RESONANCE AND LOCAL-FIELD EFFECTS IN THE CHARACTERIZATION OF MOLECULAR MONOLAYERS BY OPTICAL 2ND-HARMONIC GENERATION

Langmuir-Blodgett (LB) monolayers are ideal model systems for testing the design parameters of molecular units for ultrathin film nonlinear optical devices. The second-harmonic response of a mixed liquid crysta l-stearic acid monolayer as a function of liquid crystal concentration is used here to measure the local field in the monolayer. The effect of the local field is much larger than the resonance behaviour in this system. The apparent molecular hyperpolarizability beta in pure monol ayers is reduced by a factor of six due to the local field. The other effect of the local field is to increase the apparent tilt angle of th e liquid crystals by up to 20-degrees. These effects are significant i n the design of materials for ultrathin film nonlinear optical devices . Determination of the true beta values of an isolated chromophore sho uld be made using monolayer films where the active species is diluted by about a factor of 10 by a species with negligible polarizability, h aving first established that the active species disperses uniformly. I n this way the experimental hyperpolarizability may be directly compar ed, for example, to theoretical values from calculations of isolated m olecular clusters.