Near-field scanning optical microscopy was used to probe single nanoparticles, close-packed dimer, well-defined trimeric nanoaggregate, and separated dimer of gold nanoparticles. Surface plasmon (SP)-coupled electromagnetic (EM) field confinement at interstitials was confirmed through two-photon-induced photoluminescence (TPI-PL) imaging. A fairly well correlation between topography and metrology in nanoscale was reported with sufficient high spatial resolution. A strong excitation of SP-assisted TPI-PL was observed at gold dimer and trimeric nanoaggregate, whereas single particles and separated dimer showed negligible TPI-PL excitation. A confined distribution of TPI-PL was revealed in close-packed dimer with reference to those at isolated nanoparticles and trimer. Strong optical distribution of TPI-PL was observed at trimer, but the distribution was found broadened with reference to those at dimer. A plausible explanation of such broadened distribution was interpreted as the coalescence of surface plasmon-assisted confined optical signal of TPI-PL. Such a direct and distinguished correlation between local topography and optical field distribution with high resolution is indispensable and further addition to realize surface plasmon coupling at the interstitials. Finite different time domain analysis was carried out to elucidate the results observed herein. For a small model of three nanoobjects keeping the geometry and parameters very similar to that of the close-packed trimeric nanoaggregate under investigation, multiple confined EM fields were found at the out-of-plane polarization to the interparticle axes. With the step sizes (i.e., 50 nm), these nearby localized EM near-field-mediated optical distributions open up a possibility to coalescence and thus get broadened.
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