Nonlinear polarizability of correlated one-dimensional systems

Polymers have been found to be materials with very nonlinear optical properties. Previous theories of polymer polarizability have used noninteracting electronic models. We study the effect of electronic correlations on the polarizability and hyperpolarizability of one-dimensional systems. We consider the Hubbard model for interacting systems and solve exactly finite chains in the presence of an electric field. For the case of one electron per site, the linear polarizability decreases with interaction, as expected from the decrease of charge fluctuations. The hyperpolarizability, however, shows a more interesting behavior: it first increases as a function of interaction and then decreases after reaching a maximum. We understand this effect in a spin-dependent Hartree-Fock treatment of the electronic correlations.