Exciton effects and nonlinear optical response in soliton lattice states of doped conjugated polymers

Exciton effects on conjugated polymers are investigated in the soliton lattice system. We use the Su-Schrieffer-Heeger model with long-range Coulomb interactions treated by the single-excitation configuration-interaction method. The soliton band is present in the Peierls gap of the doped system. There appears a new kind of the exciton where an electron-hole pair is excited between the soliton band and the continuum states. We find that the oscillator strengths accumulate rapidly at this exciton as the soliton concentration increases. The contribution from the lowest exciton is more than 90% at the 10% doping. The third-harmonic generation (THG) at off-resonance frequencies is calculated as functions of the soliton concentration and the chain length of the polymer. The optical nonlinearity by the THG at the 10% doping increases by the factor about 10^2 from that of the neutral system.