The third-order nonlinear optical response of noble-metal nanoparticles embedded in a dielectric matrix depends on the particle intrinsic third-order susceptibility ${\chi }_m^{\left(3\right)}$. We propose a model which allows one to calculate the hot electron contribution ${\chi }_{\text{he}}^{\left(3\right)}$ to ${\chi }_m^{\left(3\right)}$ 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 ${\chi }_{\text{he}}^{\left(3\right)}$. Finally, the variation in ${\chi }_{\text{he}}^{\left(3\right)}$ with intensity in the high excitation regime is discussed for picosecond pulses.

• Received 6 June 2008

DOI:https://doi.org/10.1103/PhysRevB.79.045410