Nonlinear optical response from excitons in soliton lattice systems of doped conjugated polymers

Abstract

Exciton effects on conjugated polymers are investigated in soliton lattice states. We use the Su-Schrieffer-Heeger model with long-range Coulomb interactions. The Hartree Fock approximation and the single-excitation configuration-interaction method are used to obtain optical absorption spectra. The third-harmonic generation (THG) at off-resonant frequencies is calculated as a function of the soliton concentration and the chain length of the polymer. The magnitude of the THG at 10% doping increases by a factor of about 102 from that of the neutral system. This is due to the accumulation of the oscillator strengths at the lowest exciton with increasing soliton concentration. The increase by the order two is common for various choices of Coulomb interaction strengths.