Theory of Molecular Nonlinear Optics

The theory of molecular nonlinear optics based on the sum-over-states (SOS) model is reviewed. The interaction of radiation with a single wtpisolated molecule is treated by first-order perturbation theory, and expressions are derived for the linear (alpha(ij)) polarizability and nonlinear (beta(ijk), gamma(ijkl)) molecular hyperpolarizabilities in terms of the properties of the molecular states and the electric dipole transition moments for light-induced transitions between them. Scale invariance is used to estimate fundamental limits for these polarizabilities. The crucial role of the spatial symmetry of both the single molecules and their ordering in dense media, and the transition from the single molecule to the dense medium case (susceptibilities. chi((1))(ij), chi((2))(ijk), chi((3))(ijkl), is discussed. For example, for beta(ijk), symmetry determines whether amolecule can support second-order nonlinear processes or not. For asymmetric molecules, examples of the frequency dispersi