Crossover Phenomenon in Third-Order Nonlinear Optical Susceptibilities of Gold Nanoparticles from Plasmons to Discrete Electronic States

Abstract

We report on the size-dependent behavior of third-order nonlinear optical susceptibilities, χ(3), of thiolate monolayer-protected Au clusters and Au nanoparticles embedded in SiO2 in the size range between 25 and 170 000 atoms. The imaginary part of χ(3) is negative and increases with increasing size above 135 Au atoms, while it is positive and increases with decreasing size below 66 Au atoms. These results reveal a crossover from the local-field effect due to surface plasmons in the metal electron system to the quantum size effect of a molecule-like electronic system. From the transient absorption spectra in the femtosecond time region, it is found that the optical nonlinearity in the quantum-size region reflects the characteristic electronic states due to geometric and electronic interactions between the core and ligand in the Au clusters.