Direct Visualization of Near-Field Distributions on a Two-Dimensional Plasmonic Chip by Scanning Near-Field Optical Microscopy

A precise understanding of the near-field distributions of plasmonic nanostructures is indispensable for their practical applications. In this study, we directly visualized near-field distributions on two-dimensional nanohole arrays covered with a gold film (plasmonic chip) by scanning near-field optical microscopy. In the near-field images, strong extinction spots were observed not only inside the nanoholes but also on the outside. We also found that the spatial characteristics outside the nanoholes are strongly dependent on the excitation wavelength. From the electromagnetic simulations, we revealed that the positions of the extinction spots in the near-field images are well correlated with the locations for the enhanced electric fields. We also found that the excitation wavelength dependency on the near-field distributions are rationalized with the classical grating coupling conditions for the plasmonic chip. These findings provide new physical insights into the near-field characteristics of the metallic nanohole arrays and are essential not only for optimizing the performances in the plasmonic nanohole arrays but also for boosting their practical applications.