Near-Field Optical Properties of Ag x Au1−x Nanoparticle Chains Embedded in a Dielectric Matrix

Abstract

We study by the finite-difference time-domain method the near-field optical properties of isolated or coupled Ag x Au1−x alloy nanoparticles shallowly buried inside dielectric matrices. The optical index of alloys is obtained experimentally using spectroscopic ellipsometry measurements from multilayered thin films fabricated by ion-beam sputtering. Then, we numerically investigate the influence of the nanoparticle composition, interparticle gap and capping-layer thickness on the amplitude and spatial extent of the electric field in the vicinity of ellipsoidal nanoparticles. Our calculations provide evidence that pure metal nanoparticles (Ag or Au) exhibit a greater field enhancement associated with a larger out-of-plane extent compared to alloy nanoparticles, an effect that is even more pronounced when the optical index of surrounding matrix is increased. Moreover, we show that the optimal gap between nanoparticles to maximize the amplitude of the electric field at the capping layer/air interface results from a delicate balance, which strongly depends on the thickness of the dielectric capping layer.