Electronic-oscillator analysis of femtosecond four-wave mixing in conjugated polyenes

Equations of motion which describe the nonlinear optical response of conjugated polyenes using a collective electronic-oscillator representation are derived. Specific signatures of electronic correlations which enter as anharmonicities and scattering between oscillators are predicted in ultrafast resonant four-wave mixing. Only few resonant oscillators need to be considered explicitly; effects of the remaining (off-resonant) oscillators are introduced via renormalized anharmonic coupling coefficients. The connection with inorganic semiconductors is established.