Correlation-function description of the ac orientational Kerr susceptibility of plastic crystals

An expression is derived for the optical-field-induced birefringence due to molecular reorientations in cubic plastic crystals. The effect is described by a nonlinear susceptibility tensor shown to be proportional to a tensorial correlation function. For most cubic point groups this correlation function has only two independent components. A decomposition into irreducible tensorial sets leads to the two physically meaningful quantities. One is labeled the tensor strength. It measures the short-range angular correlation of molecular pairs. It is a susceptibility that increases as this range increases, i.e., as the plastic-solid phase transition is approached. The second quantity is labeled the tensor anisotropy. It measures the long-range cubic anisotropy of the molecular orientations. It is related to an orientational order parameter of the plastic-liquid transition. © 1974 The American Physical Society.