Intrinsic pulse-width dependence of third-order nonlinear optical susceptibility in the femtosecond regime.

We demonstrate that the second-order coupling of local polarizations contributes significantly to the third-order nonlinear optical susceptibility chi((3)) through cascaded processes, as much as does direct third-order coupling. Temporal analysis of nonlinear polarization shows that chi((3)) depends intrinsically on pulse width if the pulse width is shorter than about 10 times the relaxation time of nonlinear polarization. Analysis of the pulse-width dependence of the third-order polarization in a femtosecond regime may differentiate the second-order local cascading process and the direct third-order process.