Characterizing the optical response of symmetric hemispherical nano-dimers

We present the first comprehensive study of a hemispherical nano-dimer interacting with an optical field. We characterize the optical response of the hemispherical dimer numerically using the finite-element method. The qualitative insight gained through the numerical analysis is enhanced with a derivation of an analytical approximation for the polarizability of a symmetric hemispherical dimer by invoking the dipole-dipole approximation. The results explain the effects of inter-particle spacing and the polarization of external excitation on the extinction spectra. Considering three configurations of hemispherical dimers, we show that both the frequency and the strength of plasmon resonances in each configuration are highly dependent on the inter-particle distance and the state of polarization of the incident light. We also show that, in the case of longitudinal polarization and an edge-to-edge orientation, the hemispherical dimer provides much better near-field confinement and produces much more enhancement of the electric field than a spherical dimer of the same volume. This result should prove useful for sensing and SERS applications.