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Plasmonic oligomers allow new ways to manipulate nonlinear optical effects such as second-harmonic generation (SHG) through collective resonances. However, earlier techniques to probe such effects have relied mostly on the use of plane waves or focused beam excitations with homogenous states-of-polarization (e.g., linear) that obviously do not match the spatial symmetries of the oligomer. Here, we investigate collective effects in the SHG from individual plasmonic oligomers using microscopy with cylindrical vector beams such as radial or azimuthal polarizations. The oligomers were prepared by electron-beam lithography. The oligomers consisted of gold nanorods that have a longitudinal plasmon resonance close to the fundamental wavelength that is used for SHG excitation and whose long axes are arranged locally such that they follow the distribution of the transverse component of the electric field of radial or azimuthal polarizations. We found that SHG from such oligomers is strongly modified by the interplay between the properties of the incident cylindrical vector beam and interparticle coupling. We find that the oligomers with radially-oriented nanorods exhibit small coupling effects. In contrast, we observed that the oligomers with azimuthally-oriented nanorods exhibit large coupling effects that lead to silencing of SHG from the whole structure. We found good qualitative agreement between our experimental findings and calculations using the method of moments. The work describes a new route to investigate coupling effects in arrangements of nanostructures and thereby to control the efficiency of nonlinear effects in these structures.
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