THE EFFECTS OF SURFACE-LAYERS ON 3RD-HARMONIC GENERATION FROM SOLUTIONS OF A NEMATOGENIC POLYMER

Third harmonic generation (THG) is used to study the third-order nonli near optical properties of nematic and isotropic solutions of poly(1,4 -phenylene-2,6-benzobisthiazole) and related small-molecule model comp ounds. Maker fringe patterns (MFP) obtained using both plane slab and wedge-shaped cells for fundamental wavelengths of 1.542 and 1.907 mu m are analyzed in terms of postulated surface layers in the isotropic p reparations. The layers are postulated to comprise regions with concen tration smaller than for the bulk sample, in which the rod-like chains tend to have their axes parallel to the plane of the surface, creatin g a negatively birefringent uniaxial nematic layer. The THG with the n ematic solution exhibits intensity with polarization components unexpe cted for the uniaxial nematic symmetry attributed from linear optical properties, along with other unexpected features in the MFP. This beha vior is attributed to the effects of surface layers, postulated to exh ibit biaxial nematic symmetry, in which the uniaxial nematic symmetry is broken in regions on the scale of a wavelength. This provides for a veraging that does not cause departure from the linear optical behavio r expected for a uniaxial nematic, but does produce effects on nonline ar optical properties. (C) 1996 John Wiley & Sons, Inc.