Unpolarized photoluminescence from d-band holes versus polarized scattering of single gold nanosponges

Gold nanosponges are gold/air percolated nanoparticles with three dimensionally intertwined gold and air filaments of sub-20 nm diameter. The internal structure of the nanosponges, i.e. the size of the gold and air filaments can be tuned by varying the fabrication parameters. Gold nanosponges exhibit multiple scattering resonances in the visible to NIR spectral range. Using dark field optical spectroscopy and scanning electron microscopy, it is feasible to determine their plasmonic response and relate it to the morphology of a specific single nanosponge. We found that nanosponges with a spherical envelope show a distinctive polarization dependence in their dark field scattering spectra. This excludes the simple assignment of an effective-medium permittivity to the nanosponges, although the gold and air filaments are clearly sub-wavelength. Instead, the individual inner morphology needs to be respected when determining the optical properties. This is also found in numerical simulations.