QUANTUM-LATTICE FLUCTUATIONS AND THE LINEAR AND NONLINEAR-OPTICAL SUSCEPTIBILITY OF DEGENERATE GROUND-STATE CONJUGATED POLYMERS

The contributions of soliton-antisoliton (SSBAR) configurations to the linear and third-order nonlinear optical response are investigated fo r conjugated polymers having a degenerate ground state.1),2) We treat the direct photoproduction of charged solitons as a nonlinear Franck-C ondon problem and obtain analytical expressions for the linear optical susceptibility, chi(1)(omega). With the help of the oscillator streng th sum rule, we decompose the pi-pi oscillator strength into two part s; a contribution where the final states are SSBAR pairs and a contrib ution where the final states are free electron-hole pairs (as in the n oninteracting rigid-lattice). The linear optical coefficients calculat ed from chi(1)(omega) are in general agreement with optical data obtai ned from trans-polyacetylene. The results imply that approximately 25% of the integrated oscillator strength of trans-polyacetylene arises f rom the direct photoproduction of solitons. A parallel treatment of th e generalized third-order nonlinear optical susceptibility, chi(3)(ome ga), is presented, demonstrating that, for any third-order process, co ntributions arising from neutral SSBAR pair configurations as intermed iate states are one to two orders of magnitude larger than the corresp onding rigid-lattice contribution. This mechanism for chi(3) is enable d by nonlinear zero-point motion which provides a finite Franck-Condon overlap between the ground and Sg excited state lattice wavefunctions . The large contribution to chi(3) from the SSBAR intermediate states results from the large transition dipole moment between the free elect ron-hole pair excited states of B(u) symmetry and the A(g) symmetric n eutral SSBAR excited state. This enhanced transition dipole moment is a consequence of the large virtual shifts of oscillator strength assoc iated with the localized Sg 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.