7 research outputs found
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
Gap-Plasmon-Enhanced Nanofocusing Near-Field Microscopy
We
report the observation of coherent light scattering from nanometer-sized
gap regions in a nanofocusing scanning near-field optical microscope.
When approaching a nanofocusing gold taper to the surface of a thin
semitransparent gold film and detecting in transmission, we find a
steep increase in scattering intensity over the last 5 nm in a near-field
signal selected in <i>k</i>-space. This is confirmed as
a signature of highly confined gap plasmons by detailed comparisons
to finite element method simulations. The simulations reveal that
the confinement is adjustable via the underlying probeāsample
distance control scheme even to levels well below the taper apex radius.
This controlled experimental realization of gap plasmons and the extraction
of their signature in a scanning probe microscope pave the way toward
broadband spectroscopy at and below single-nanometer length scales,
using parallel detection at multiple wavelengths, for instance, in
transient absorption or two-dimensional spectroscopy
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 5Ā° to 47Ā° 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 ā¼20Ā°), 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 ā¼10Ā°). For intermediate opening angles, a gradual
transition of these two mechanisms was observed
Suppression of Radiative Damping and Enhancement of Second Harmonic Generation in Bullās Eye Nanoresonators
We report a drastic increase of the
damping time of plasmonic eigenmodes
in resonant bullās eye (BE) nanoresonators to more than 35
fs. This is achieved by tailoring the groove depth of the resonator
and by coupling the confined plasmonic field in the aperture to an
extended resonator mode such that spatial coherence is preserved over
distances of more than 10 Ī¼m. Experimentally, this is demonstrated
by probing the plasmon dynamics at the field level using broadband
spectral interferometry. The nanoresonator allows us to efficiently
concentrate the incident field inside the central aperture of the
BE and to tailor its local optical nonlinearity by varying the aperture
geometry. By replacing the central circular hole with an annular ring
structure, we obtain 50-times higher second harmonic generation efficiency,
allowing us to demonstrate the efficient concentration of long-lived
plasmonic modes inside nanoapertures by interferometric frequency-resolved
autocorrelation. Such a light concentration in a nanoresonator with
high quality factor has high potential for sensing and coherent control
of light-matter interactions on the nanoscale
Toward Plasmonics with Nanometer Precision: Nonlinear Optics of Helium-Ion Milled Gold Nanoantennas
Plasmonic nanoantennas are versatile
tools for coherently controlling
and directing light on the nanoscale. For these antennas, current
fabrication techniques such as electron beam lithography (EBL) or
focused ion beam (FIB) milling with Ga<sup>+</sup>-ions routinely
achieve feature sizes in the 10 nm range. However, they suffer increasingly
from inherent limitations when a precision of single nanometers down
to atomic length scales is required, where exciting quantum mechanical
effects are expected to affect the nanoantenna optics. Here, we demonstrate
that a combined approach of Ga<sup>+</sup>-FIB and milling-based He<sup>+</sup>-ion lithography (HIL) for the fabrication of nanoantennas
offers to readily overcome some of these limitations. Gold bowtie
antennas with 6 nm gap size were fabricated with single-nanometer
accuracy and high reproducibility. Using third harmonic (TH) spectroscopy,
we find a substantial enhancement of the nonlinear emission intensity
of single HIL-antennas compared to those produced by state-of-the-art
gallium-based milling. Moreover, HIL-antennas show a vastly improved
polarization contrast. This superior nonlinear performance of HIL-derived
plasmonic structures is an excellent testimonial to the application
of He<sup>+</sup>-ion beam milling for ultrahigh precision nanofabrication,
which in turn can be viewed as a stepping stone to mastering quantum
optical investigations in the near-field