Plasmonic Resonances in Ag-Au Nanosphere Heterodimers: How Accurate Are the Approximate Analytical Descriptions?

Modeling and computation of the optical response of noble metal nanoparticles and their assemblies have played a pivotal role in plasmonics research. The methodologies adopted range from analytical approaches such as the Mie theory and the quasi-static approximation to the computationally intensive approaches such as the finite element method, T-matrix method, finite-difference time-domain (FDTD) method, and the boundary element method. Metal nanosphere homodimers have been employed by researchers as prototype systems for discerning the fundamental aspects of plasmon coupling, leading to the emergence of applications such as plasmon rulers and surface-enhanced spectroscopy. In contrast, nanosphere heterodimers have received much less attention. Herein, we employ analytical approaches based on the electrostatic and the electrodynamic polarizabilities as well as the Mie theory to describe the optical resonances in Ag-Au nanosphere heterodimers within the framework of the coupled dipole approximation. The computationally intensive FDTD method is further used as a benchmark to assess the performance of the analytical methodologies. The incorporation of the radiative damping and retardation effects by way of electrodynamic dimer polarizabilities results in very good agreement of the spectral peak positions as well as the intensity patterns with those obtained from the FDTD simulations. The computational costs associated with the analytical approaches are much lower than those of the FDTD method, and therefore such approaches are immensely valuable in plasmonics research.