Characterization of high order modes of plasmonic antenna formed by nanoparticle/thin film hybrid structures

Abstract

The plasmonic modes of a nano-antenna formed by a nanoparticle/thin film hybrid system are investigated. Plasmonic nano-antennas are well-known for their capabilities to concentrate electromagnetic wave into extreme small region and couple the emission from active materials in proximity to the antennas into far-field region. Previously, we have shown through direct measurement of emission profile images that the nano-antennas not only enhance Raman emission but also systematically direct inelastic emission to the far-field through the dipole mode. We also showed that high order modes of the hybrid structure can be detected. Here, the higher order plasmonic modes are further characterized through simulations of spectrum and scattering profile. Through spectral simulation with improved resolution, two distinct modes are found to contribute to the broad band high order mode. One has dipole-like behavior and the other has quadrupolelike behavior. The modes are characterized both through near-field distribution and far-field scattering profiles. The quadrupole-like mode can be excited by both p- and s-polarized light whereas the dipole-like mode is only excited by ppolarized light. These high order modes are not as bright as the dipole mode in the far-field spectrum but have substantial near field enhancement which can be utilized for surface-enhancing spectroscopy and sensing. In addition, characterization of high order modes may serve to clarify the interaction between nano-antenna and active materials and will lead to design rules for applications of active plasmonic structures in integrated optical circuits.