THE EFFECT OF THE LOCAL-FIELD ON THE OPTICAL 2ND-HARMONIC RESPONSE OFMIXED LIQUID-CRYSTAL STEARIC-ACID MONOLAYERS

Optical second-harmonic generation (SHG) from mixed liquid crystal-ste aric acid molecular monolayers, as a function of liquid crystal concen tration, is reported. It is shown that, in concentrated monolayers, a substantial local-field effect exists which can be described, to a fir st approximation, by a point dipole model. The apparent molecular hype rpolarizability in pure monolayers of these liquid crystals is reduced by a factor of between six and eight, due to this local field. This i s an important consideration in the design of materials for ultra-thin -film non-linear optical devices. Hyperpolarizability measurements sho uld be made using monolayer films where the active species is diluted by about a factor of ten by a species with negligible polarizability, such as a fatty acid, having first taken care to establish that the ac tive species disperses uniformly. In this way the experimental hyperpo larizability may be directly compared to theoretical values from calcu lations of isolated molecular clusters. These LB monolayers are ideal model systems for testing theoretical approaches to the local-field pr oblem, and progress in this area is important in understanding the opt ical response of surfaces and interfaces in general, and those of semi conductors in particular.