SOLITON-ANTISOLITON CONFIGURATIONS AND THE LINEAR AND NONLINEAR-OPTICAL RESPONSE OF DEGENERATE-GROUND-STATE CONJUGATED POLYMERS

The contributions of soliton-antisoliton (SSBAR) configurations to the linear and third-order nonlinear optical response are' investigated for conjugated polymers having a degenerate ground state. We treat the direct photoproduction of charged solitons as a nonlinear Franck-Condon problem and obtain analytical expressions for the linear optical susceptibility chi(1)(omega). With the help of the oscillator strength sum rule, we decompose the pi - pi* oscillator strength into two parts; a contribution where the final states are charged SSBAR pairs and a contribution where the final states are free electron-hole pairs (as in the noninteracting rigid lattice). The linear optical coefficients calculated from chi(1)(omega) are in general agreement with optical data obtained from trans-polyacetylene. The results imply that approximately 25% of the integrated oscillator strength of trans-polyacetylene arises from the direct photoproduction of solitons. A parallel treatment of the generalized third-order nonlinear optical susceptibility chi(3)(omega(sigma)) is presented, demonstrating that, for any third-order process, contributions arising from neutral SSBAR pair configurations as intermediate states are one to two orders of magnitude larger than the corresponding rigid-lattice contribution. This mechanism for chi(3) is enabled by nonlinear zero-point motion which provides a finite Franck-Condon overlap between the ground and SSBAR excited state lattice wave functions. The large contribution to chi(3) from the SSBAR intermediate states results from the large transition dipole moment between the free electron-hole pair excited states of B(u) symmetry and the A(g) symmetric neutral SSBAR excited state. This enhanced transition dipole moment is a consequence of the large virtual shifts of oscillator strength associated with the localized SSBAR electron-lattice configuration. The third-harmonic conversion efficiency chi(3)(3omega) is further enhanced by a condition unique to degenerate-ground-state systems, simultaneous two- and three-photon resonance.