Unveiling and Imaging Degenerate States in Plasmonic Nanoparticles with Nanometer Resolution

Abstract

Metal nanoparticles host localized plasmon excitations that allow the manipulation of optical fields at the nanoscale. Despite the availability of several techniques for imaging plasmons, direct access into the symmetries of these excitations remains elusive, thus hindering progress in the development of applications. Here, we present a combination of angle-, polarization-, and space-resolved cathodoluminescence spectroscopy methods to selectively access the symmetry and degeneracy of plasmonic states in lithographically fabricated gold nanoprisms. We experimentally reveal and spatially map degenerate states of multipole plasmon modes with nanometer spatial resolution and further provide recipes for resolving optically dark and out-of-plane modes. Full-wave simulations in conjunction with a simple tight-binding model explain the complex plasmon structure in these particles and reveal intriguing mode-symmetry phenomena. Our approach introduces systematics for a comprehensive symmetry characterization of plasmonic states in high-symmetry nanostructures.

Electron-beam spectroscopy for nanophotonics

“…60 Differences in the angular distribution of the CL emission can also yield information on plasmon mode symmetry. 61 This interference is a direct demonstration of the coherent nature of electron-beam excitation of multiple resonant plasmonic modes by a single electron. Furthermore, in CL polarimetry the four Stokes parameters can be derived from six independent polarization-filtered measurements, enabling spatial and angular mapping of the full polarization state of CL.…”

Graphene Plasmonics: Fully Atomistic Approach for Realistic Structures

“…Finally, it is also worth noticing that ωFQ correctly reproduces the degeneracy between x - and y -polarizations (see Figure S5 in the Supporting Information), a feature which has recently been reported for similar geometries. 37 …”

Polarized cathodoluminescence for strain measurement

“…Polarization resolved CL, for its part, has been implemented within scanning electron microscopes (SEMs) and transmission electron microscopes (TEMs) for the study of nanowires, 32,33 plasmons, 34,35 and the effect of polarized electrons. 36 However, direct strain measurement from the DOP of CL has not been reported to our knowledge.…”