A classical model of the upper bounds of the cascading contribution to   the second hyperpolarizability
  We investigate whether microscopic cascading of second-order nonlinearities of two molecules in the side-by-side configuration can lead to a third-order molecular nonlinear-optical response that exceeds the fundamental limit. We find that for large values of the second hyperpolarizability, the side-by-side configuration has a cascading contribution that lowers the direct contribution. However, we do find that there is a cascading contribution to the second hyperpolarizability when there is no direct contribution. Thus, while cascading can never lead to a larger nonlinear-optical response than for a single molecule with the same number of electrons, it may provide design flexibility in making large third-order susceptibility materials when the molecular second hyperpolarizability vanishes 
Nathan J. Dawson, Benjamin R. Anderson, Jennifer L. Schei, and Mark G.   Kuzyk
physics.optics physics.atm-clus physics.chem-ph