Nonlinear optical properties of linear chains and electron-correlation effects

Abstract

The large third-order nonlinear optical properties ${\ensuremath{\gamma}}_{\mathrm{ijkl}}(\ensuremath{-}{\ensuremath{\omega}}_{4};{\ensuremath{\omega}}_{1},{\ensuremath{\omega}}_{2},{\ensuremath{\omega}}_{3})$ observed for conjugated organic linear chains, such as polyenes, are explained by strong electron-correlation behavior in virtual, two-photon $^{1}A_{g}$ states. The dominant chain axis component ${\ensuremath{\gamma}}_{\mathrm{xxxx}}$ is found to possess a power-law dependence on the chain length with an exponent of 4.6 \ifmmode\pm\else\textpm\fi{} 0.2, independent of chain conformation. Good agreement in sign and magnitude is obtained between calculated and experimental gas-phase values of ${\ensuremath{\gamma}}_{\mathrm{ijkl}}(\ensuremath{-}2\ensuremath{\omega};\ensuremath{\omega},\ensuremath{\omega},0)$.