Nonlinear optical spectra of conjugated polymers: Effect of long-range coulomb interactions

Abstract

Nonlinear optical spectra of conjugated polymers are theoretically studied in the Su-Schrieffer-Heeger model supplemented by long-range Coulomb interactions. Excitonic correlation for electron-hole excitations in explicitly taken into account by a standard method. Nonlinear susceptibilities χ (3) are calculated numerically with a standard sum-over-states method for finite chains (up to 1000 sites). Using moderate interaction strength, our calculations can reproduce many distinct features observed in the linear and nonlinear spectra (two-photon absorption, third-harmonic generation, and electroabsorption) of polydiacetylenes and some other polymers. Especially, a hump in the spectrum of third-harmonic generation on the high energy side of its main peak is shown to be due to the three-photon resonance to weakly bound or unbound electron-hole states around the exciton ionization threshold.