THE EFFECT OF SIDE-CHAIN SUBSTITUENTS ON 3RD-ORDER OPTICAL NONLINEARITY OF CONJUGATED POLYMERS - A THEORETICAL-STUDY

We present a detailed investigation concerning the influence of phenyl side groups, adjacent to a carbon backbone, on the third-order nonlin ear optical susceptibility chi((3)) (omega) Of conjugated polymers. A theoretical calculation of chi((3)) (omega) is carried out within the simple two-band model of the electronic structure of the polymer based on a tight-binding approximation. Our treatment takes into considerat ion two sources of optical nonlinearity: the interband transitions of rr-electrons and combined (interband-intraband) ones. The influence of the substituent groups upon the electronic structure of the polymer i s modeled by changing the Coulomb integral on the sites of the main ch ain to which the side groups are attached. We adopt the Genkin-Mednis approach to derive analytical expressions for chi((3)) (omega) respons ible for the third-harmonic generation, the intensity-dependent index of refraction and the two-photon absorption. The spectra chi((3)) (ome ga) are evaluated numerically for all these effects and the resonance structure of the spectra is analyzed depending on the distance between the side groups and their coupling strength with the backbone. It is found that the interaction between the side groups and the backbone re sults in a change of the sign of chi((3)) (0) from positive (which is predicted for polyacetylene) to negative and leads to an enhancement o f the chi((3)) (0) coefficient with increasing side-group separation a long the backbone. The resonance behavior of chi((3)) (omega) in the t ransparency region of the polymer also reveals remarkable differences from that observed experimentally in polyacetylene. Our results show t hat the optimal positioning of the substituent groups along the backbo ne may be promising for engineering new polymer materials with signifi cantly enhanced nonlinear optical properties. (C) 1998 Elsevier Scienc e S.A. All rights reserved.