67 research outputs found
Quantum Plasmonics: Optical Properties of a Nanomatryushka
Quantum
mechanical effects can significantly reduce the plasmon-induced
field enhancements around nanoparticles. Here we present a quantum
mechanical investigation of the plasmon resonances in a nanomatryushka,
which is a concentric coreâshell nanoparticle consisting of
a solid metallic core encapsulated in a thin metallic shell. We compute
the optical response using the time-dependent density functional theory
and compare the results with predictions based on the classical electromagnetic
theory. We find strong quantum mechanical effects for coreâshell
spacings below 5 Ă
, a regime where both the absorption cross
section and the local field enhancements differ significantly from
the classical predictions. We also show that the workfunction of the
metal is a crucial parameter determining the onset and magnitude of
quantum effects. For metals with lower workfunctions such as aluminum,
the quantum effects are found to be significantly more pronounced
than for a noble metal such as gold
Mechanisms of Fano Resonances in Coupled Plasmonic Systems
Fano resonances in hybridized systems formed from the interaction of bright modes only are reported. Despite precedent works, we demonstrate theoretically and experimentally that Fano resonances can be obtained by destructive interference between two bright dipolar modes out of phase. A simple oscillator model is provided to predict and fit the far-field scattering. The predictions are verified with numerical calculations using a surface integral equation method for a wide range of geometrical parameters. The validity of the model is then further demonstrated with experimental dark-field scattering measurements on actual nanostructures in the visible range. A remarkable set of properties like crossings, avoided crossings, inversion of subradiant and superradiant modes and a plasmonic equivalent of a bound state in the continuum are presented. The nanostructure, that takes advantage of the combination of Fano resonance and nanogap effects, also shows high tunability and strong near-field enhancement. Our study provides a general understanding of Fano resonances as well as a simple tool for engineering their spectral features
Plasmon Blockade in Nanostructured Graphene
Among the many extraordinary properties of graphene, its optical response allows one to easily tune its interaction with nearby molecules <i>via</i> electrostatic doping. The large confinement displayed by plasmons in graphene nanodisks makes it possible to reach the strong-coupling regime with a nearby quantum emitter, such as a quantum dot or a molecule. In this limit, the quantum emitter can introduce a significant plasmonâplasmon interaction, which gives rise to a plasmon blockade effect. This produces, in turn, strongly nonlinear absorption cross sections and modified statistics of the bosonic plasmon mode. We characterize these phenomena by studying the equal-time second-order correlation function <i>g</i><sup>(2)</sup>(0), which plunges below a value of 1, thus revealing the existence of nonclassical plasmon states. The plasmon-emitter coupling, and therefore the plasmon blockade, can be efficiently controlled by tuning the doping level of the graphene nanodisks. The proposed system emerges as a new promising platform to realize quantum plasmonic devices capable of commuting optical signals at the single-photon/plasmon level
Relaxation of Plasmon-Induced Hot Carriers
Plasmon-induced hot carrier generation
has attracted much recent attention due to its promising potential
in photocatalysis and other light harvesting applications. Here we
develop a theoretical model for hot carrier relaxation in metallic
nanoparticles using a fully quantum mechanical jellium model. Following
pulsed illumination, nonradiative plasmon decay results in a highly
nonthermal distribution of hot electrons and holes. Using coupled
master equations, we calculate the time-dependent evolution of this
carrier distribution in the presence of electronâelectron,
electronâphoton, and electronâphonon scattering. Electronâelectron
relaxation is shown to be the dominant scattering mechanism and results
in efficient carrier multiplication where the energy of the initial
hot electronâhole pair is transferred to other multiple electronâhole
pair excitations of lower energies. During this relaxation, a small
but finite fraction of electrons scatter into luminescent states where
they can recombine radiatively with holes by emission of photons.
The energy of the emitted photons is found to follow the energies
of the electrons and thus redshifts monotonically during the relaxation
process. When the energies of the electrons approach the Fermi level,
electronâphonon interaction becomes dominant and results in
heating of the nanoparticle. We generalize the model to continuous-wave
excitation and show how nonlinear effects become important when the
illumination intensity increases. When the temporal spacing between
incident photons is shorter than the relaxation time of the hot carriers,
we predict that the photoluminescence will blueshift with increasing
illumination power. Finally, we discuss the effect of the photonic
density of states (Purcell factor) on the luminescence spectra
Porous Au Nanoparticles with Tunable Plasmon Resonances and Intense Field Enhancements for Single-Particle SERS
Porous Au nanoparticles with fine-controlled overall particle sizes
have been fabricated using a kinetically controlled seed-mediated
growth method. In contrast to spherical Au nanoparticles with smooth
surfaces, the porous Au nanoparticles exhibit far greater size-dependent
plasmonic tunability and significantly intensified local electric
field enhancements exploitable for single-particle plasmon-enhanced
spectroscopies. The effects of the nanoscale porosity on the far-
and near-field optical properties of the nanoparticles have been investigated
both experimentally by optical extinction and single-nanoparticle
Raman spectroscopic measurements and theoretically through finite-difference
time-domain calculations
Electron Energy-Loss Spectroscopy Calculation in Finite-Difference Time-Domain Package
Electron energy-loss spectroscopy
(EELS) is a unique tool that
is extensively used to investigate the plasmonic response of metallic
nanostructures. We present here a novel approach for EELS calculations
using the finite-difference time-domain (FDTD) method (EELS-FDTD).
We benchmark our approach by direct comparison with results from the
well-established boundary element method (BEM) and published experimental
results. In particular, we compute EELS spectra for spherical nanoparticles,
nanoparticle dimers, nanodisks supported by various substrates, and
a gold bowtie antenna on a silicon nitride substrate. Our EELS-FDTD
method can be easily extended to more complex geometries and configurations.
This implementation can also be directly exported beyond the FDTD
framework and implemented in other Maxwellâs equation solvers
Asymmetric Aluminum Antennas for Self-Calibrating Surface-Enhanced Infrared Absorption Spectroscopy
While
there has been a tremendous increase of recent interest in noble metal-based
antennas as substrates for surface-enhanced infrared absorption spectroscopy,
more abundant and manufacturable metals may offer similar or additional
opportunities for this mid-infrared sensing modality. Here we examine
the feasibility of aluminum antennas for SEIRA, by designing and fabricating
asymmetric aluminum cross antennas with nanometer-scale gaps. The
asymmetric cross design enables the simultaneous detection of multiple
infrared vibrational resonances over a broad region of the mid-infrared
spectrum. The presence of the Al<sub>2</sub>O<sub>3</sub> amorphous
surface oxide layer not only passivates the metal antenna structures
but also enables a very straightforward covalent binding chemistry
for analyte molecules to the antenna through multiple approaches,
in this case by the use of carboxylic acid functional groups. The
aluminumâoxygen stretching mode of the oxide can be used as
a self-calibration standard to quantify the number of analyte molecules
on the antenna surface
Fabrication of Elliptical Nanorings with Highly Tunable and Multiple Plasmonic Resonances
Herein, a new and facile patterning method is demonstrated
for
the scalable fabrication of gold elliptical rings (ERs) in a controlled
manner over large areas. In this method, well-ordered hexagonally
arrayed polystyrene (PS) rings, fabricated by colloidal lithography,
were used as masters to generate polyÂ(dimethylsiloxane) (PDMS) stamps
with circular apertures. The stamps were then stretched and utilized
as molds for creating elliptical PS rings by a capillary filling process.
Through subsequent reactive ion etching and chemical wet-etching,
the elliptical PS rings could be readily transferred into an underlying
gold film, leading to the formation of gold ERs. Since the aspect
ratio (AR) of the elliptical PS rings could be controlled by varying
the applied strain during the capillary filling process, gold ERs
with different ARs could be fabricated in a scalable manner. The optical
properties of the gold ERs were characterized by UVâvis/NIR
and IR extinction measurements. The ERs exhibited only odd modes of
polarization-dependent plasmonic resonances at normal incidence. The
experiments and corresponding theoretical studies illustrated that
all resonant modes could be tuned across a broad spectral range from
the visible to the mid infrared (550â4700 nm) by simply varying
the AR of the ERs. Moreover, the experimental data were confirmed
by COMSOL simulations
Fabrication of Split-Rings via Stretchable Colloidal Lithography
Herein,
a facile new patterning method is demonstrated for creating pairs
of split-ring resonators (SRRs) in a scalable manner over large surface
areas. This method is based on a novel variation of colloidal lithography
called stretchable colloidal lithography (SCL), which combines conventional
colloidal lithography and stretchable polyÂ(dimethylsiloxane) (PDMS)
molds. To fabricate SRRs, arrays of circular polystyrene (PS) rings
were fabricated by conventional colloidal lithography. The circular
ring features could be transferred to a PDMS stamp, forming negative
features, circular apertures on the stamp. The PDMS stamp was then
stretched, thereby transforming the circular apertures into elliptical
ones. By using the stretched PDMS molds for polymer imprinting, elliptical
rings with nonuniform heights could be fabricated. Each elliptical
ring could be transformed into a pair of PS SRRs by controlled O<sub>2</sub> reactive ion etching. Through a subsequent chemical wet etching
step, PS SRRs could be readily transferred into an underlying gold
film. The SRRs exhibited multiple modes of polarization-dependent
plasmonic resonances in the visible and infrared spectral regions.
Experiments and corresponding theoretical modeling demonstrated that
these multiple resonances could be tuned in a predictable manner.
All optical data compared well with results from electromagnetic simulations
Standing Wave Plasmon Modes Interact in an Antenna-Coupled Nanowire
In
a standing wave optical cavity, the coupling of cavity modes,
for example, through a nonlinear medium, results in a rich variety
of nonlinear dynamical phenomena, such as frequency pushing and pulling,
mode-locking and pulsing, modal instabilities, even complex chaotic
behavior. Metallic nanowires of finite length support a hierarchy
of longitudinal surface plasmon modes with standing wave properties:
the plasmonic analog of a FabryâPeÌrot cavity. Here we
show that positioning the nanowire within the gap of a plasmonic nanoantenna
introduces a passive, hybridization-based coupling of the standing-wave
nanowire plasmon modes with the antenna structure, mediating an interaction
between the nanowire plasmon modes themselves. Frequency pushing and
pulling, and the enhancement and suppression of specific plasmon modes,
can be controlled and manipulated by nanoantenna position and shape
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