Cherenkov Radiation in a Bianisotropic Medium with Magnetoelectric Effect

Cherenkov radiation is studied analytically in a dispersive bianisotropic medium with magnetoelectric effect. This study is based on the vector-potential approach. Notable features such as double cones of Cherenkov radiation in magnetoelectric media are studied and the angle of propagation cones are calculated, which allow for characterizing magnetoelectric materials. We also analyze emitted electromagnetic fields in the spectral domain to obtain radiated power and momentum-resolved electron energy-loss spectra. The impact of magnetoelectric coefficients on characteristics of the Cherenkov radiation is also investigated in order to introduce parameters for the characterization of magnetoelectric materials

Long-Range Coupling of Toroidal Moments for the Visible

Dynamic toroidal multipoles are the third independent family of elementary electromagnetic sources in addition to electric and magnetic multipoles. Whereas the dipole–dipole coupling in electric and magnetic multipole families has been well studied, such fundamental coupling effects in the toroidal multipole family have not yet been experimentally investigated. Here we propose a plasmonic decamer nanocavity structure to realize transverse coupling between magnetic toroidal dipoles. The coupling effect was investigated both experimentally and theoretically, by means of electron energy-loss spectroscopy and energy-filtered transmission electron microscopy, together with finite-difference time-domain calculations. We observe that the coupling causes a reorientation of the magnetic moment loops surrounding the initial toroidal moments. This coupling results in three eigenstates of this toroidal system. The underlying coupling mechanism is qualitatively demonstrated. Our investigations pave the way toward a better understanding of coupling phenomena of toroidal moments and will bias applications in the long-range ordering of moments in metamaterials, e.g., for transfer of electromagnetic energy using toroidal moments (by analogy with chain metallic waveguides)

Toroidal Plasmonic Eigenmodes in Oligomer Nanocavities for the Visible

Plasmonics has become one of the most vibrant areas in research with technological innovations impacting fields from telecommunications to medicine. Many fascinating applications of plasmonic nanostructures employ electric dipole and higher-order multipole resonances. Also magnetic multipole resonances are recognized for their unique properties. Besides these multipolar modes that easily radiate into free space, other types of electromagnetic resonances exist, so-called toroidal eigenmodes, which have been largely overlooked historically. They are strongly bound to material structures and their peculiar spatial structure renders them practically invisible to conventional optical microscopy techniques. In this Letter, we demonstrate toroidal modes in a metal ring formed by an oligomer of holes. Combined energy-filtering transmission electron microscopy and three-dimensional finite difference time domain analysis reveal their distinct features. For the study of these modes that cannot be excited by optical far-field spectroscopy, energy-filtering transmission electron microscopy emerges as the method of choice. Toroidal moments bear great potential for novel applications, for example, in the engineering of Purcell factors of quantum-optical emitters inside toroidal cavities

Wedge Dyakonov Waves and Dyakonov Plasmons in Topological Insulator Bi₂Se₃ Probed by Electron Beams

Bi₂Se₃ has recently attracted a lot of attention because it has been reported to be a platform for the realization of three-dimensional topological insulators. Due to this exotic characteristic, it supports excitations of a two-dimensional electron gas at the surface and, hence, formation of Dirac-plasmons. In addition, at higher energies above its bandgap, Bi₂Se₃ is characterized by a naturally hyperbolic electromagnetic response, with an interesting interplay between type-I and type-II hyperbolic behaviors. However, still not all the optical modes of Bi₂Se₃ have been explored. Here, using mainly electron energy–loss spectroscopy and corresponding theoretical modeling we investigate the full photonic density of states that Bi₂Se₃ sustains, in the energy range of 0.8 eV–5 eV. We show that at energies below 1 eV, this material can also support wedge Dyakonov waves. Furthermore, at higher energies a huge photonic density of states is excited in structures such as waveguides and resonators made of Bi₂Se₃ due to the hyperbolic dispersion

Excitation of Mesoscopic Plasmonic Tapers by Relativistic Electrons: Phase Matching <i>versus</i> Eigenmode Resonances

We investigate the optical modes in three-dimensional single-crystalline gold tapers by means of electron energy-loss spectroscopy. At the very proximity to the apex, a broad-band excitation at all photon energies from 0.75 to 2 eV, which is the onset for interband transitions, is detected. At large distances from the apex, though, we observe distinct resonances with energy dispersions roughly proportional to the inverse local radius. The nature of these phenomena is unraveled by finite difference time-domain simulations of the taper and an analytical treatment of the energy loss in fibers. Our calculations and the perfect agreement with our experimental results demonstrate the importance of phase-matching between electron field and radiative taper modes in mesoscopic structures. The local taper radius at the electron impact location determines the selective excitation of radiative modes with discrete angular momenta

Excitation of Mesoscopic Plasmonic Tapers by Relativistic Electrons: Phase Matching <i>versus</i> Eigenmode Resonances

We investigate the optical modes in three-dimensional single-crystalline gold tapers by means of electron energy-loss spectroscopy. At the very proximity to the apex, a broad-band excitation at all photon energies from 0.75 to 2 eV, which is the onset for interband transitions, is detected. At large distances from the apex, though, we observe distinct resonances with energy dispersions roughly proportional to the inverse local radius. The nature of these phenomena is unraveled by finite difference time-domain simulations of the taper and an analytical treatment of the energy loss in fibers. Our calculations and the perfect agreement with our experimental results demonstrate the importance of phase-matching between electron field and radiative taper modes in mesoscopic structures. The local taper radius at the electron impact location determines the selective excitation of radiative modes with discrete angular momenta

Reflection and Phase Matching in Plasmonic Gold Tapers

We investigate different dynamic mechanisms, reflection and phase matching, of surface plasmons in a three-dimensional single-crystalline gold taper excited by relativistic electrons. Plasmonic modes of gold tapers with various opening angles from 5° to 47° are studied both experimentally and theoretically, by means of electron energy-loss spectroscopy and finite-difference time-domain numerical calculations, respectively. Distinct resonances along the taper shaft are observed in tapers independent of opening angles. We show that, despite their similarity, the origin of these resonances is different at different opening angles and results from a competition between two coexisting mechanisms. For gold tapers with large opening angles (above ∼20°), phase matching between the electron field and that of higher-order angular momentum modes of the taper is the dominant contribution to the electron energy-loss because of the increasing interaction length between electron and the taper near-field. In contrast, reflection from the taper apex dominates the EELS contrast in gold tapers with small opening angles (below ∼10°). For intermediate opening angles, a gradual transition of these two mechanisms was observed