Numerical coupled Liouville approach: the dependence of the second hyperpolarizability on field intensity and the size of linear molecular aggregates

The second hyperpolarizabilities (γ) of linear molecular aggregates composed of two-state molecules are calculated using the numerical coupled Liouville approach, which can describe the intermolecular interaction by using the propagation of retarded electric fields among the monomers. The γ values for intermediate-sized molecular aggregates under near-resonant intense external fields are found to exhibit abrupt changes as the field intensity increases. The spatial variations in γ values are investigated for several sizes of linear aggregates by elucidating the γ value at each molecule site under fields with different intensity.