Abstract
Molecules and materials with negative real parts of third-order nonlinear optical (NLO) polarizability Re(γ) are rare despite their advantages for applications such as all-optical switching. Although plasmonic metal nanoparticles can have a negative Re(γ) that increases in magnitude with a decrease in size, these properties are not yet understood from a quantum mechanical perspective. Here, we use quantum chemical approaches to model the NLO properties of prototypical silver nanoclusters. In linear Ag nanowires, the longitudinal excited-state properties are analogous to those of polyenes, leading to a positive Re(γ) that increases with nanowire length. In contrast, the transverse modes show plasmon-like mixing of excitations in the main one-photon excited state, leading to a negative Re(γ). The tetrahedral Ag20 cluster likewise has a negative Re(γ) due to plasmon-like mixing of excitations. On the basis of these results, we propose a new approach to obtain a large negative Re(γ) by identifying and tuning the structural features that contribute to plasmon-like mixing of excitations in the absorbing states.