New Physical Methods for a Space Resolved Mapping of the Macroscopic Polarisation in Molecular Crystals and a Stochastic Theory for Understanding

Abstract

The spatial distribution of polarity in molecular crystals built up by dipolar entities was imaged by two new techniques: Scanning pyroelectric microscopy (SPEM) and phase-sensitive second harmonic microscopy (PS-SHM). SPEM maps the variation in electrical polarisation due to a change in temperature (pyroelectric effect). The PS-SHM contrast is caused by a change of the sign of the molecular hyperpolarisability (nonlinear optical effect). A spatial inhomogeneity of polar properties in molecular crystals is predicted by a 2D Ising-type model describing 180° orientational disorder at the crystal-nutrient interface. As a result of (slow) growth, centric and acentric seed crystals can continuously transform into bipolar (twinned) crystals showing an inhomogeneous spatial distribution of polarity in the as-grown state.