Langmuir-Blodgett films and non-linear optics

Abstract

Recent work on the non-linear optical and electro-optical properties of Langmuir-Blodgett monolayers and multilayers is reviewed. The fundamental theory of non-linear optical susceptibilities and interactions is discussed, as well as their relationship to molecular hyperpolarizabilities. Among the non-linear optical experiments discussed are second and third harmonic generation, d.c. electric field induced second harmonic generation and the Pockels linear electro-optic effect. It is shown that surface second harmonic generation is a powerful tool for studying the orientation of molecules and their hyperpolarizabilities. Second harmonic generation from multilayers is also useful in understanding their structure and interactions. The interactions between monolayers and metallic surfaces are also studied by surface-plasmon-enhanced second harmonic generation. Finally, third harmonic generation is used to study the electronic spectra and resonances (two- and three-photon absorption) on very thin multilayers of polydiacetylenes. It is concluded that non-linear optics is a powerful and useful tool for the study of Langmuir-Blodgett films, and that there is a good understanding of fundamental theoretical and experimental problems. However, the details of the interpretation of non-linear optical experiments are not fully understood. Consistent results are not always obtained from the same sample. The internal optical electric field needs to be understood more fully. Electro-optic studies are still in their infancy, and they would complement the non-linear optical studies.