Electronic structure and third-order nonlinear optical response of conjugated polymer chains bearing periodically arranged side groups

The electronic structure of an infinite one-dimensional polyene chain bearing periodically arranged phenyl side groups is calculated using the Green function method within a tight-binding (Hückel) model. It is shown that the side groups have a striking effect on the energy band structure: they split the energy bands of the polyene chain into a set of minibands and minigaps, the number and width of which can be governed by changing the distance between the side groups and the degree of their conjugation with the polyene backbone. We calculated the third-order nonlinear optical susceptibility χ(3) caused by non-parabolicity of the electron dispersion law in the lowest conduction miniband which is supposed to be partially filled with π-electrons. It is found that χ(3) is directly proportional to the width of the miniband and shows a four-power dependence on the distance between the side groups. The numerical estimates indicate that in the transparency region χ(3) can reach values comparable to those of inorganic materials with strong nonlinear optical properties.