Environmental effects on molecular response in materials for non-linear optics

The concept of a molecule in a material is discussed. The properties of such entities differ from those of the free molecules. These differences are explored with particular reference to non-linear optical response. Molecular distortion induces changes exemplified by a non-zero static first hyperpolarizability β for the S₈ molecules in the crystalline complex CHI₃ · 3S₈, which dominates the net response. Strong permanent electric fields modify the response, but for urea can be calculated accurately enough to reproduce the refractive indices and quadratic susceptibility reasonably accurately. Non-uniform fields also have significant effects especially in combination with non-uniform response, as shown by calculations on model Langmuir-Blodgett films and on surface-induced second-harmonic generation (SHG). Orbital confinement tends to suppress response but may be less important in large molecules. In general, good quantitative theories of these factors are now available.