Nonlinear optical susceptibilities of conjugated polymers: Damping, resonances, and scaling laws

Third-order nonlinear optical spectra ${\mathrm{\chi }}^{\mathrm{}\left(3\right)\mathrm{}}$ of conjugated polymers are theoretically studied within the treatment of double-excitation configuration interaction for the Pariser-Parr-Pople model. We consider a realistic situation of dephasing rates being much larger than depopulation rates. It is shown that, on the low-energy side of the one-photon resonance, the absolute value of ${\mathrm{\chi }}^{\mathrm{}\left(3\right)\mathrm{}}$(-ω;ω,-ω,ω) is approximately proportional to the linear absorption spectrum α(ω). Additional peaks show up in the ${\mathrm{\chi }}^{\mathrm{}\left(3\right)\mathrm{}}$ spectrum at frequencies corresponding to energy differences among excited states. Their intensities can become comparable to and even stronger than ordinary two-photon absorption peaks in the same energy region. The same is true for the electromodulation spectrum ${\mathrm{\chi }}^{\mathrm{}\left(3\right)\mathrm{}}$(-ω;ω,0,0). In this situation the product of ${\mathrm{\chi }}^{\mathrm{}\left(3\right)\mathrm{}}$(-ω;ω,0,0) and α(ω) is shown to be approximately proportional to ${\mathrm{\chi }}^{\mathrm{}\left(3\right)\mathrm{}}$(-ω;ω-ω,ω). © 1996 The American Physical Society.