The effects of size quantization on the nonlinear optical response of Ag nanoparticles are experimentally studied by spectroscopic ellipsometry and femtosecond spectroscopic pump-and-probe techniques. In the vicinity of a localized surface-plasmon resonance (2.0–3.5 eV), we have investigated the optical nonlinearity of Ag particles embedded in silica glass for particle diameters ranging from 3.0 to 16 nm. The intrinsic third-order optical susceptibility ${\chi }_m^{\left(3\right)}$ of Ag particles exhibited significant spectral and size dependences. These results are explained as quantum and dielectric confinements and are compared to the results of theoretical quantum finite-size effects calculation for metallic particles. In light of these results, we discuss the contribution of interband transitions to the size dependence of ${\chi }_m^{\left(3\right)}$. Quantum size effects lead to an increase in nonlinearity in small Ag particles.

• Received 21 April 2014
• Revised 27 August 2014

DOI:https://doi.org/10.1103/PhysRevB.90.125417