Fine-tuning of plasmonics by Au@AuY/Au core–shell nanoparticle monolayer for enhancement of third-order nonlinearity

The manipulation of plasmonics on noble metal nanoparticles (NPs) is of great interest in developing nonlinear photonic devices, such as all-optical switches and frequency combs. An [email protected]/Au-shell nanoparticle ([email protected]/Au NP) monolayer is proposed for the fine-tuning of plasmonics and enhanced third-order nonlinearity. Based on the different thermodynamic mechanisms of Au and Y ions, the compact [email protected]/Au core–shell architectures are designed and surface-modified in fused silica (SiO2) with enhanced free electron density, mobility, and quantum size effect. The flexible modulation of plasmonics is realized, resulting in significant absorption enhancement (165% for interband absorption and 38% for free electron absorption, respectively) and fine-tuning of the localized surface plasma resonance (LSPR) band. In addition, the physical mechanism is investigated by density functional theory (DFT) and Mie theory, which reveals a transition from size-independence to size-dependence of LSPR owing to the synergistic effect of multiple physical factors such as free electron density and mobility. With the above advantages, the third-order nonlinearity is enhanced by 4.4 times compared with traditional Au NPs. It indicates the significant potential of [email protected]/Au core–shell NP monolayer in the performance improvement of nonlinear photonic devices.