Plasmonic mode mixing in nanoparticle dimers with nm-separations via substrate-mediated coupling

We fabricate arrays of metallic nanoparticle dimers with nanometer separation using electron beam lithography and angle evaporation. These "nanogap" dimers are fabricated on thin silicon nitride membranes to enable high resolution transmission electron microscope imaging of the specific nanoparticle geometries. Plasmonic resonances of the pairs are characterized by dark-field scattering micro-spectroscopy, which enables the optical scattering from individual nanostructures to be measured by using a spatially-filtered light source to illuminate a small area. Scattering spectra from individual dimers are correlated with transmission electron microscope images and finite-difference time-domain simulations of their electromagnetic response, with excellent agreement between simulation and experiment. We observe a strong polarization dependence with two dominant scattering peaks in spectra taken with the polarization aligned along the dimer axis. This response arises from a unique Fano interference, in which the bright hybridized modes of an asymmetric dimer are able to couple to the dark higherorder hybridized modes through substrate-mediated coupling. The presence of this interference is strongly dependent on the nanoparticle geometry that defines the plasmon energy profile but also on the intense localization of charge at the dielectric surface in the nanogap region for separations smaller than 6 nm.