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In molecular quantum aggregates, dipole-dipole coupling between chromophores may lead to excitonic states and energy transport. We investigate, as another class of aggregates, coupled metallic nanostructures for which the dipole-dipole interaction is based on multielectron plasmonic excitations rather than single-electron transitions. Nonetheless, analogous phenomena may be observed, such as the emergence of dark and bright modes, long-living temporal coherences and coherent beating signals. For the time-resolved investigation of such systems we have developed coherent two-dimensional (2D) nanoscopy [Science 333, 1723 (2011)], which provides 2D spectral-temporal information with simultaneous spatial resolution below the optical diffraction limit. This is achieved in a combination of the 2D scheme with photoemission electron microscopy. In contrast to near-field spectroscopy methods, wide-field illumination provides extended images, and thus the individual emitters of a nanostructure aggregate can be analyzed simultaneously yet separately. |