Quantum Lattice Fluctuations and the Linear and Nonlinear Optical Susceptibility of Degenerate Ground State Conjugated Polymers

The contributions of soliton-antisoliton (SS) 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, χ(1)(ω). With the help of the oscillator strength sum rule, we decompose the π-π* oscillator strength into two parts; a contribution where the final states are SS 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 χ(1)(ω) 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, χ(3)(ωσ), is presented, demonstrating that, for any third-order process, contributions arising from neutral SS pair configurations as intermediate states are one to two orders of magnitude larger than the corresponding rigid-lattice contribution. This mechanism for χ(3) is enable by nonlinear zero-point motion which provides a finite Franck-Condon overlap between the ground and SS excited state lattice wavefunctions. The large contribution to χ(3) from the SS intermediate states results from the large transition dipole moment between the free electron-hole pair excited states of Bu symmetry and the Ag symmetric neutral SS excited state. This enhanced transition dipole moment is a consequence of the large configuration. The third-harmonic conversion efficiency χ(3)(3ω) is further enhanced by a condition unique to degenerate ground state systems, simultaneous two and three-photon resonance.