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Nanometric Interfacial Polarization Phenomena in Organic Monolayer Films Studied by Maxwell Displacement Current-Second Harmonic Generation Spectroscopy

Optical second-harmonic generation (SHG) and Maxwell displacement current (MDC) spectroscopy for the study of the dielectric polarization phenomena in organic monolayer films is described. It is shown that a spontaneous polarization and a nonlinear polarization are induced in monolayers due to symmetry breaking at the interface, and this leads to the generation of an MDC and an SH signal, respectively. The spontaneous and nonlinear polarizations are specified using orientational order parameters expressed using Legendre polynomials of orientational angle. The MDC-SHG experiments were carried out for mesogenic liquid crystal (LC) monolayers of 4'-n-octyl-4-cyanobiphenyl (8CB) on the air-water interface. It is shown that an MDC and an SH signal were generated reversibly in 8CB monolayers by compression and expansion.