Second Harmonic Generation by Merocyanine in Mesomorphic Films. An Anomalous Electrostatic Field Effect

Poled thin films of merocyanine obtained by photo- or thermochromic conversion of spiropyran incorporated in various mesogenic molecules, including liquid-crystal polymers, quasi-liquid crystals, and blends, exhibit strong second harmonic generation (SHG). As well as the “conventional” SHG deriving from the χ⁽²⁾_xxx component, where x is the poling direction (in the plane of the film), the films exhibit an even stronger anomalous “perpendicular” SHG via χ⁽²⁾_zzz hyperpolarizability when an electric field in the x direction is reapplied to the samples at room temperature. It is believed that this anomalous SHG arises from merocyanine aggregates stacked in the z direction being rotated by the field.