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A method is presented for the characterization of locally enhanced fields at laser-irradiated metal nanostructures. Excitation with 120 fs laser pulses gives rise to photoluminescence mediated by two-photon absorption. A metal tip used to locally scatter the photoluminescence renders a map of regions with high field strengths. Near-field photoluminescence images of particle clusters reveal the dipole nature of the electromagnetic field surrounding the particles. Spectra acquired with and without the presence of the tip show no significant shift of the surface plasmon resonance of the particle clusters, confirming that the tip acts as a passive probe.
Bouhélier et al. (Thu,) studied this question.