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

Third-order nonlinear optical spectra chi((3)) 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 chi((3))(-omega;omega,-omega,omega) is approximately proportional to the linear absorption spectrum alpha(omega). Additional peaks show up in the chi((3)) 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 chi((3))(-omega;omega,O,O). In this situation the product of chi((3))(-omega;omega,O,O) and alpha(omega) is shown to be approximately proportional to chi((3))(-omega;omega-omega,omega).