Nonlinear optical studies of monomolecular films under pressure

Nonlinear optical techniques (second-harmonic and sum-frequency generation) have been used to study the structure of organic molecules that are confined and compressed between a lens and a flat surface. The molecules studied include self-assembled monolayers of n-octadecyltriethoxysilane and Langmuir-Blodgett films of stearic acid, octadecylalcohol, octadecylamine, and a liquid-crystal molecule 4’-n-octyl-4-cyanobiphenyl (8CB). The contact area created by elastic deformation of the flat surface and lens under pressure was large enough to contain the entire laser beam (>100 μm radius at ≊10 MPa for R=15 cm). Under these conditions, the sum-frequency generation (from ${\mathrm{CH}}_3$ and OH stretch modes) and second-harmonic generation (8CB) signals were found to decrease by a factor between 100 and 1000 times the original signal. This indicates vanishing of the second-order monolayer susceptibility due to disorder of the head groups and/or flattening of the molecular axis so that they lie parallel to the surface. The phenomenon was reversible and the nonlinear signals recovered completely upon removal of the pressure.