Liquid Crystal Alignment Layers: Non-destructive Determination of Ordering with Non-linear Optical Spectroscopy

Objective: Develop non-destructive, optical probes of substrate characteristics relevant to the alignment of liquid crystal adsorbates.

Problem: The fabrication of liquid crystal displays currently involves the empirically optimized mechanical preparation of a polymeric substrate to induce alignment in a contacting liquid crystal. Optimization and control of the preparation of these alignment layers is significantly hampered by the absence of both non-destructive diagnostics of the prepared layer and a fundamental understanding of the liquid crystal/alignment layer interaction.

Approach: In centrosymmetric media, second harmonic generation (SHG) is symmetry forbidden, imparting extreme surface sensitivity to this nonlinear optical (NLO) technique. Additionally, SHG is more sensitive to interface structure than linear optical techniques by virtue of the transformation properties of the relevant mixing coefficients. SHG has been employed by various researchers to study adsorbed liquid crystal (LQ) monolayers on mechanically processed substrates with great success. There are essentially no reports in the literature of SHG studies of the LQ alignment layers in the absence of LQ monolayers. Characterization of the alignment layer itself should lead to a greater understanding of the fundamental interactions important to LQ assembly, and possibly result in a useful analytic tool for the processing of LQ films.
 

Results and Future Plans: In FY95 SHG studies were performed on LQ alignment layers consisting of a thin flim of polyimide copolymer doped with a dye to enhance the SHG response. Shown in the figure is the second harmonic at 532 nm detected following 1064 nm irradiation of a heavily buffed sample as a function of the orientation of the buffing direction with respect to the plane-of-incidence and the 532 nm and 1064 nm polarizations. The data is presented as a polar plot.  Clearly the SHG technique has adequate sensitivity for characterization of alignment layers containing dye dopants. Analysis of the SHG data establishes that the layer has Cs symmetry, i.e. the buffing not only establishes a preferred alignment, but actually orients the film. Additional studies on a series of films with varying degrees of buffing are underway to quantitatively correlate the asymmetry of the alignment layers with their ability align LQ films.

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