Characterization of Surface Plasmons and Toroidal Moments Using Relativistic Electrons

Plasmonics is one of the research fields in nano-optics with the emphasis on resonant light–matter interactions. Plasmonics has attracted tremendous attention by exhibiting the capability of focusing electromagnetic fields and confining the field beyond the diffraction limit, for enhancing light–matter interactions on the nanoscale. Therefore, plasmons have been applied in the fields of near-field imaging, biosensing, light harvesting, light nanofocusing and emitting, medicine thermotherapy, etc. One of the fundamental investigations is to characterize plasmonic phenomena of a single nano-object in order to systematically quantify the influence of variables in a controlled way. It requires not only the good control of nanofabrication but also an effective and comprehensive characterization tool with a spatial resolution on the nanoscale. Here, electron energy-loss spectroscopy, energy-filtered transmission electron microscopy, and cathodoluminescence spectroscopy are applied as they are the pioneering methods to ´observe´ plasmonic phenomena on the nanoscale, owing to the great instrumentation improvement in the electron energy monochromator and the stability of transmission electron microscopes. Three-dimensional gold tapers play an important role in nano-optics. They possess the capability of light nanofocusing by transforming surface plasmon polaritons on the shaft to localized surface plasmons at the apex. In this thesis, I employed electron energy-loss spectroscopy and energy-filtered transmission electron microscopy to resolve discrete plasmonic modes in this transformation region beyond the range of optical microscopy. The link and distinction of the underlying physics of the observed modes were disentangled by systematically investigating the plasmonic modes of gold tapers with different opening angles, in combination with numerical finite-difference time-domain simulations. These results suggested that there were two main coexisting mechanisms, namely reflection and phase matching, mutually contributing to the observed plasmonic modes. The dominance from reflection to phase matching was modulated when increasing the interaction length between the fast electrons and the taper near-field. Additionally, the radiation properties of the plasmonic modes in gold tapers are further investigated by using cathodoluminescence spectroscopy. The results are helpful in designing gold tapers as nanofocusing waveguides and as point sources for photon emission. Another employment of electron energy-loss spectroscopy in this thesis is to explore the fundamental electromagnetic properties of the third family of elementary electromagnetic sources, namely toroidal moments. Despite the infancy of the field, dynamic toroidal moments have recently triggered increasing research interest initiated by their peculiar symmetry character, i.e., having odd parity under time- and space-inversion symmetry operations. Metamaterial engineering makes the dominant toroidal dipole responses detectable without being masked by electric or magnetic dipoles. A toroidal dipole response can be achieved in the optical regime via plasmon-induced displacement currents. One fundamental question is, whether single dynamic toroidal dipoles radiate to the far-field. Theoretical developments have renewed the understanding of the radiative properties of toroidal dipoles, however there is still lack of experimental evidence. I have experimentally investigated the far-field radiation of toroidal dipole moments in a plasmonic heptamer nanocavity by cathodoluminescence spectroscopy. On the other hand, the present focus of this field is on the novel optical phenomena of a single toroidal dipole resonance and its interactions with electric and magnetic multipoles. Differently, I am interested in the fundamental toroidal dipole–dipole coupling, as the coupling effect tailors the optical response and can be adopted as building and manipulating element for designing potential devices. The transverse coupling of toroidal dipoles was carried out on a plasmonic decamer nanocavity. Here, I experimentally characterized the pronounced coupled toroidal modes by electron energy-loss spectroscopy and visualized them by energy-filtered transmission electron microscopy. The coupling mechanism was further illustrated via theoretical analysis, and a simplified toroidal dipole–dipole interaction model was therefore proposed in a qualitative way. The finding paves the way for further research and exploitation in the fields of nano-optics and meta-devices.