Wavelength-Dependent Optical Force Imaging of Bimetallic Al-Au Heterodimers

Many important applications of nanometer-scale metallic complexes arise from the light-induced, near-field interactions between their component structures. Here we examine the near-field interactions in bimetallic Al-Au plasmonic nanodisk heterodimers, where the coupling between the primitive plasmons of nanostructures composed of two different metals is studied. Understanding the correlations between nanoparticle morphology and near-field optical properties, particularly for nanostructures composed of two different metals, requires spectrally resolved near-field spatial information. An ideal tool for such investigations is the recently developed photoinduced force microscopy, where the electromagnetic forces between an optically excited plasmonic nanostructure and an adjacent scanning nanoscale tip are measured. Using this approach, we visualize the wavelength-dependent near-field interactions in these bimetallic heterodimers. This system provides a prime example of the diabatic, antenna-reactor picture of plasmon coupling where for a given wavelength the more resonant primitive "driving" plasmon induces a response, the "forced" plasmon, in the off-resonant component. We critically examine spectrally resolved tip-nanostructure forces, comparing experiment with theory, for tips and nanoscale structures of realistic dimensions relative to frequently used approximations for tip geometries. The contrasting effects of dielectric versus metallic tips on acquired spectral force profiles are also examined.