Imaging of Localized Surface Plasmon by Apertureless Scanning Near-Field Optical Microscopy

We measured the distribution of surface plasmon polaritons (SPPs) on cross grooves fabricated in Au film by apertureless scanning near-field optical microscopy (aSNOM). SNOM signals around horizontal and vertical grooves had double peaks and a single peak, respectively. To understand these different behaviors, we carried out simulations by the finite element method. From the simulation results, we found that the double peaks that appeared near the left and right edges of the horizontal groove were due to localized surface plasmons (LSPs). On the other hand, the strong signal that appeared near the center of the vertical groove was due to channel plasmon polariton. The SPP excitation depended on the direction of the electric field of the incident light and the cross sections of the grooves. This experiment reveals that aSNOM is a powerful tool for observing the distribution of SPPs on the nanoscale.