Nonlinear optics

Conjugated polymers and other systems with delocalized electrons exhibit large nonresonant third-order nonlinear optical responses. However, the magnitude of the response still falls short of that needed for many proposed technological uses. We will present a series of model calculations that explore the origins and structure-property relationships of the hyperpolarizability in these systems. Our reference point is Huckel, or tight-binding, theory. While Huckel theory does predict a large nonlinear optical response for these materials, it fails to correctly describe many qualitative features of the excited electronic states that mediate the response. Our "scattering formalism" for third-order nonlinear optical calculations is the first method that can include electron-correlation in calculations of long polymer chains, in a manner that reproduces features such as low lying Ag symmetry states. The results indicate that while electron correlation does have large effects on individual electronic states, the nonresonant response is almost exclusively a function of (i) the optical gap and (ii) the bandwidth of the material. Furthermore, the functional dependence on these two properties is nearly identical to the functional dependence predicted by Huckel theory.