Nonlinear optical properties of linear chains and electron-correlation effects

The large third-order nonlinear optical properties γᵢⱼₖₗ(-ω₄;ω₁,ω₂,ω₃) observed for conjugated organic linear chains, such as polyenes, are explained by strong electron-correlation behavior in virtual, two-photon ¹Ag states. The dominant chain axis component γₓₓₓₓ is found to possess a power-law dependence on the chain length with an exponent of 4.6 ±{} 0.2, independent of chain conformation. Good agreement in sign and magnitude is obtained between calculated and experimental gas-phase values of γᵢⱼₖₗ(-2ω;ω,ω,0).