Electron Energy-Loss Spectroscopy of Multipolar Edge and Cavity Modes in Silver Nanosquares

The characterization of surface plasmon resonances supported by metallic nanostructures requires high spatial and energy resolution. In the past few years, electron energy loss spectroscopy (EELS) has emerged as a very powerful tool to accomplish this task. In this work, we demonstrate the power of this technique for probing and imaging resonances of metallic nanostructures by analyzing the plasmonic response of silver nanosquares of sizes ranging from 230 nm up to 1 mu m. Because of the relatively large size of these structures, we find that, despite their simple geometry, these systems can support a large variety of multipolar modes, which can only be detected and imaged thanks to the high spatial and energy resolution achieved by pushing EELS to its limits. The experimental results are supported by rigorous theoretical calculations that allow a detailed interpretation of the EELS measurements. In particular, we were able to map, with high level of detail, edge and high-order cavity modes. Furthermore, by calculating the scattering cross-section of these nanostructures, we confirm that most of the observed modes are dark and thus remain hidden in optical measurements, thus demonstrating the power of EELS as a unique tool for probing and imaging a large range and variety of plasmonic resonances of metallic nanostructures.