Probing the symmetry and phase of localised surface plasmon resonances with singular electron probes
Plasmonics, the science and technology of the interaction of light with metallic objects, is funda- mentally changing the way we can detect, generate and manipulate light at the nanoscale. While the field is progressing swiftly thanks to the availability of nanoscale manufacturing and analysis methods, fundamental properties such as the symmetries of the plasmonic excitations cannot be accessed by direct measurements, leading to a partial and sometimes incorrect understanding of their properties. Here we overcome this limitation by deliberately shaping the wave–function of a free electron beam to match the symmetry of the plasmonic excitations in a modified transmission electron microscope. We show experimentally and theoretically that this offers selective detection of specific plasmon modes within metallic nanoparticles while filtering out modes with other symmetries. This method shows some resemblance to the widespread use of polarised light for the selective excitation of plasmon modes but adds the advantage of locally probing the response of individual plasmonic objects and, in principle, a far wider range of symmetry selection criteria.