Here we show near-field transmisson images of a gold nano disc (r = 400nm, h = 35nm), obtained via Scanning Near-field Optical Microscopy (SNOM), at two different wavelengths. Images produced using the theory of principal modes (top row) provide a good match to the experiments (bottom row). We observe, in both sets of images, deeply sub-wavelength spatial features of the optical patterns, which evolve with wavelength. The theory of principal modes enables us to examine the mode structure of these patterns, which mainly consist of two (pairs of) modes. In the graph (far right) we show that these two distinct mode patterns coincide with the local minima (A) and maxima (B) of the sensitivity (to excitation) of the dominant mode for the m=0 channel. At the same time, the dominant mode of the m=1 channel behaves conversely, and has a resonance close to the shorter wavelength (A). This simple mechanism is sufficient to explain the complex behaviour observed in these images.
K. Imura, K. Ueno, H. Misawa, H. Okamoto, D. McArthur, B. Hourahine, and F. Papoff, “Plasmon modes in single gold nanodiscs,” Opt. Express 22, 12189-12199 (2014)