Nonlinear optical susceptibilities of conducting polymers

Abstract

By use of the Genkin-Mednis approach, a general formalism of the nonlinear optical susceptibilities has been derived for one-dimensional electron-lattice systems. Based on the Su-Schrieffer-Heeger model, we get an analytic expression of the third-harmonic generation ${\mathrm{\chi }}^{\mathrm{}\left(3\right)\mathrm{}}$(ω) of conducting polymers. After considering the effects of finite lifetime of the excited states, the cusp at ħω=Δ is greatly depressed so that it becomes too small to interpret the observed peak of ${\mathrm{\chi }}^{\mathrm{}\left(3\right)\mathrm{}}$(ω) at 0.9 eV. The case of nondegenerate polymers and the effect of electron-electron interaction in the unrestricted Hartree-Fock approximation are also discussed. Our results imply that the electron interaction would play an important role in the nonlinear optical properties of conducting polymers.