that a short-range molecular interaction is responsible for alignment of the first monolayer, which then aligns the bulk via an epitaxylike interaction. Results on polymers with various structures and compositions and rubbed with a variety of rubbing strengths are presented. For the other surface treatments, the first monolayer is isotropically distributed, indicating that a bulk elastic interaction is responsible for the bulk alignment.