2-PHOTON RESONANCE IN THG OF POLYMER

The THG spectrum chi(3)(omega) of trans-polyacetylene has two peaks at 0.6 and 0.9eV. While the first peak is unanimously understood to orig inate from the three-photon resonance across the energy gap, how to in terpret the second peak is a controversial issue. Both the single-part icle and correlated electron theories show that the two-photon resonan ce 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 prese nts a microscopic theory of the excitation-dependent damping, from whi ch the THG spectrum chi(3)(omega) can be calculated and agrees quantit atively with the experimental data.