Observation of the plasmon mode transition from triangular to hexagonal nanoplates.

The geometrical shape of a metal nanostructure plays an essential role in determining the optical functionality of plasmonic cavity modes. Here, we investigate the geometrical modification effect on plasmonic cavity modes induced in two-dimensional gold nanoplates. We perform near-field transmission measurements on triangular and tip-truncated triangular nanoplates and reveal that the plasmonic cavity modes are qualitatively consistent with each other as long as the snipping size is not significant. To elucidate the tip-truncation effect on plasmonic cavity modes in detail, we carry out numerical simulations for nanoplates with various snipping sizes and find that tip truncation affects not only the optical selection rules but also the energy relation for the plasmonic cavity modes. These findings provide a foundation for the rational design of plasmonic cavities with desired optical functionality.