Second-order nonlinear-optical processes in orientationally ordered materials: relationship between molecular and macroscopic properties

Liquid crystals and polymer glasses can be formed into orientationally ordered materials by raising the temperature of the material to a temperature at which molecular motion is greatly enhanced, applying an external aligning field, and then cooling with the field applied. The resulting material exhibits second-order nonlinear-optical effects. In this paper, the relationship between the molecular hyperpolarizability and the macroscopic susceptibility is presented. The susceptibility is seen to depend on the microscopic order parameters commonly associated with liquid crystals and is discussed in the limits of one-dimensional molecules and poled polymer glasses. Agreement is found between the theory and second-harmonic-generation measurements of polymer glasses. Results of electro-optic measurements are compared with second-harmonic-generation measurements that suggest that the electro-optic effect is mostly electronic in origin.