Nonlinear Photoluminescence from Gold Nanoparticle Thin Films Studied by Scanning Near-Field Optical Microscopy

We investigate the nonlinear photoluminescence (PL) from gold nanoparticle films by scanning near-field optical microscopy. We visualize the spatial and spectral distributions of the PL active sites beyond the diffraction limit of light. From the multivariate analysis, we find that PL originates from at least two spectral components: the one showing a peak near 630 nm and the other one monotonous increase toward 700 nm. The former component is attributed to PL from the recombination of electron and hole pairs. The latter component is ascribed to the radiative decay of plasmons excited by the radiated photons. We also perform spatiotemporal imaging of the sample and demonstrate that the hot electrons are responsible for the PL. From these near-field imaging, we conclude that the PL from two spectral components are ascribable to electron-hole recombination and the radiative decay of plasmons.