Influence of laser pulse characteristics on the hot electron contribution to the third-order nonlinear optical response of gold nanoparticles

The third-order nonlinear optical response of noble-metal nanoparticles embedded in a dielectric matrix depends on the particle intrinsic third-order susceptibility χₘ⁽³⁾. We propose a model which allows one to calculate the hot electron contribution χₕₑ⁽³⁾ to χₘ⁽³⁾ in the case of gold. This phenomenon stems from the modification of the conduction-electron distribution induced by an optical excitation, and is significant when picosecond or subpicosecond laser pulses are considered. We show, in the case of a weak perturbation, the importance of the athermal regime for pulse widths lower than about 1 ps. Applying this model to two different samples, we then highlight the strong influence of the linear optical properties of the material on the spectral dispersion of χₕₑ⁽³⁾. Finally, the variation in χₕₑ⁽³⁾ with intensity in the high excitation regime is discussed for picosecond pulses.