Two-photon resonance in THG of polymer

The THG spectrum φ³(ω) of trans-polyacetylene has two peaks at 0.6 and 0.9e V. While the first peak is unanimously understood to originate from the three-photon resonance across the energy gap, how to interpret the second peak is a controversial issue. Both the single-particle and correlated electron theories show that the two-photon resonance can give the second peak, but its position is higher than 0.9eV. A phenomenological consideration indicates that the excitation-dependent damping shifts the second peak to the lower energy. This paper presents a microscopic theory of the excitation-dependent damping, from which the THG spectrum φ³(ω) can be calculated and agrees quantitatively with the experimental data.