Imaging optical near fields at metallic nanoscale voids

Direct imaging of the electromagnetic fields generated by plasmon excitation at gold nanoscale voids is achieved with high spatial resolution using scanning near-field optical microscopy (SNOM). The spatial intensity distribution of the electromagnetic field in spherical Au void structures strongly depends on the light excitation wavelength and on the diameter and degree of truncation of the voids. Comparing the SNOM results with reflectivity measurements and boundary element calculations allows for a clear identification of the plasmon modes being activated in each case. The unique mapping of the high-field regions provides key information for the use of void nanostructures as substrates for surface-enhanced Raman scattering