2 research outputs found
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)
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