ELECTRONIC-STRUCTURE AND 3RD-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 t he Green function method within a tight-binding (Huckel) model. It is shown that the side groups have a striking effect on the energy band s tructure: they split the energy bands of the polyene chain into a set of minibands and minigaps, the number and width of which can be govern ed by changing the distance between the side groups and the degree of their conjugation with the polyene backbone. We calculated the third-o rder nonlinear optical susceptibility chi((3)) caused by non-parabolic ity of the electron dispersion law in the lowest conduction miniband w hich is supposed to be partially filled with pi-electrons. It is found that chi((3)) is directly proportional to the width of the miniband a nd shows a four-power dependence on the distance between the side grou ps. The numerical estimates indicate that in the transparency region c hi((3)) can reach values comparable to those of inorganic materials wi th strong nonlinear optical properties.