Electron energy losses in Ag nanoholes—from localized surface plasmon resonances to rings of fire

Using valence-loss energy-filtering transmission electron microscopy, we measure the optical response of an arrangement of nanoholes in a Ag film over a wide range of wavelengths (0.5-2 microm). This technique allows us to detect the full transition from localized excitations to traveling guided modes with a spatial resolution of a few nanometers. By the well-defined arrangement of the nanoholes, we can clearly distinguish coupling effects from single-hole resonances. This type of experiment is ideally suited for the future design of biosensors and metamaterials.