Dipole-like and quadrupole-like reflection modes for Ag nanocube arrays on dielectric substrates

Abstract

Localized surface plasmons (LSPs) have a wide range of applications in enhancing the performance of optoelectronic devices. For those applications, LSPs are often located on the surfaces or interfaces between dielectric mediums. Hence, it is necessary to investigate interaction between LSPs and interface. In this paper, we investigate the far-field and near-field LSP behaviors of silver nanocube arrays on a dielectric substrate. Finite-difference time-domain simulation results demonstrate that, when light is incident normally from the vacuum, a much deeper dip between the two peaks corresponding to bonding and anti-bonding modes of the LSPs generated in the metal nanocubes on dielectric mediums, comparing to the case that incident light is from the substrate. The charge distribution diagram shows that these behaviors can be described as a dipole-like mode and a quadrupole-like mode. A model based on the Fano interferences using modified Fresnel equations is employed to explain the physical mechanism of these behaviors. It reveals that this phenomenon is caused by the phase difference between the superimposed dipolar modes generated on the upper and lower interfaces of nanocube. A simplified mathematical model has been built to illustrate that symmetric dips can arise from the Fano interference between two discrete states and one continuum state.