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Plasmonic dark modes excited by strongly focused illumination

Dark plasmon modes in metal nanoparticle systems are usually excited by non-optical means. We show that strongly focused illumination can lead to excitation of dark modes. We first use rigorous vectorial diffraction theory to compute the distribution of light at the focus and then numerically calculate the response of single particles and particle dimers. Controlling the distribution of light arriving at the focusing lens by pupil filters enables enhancing the excitation of dark modes. Overall, these results present guidelines for the excitation of dark plasmon modes using standard optical instrumentation.