Monolayer dielectrics and generation of Maxwell-displacement current and optical second harmonics

Optical second harmonic generation (SHG) and Maxwell displacement current (MDC) spectroscopy for the exploration of the dielectric polarization phenomena in organic monolayer film are described. It is shown that MDC and SHG are generated from the monolayers due to their noncentrosymmetric structure; the spontaneous polarization and the nonlinear polarization are the main contributions to the MDC and SHG, respectively. The developed MDC measurement coupled with the SHG measurement are shown to be helpful for the study of the orientation order and the phase transition of the monolayers, and thus helpful for the understanding of surface polarization phenomena. In order to show the effect of the SHG-MDC spectroscopy, the dielectric polarization of organic monolayer with C_∞-symmetry is theoretically analyzed with a simple rod-like model. The spontaneous and nonlinear polarization induced in this modeled monolayer system is expressed in orientational order parameters defined by the thermodynamic averages of Legendre polynomials of the orientation angle. The experiments on the SHG and MDCs of mesogenic liquid crystal (LC) monolayers of 4´n-octyl-4-cyanobiphenyl (8CB) on the air-water interface are shown. Finally, piezoelectric effect and flexoelectric effect in monolayers are briefly discussed.