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Nano Res
1
Plasmonic mode mixing in nanoparticle dimers with
nm-separations via substrate-mediated coupling
Jesse Theiss, Mehmet Aykol, Prathamesh Pavaskar, and Stephen B. Cronin ()
Nano Res., Just Accepted Manuscript • DOI: 10.1007/s12274-014-0499-7
http://www.thenanoresearch.com on May 20, 2014
© Tsinghua University Press 2014
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Nano Research
DOI 10.1007/s12274-014-0499-7
Plasmonic mode mixing in nanoparticle dimers with
nm-separations via substrate-mediated coupling
Jesse Theiss, Mehmet Aykol, Prathamesh Pavaskar, and
Stephen B. Cronin*
University of Southern California, United States
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Metallic nanoparticle dimers with nanometer separations are
characterized by dark-field scattering micro-spectroscopy and
transmission electron microscopy. Experiment and numerical
simulations show that the dimers exhibit a
polarization-dependent Fano interference resulting from
substrate-mediated coupling of the hybridized plasmon modes
in the asymmetric nanoparticle pairs.
Stephen B. Cronin, http://www.usc.edu/cronin/
Perpendicular
Parallel
Plasmonic mode mixing in nanoparticle dimers with
nm-separations via substrate-mediated coupling
Jesse Theiss, Mehmet Aykol, Prathamesh Pavaskar, and Stephen B. Cronin ()
Received: day month year
Revised: day month year
Accepted: day month year
(automatically inserted by
the publisher)
© Tsinghua University Press
and Springer-Verlag Berlin
Heidelberg 2014
KEYWORDS
Plasmonics,
Fano interference,
TEM, Nanogap
Dark-field spectroscopy
ABSTRACT
We fabricate arrays of metallic nanoparticle dimers with nanometer separation
using electron beam lithography and angle evaporation. These “nanogap”
dimers are fabricated on thin silicon nitride membranes to enable high
resolution transmission electron microscope imaging of the specific
nanoparticle geometries. Plasmonic resonances of the pairs are characterized by
dark-field scattering micro-spectroscopy, which enables the optical scattering
from individual nanostructures to be measured by using a spatially-filtered
light source to illuminate a small area. Scattering spectra from individual
dimers are correlated with transmission electron microscope images and
finite-difference time-domain simulations of their electromagnetic response,
with excellent agreement between simulation and experiment. We observe a
strong polarization dependence with two dominant scattering peaks in spectra
taken with the polarization aligned along the dimer axis. This response arises
from a unique Fano interference, in which the bright hybridized modes of an
asymmetric dimer are able to couple to the dark higher-order hybridized
modes through substrate-mediated coupling. The presence of this interference
is strongly dependent on the nanoparticle geometry that defines the plasmon
energy profile but also on the intense localization of charge at the dielectric
surface in the nanogap region for separations smaller than 6 nm.
Plasmonic excitations take advantage of the strong
interaction between light and metal surfaces to
provide nanoscale confinement and localization in
subwavelength dimensions while providing high
field enhancement. This area of research has
exploded in the last decade due to our ability to
fabricate, simulate, and microscopically image such
nanostructures. Plasmonics has found numerous
applications in areas such as biological and chemical
sensing [1-4], surface-enhanced Raman spectroscopy
(SERS) [5-7], cancer therapy [8], solar energy
conversion [9-13], photodetectors [14, 15], lasers
[16-18], waveguiding [19], single photon sources [20],
and magnetic recording [21]. Advances in fabrication
Nano Research
DOI (automatically inserted by the publisher)
Research Article
Address correspondence to Stephen B Cronin, [email protected]
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2 Nano Res.
tools (e.g., electron beam and ion beam lithography)
and characterization techniques (e.g., scanning and
transmission electron microscopy, near-field
scanning microscopy, atomic force microscopy,
electron energy loss spectroscopy /
cathodoluminescence) are also providing new insight
into the character of plasmons at the nanoscale
[22-24]. It is very important to both fabricate and
calculate the electromagnetic response of these
nanostructures as their size and separation reach the
scale of a single nanometer in order to further
improve their performance in these applications.
When two metallic nanoparticles (NPs) are brought
into close proximity, the local electric field intensity
scales dramatically with decreasing separation. The
high fields created by plasmonic nanostructures may
be used to greatly enhance physical processes like
vibrational Raman scattering [5-7], carrier generation
[25-27], and nonlinear optical effects [28-32].
Presently, there is no reliable method for producing
consistent sub 2-nm gap sizes between nanoparticles
with a larger macroscopic order suitable for device
applications. Top-down lithographic methods are
still limited to reliable resolutions on the order of ten
nanometers despite the recent advances in fabrication.
While higher resolution is achievable down to a size
of 2 nm, the tools and materials used can often limit
the utility of the fabrication for practical devices, due
to substrate limitations and thin resist layers [23, 33].
Other methods such as electromigration can be used
in nanometer-separated electrode configurations but
each junction must be created independently with a
lack of control in the exact position of the formed
nanogap [34-36]. A self-aligned technique offers
parallelism without restriction to electrode
configurations through two-step lithography and a
sacrificial cap to create spacings of 2 – 10 nm, but
non-uniform growth of the cap layer results in
geometric inconsistency in the physical gap size [35,
37]. Bottom-up fabrication techniques such as
chemical and DNA functionalization offer a more
reliable method of controlling the spacing between
two or more nanoparticles but lose large scale order
offered by top-down methods, requiring multiple
level self-assembly techniques to avoid random
placement and orientation once dispersed on a
substrate [38-42]. These functionalized techniques
may also limit the accessible hot spot area resulting
in a lower utilization of the field enhancement
properties.
Our previous work demonstrated an angle
evaporation technique with top-down lithography
able to produce two metal nanoparticles with
separations on the order of a single nanometer [43].
These nanostructures (nanoparticle dimers) were
characterized by Raman spectroscopy and high
resolution transmission electron microscopy
(HRTEM). Finite-difference time-domain (FDTD)
simulations showed that the technique could
produce nanoparticles with SERS enhancement
factors as high as 1010. In the work presented here,
similar nanostructures are isolated and measured
using dark-field micro-spectroscopy with correlated
HRTEM imaging of each individual dimer. We use
this combination of techniques to systematically
study the effects of the precise geometry on the
optical scattering of such asymmetric dimers. We
model the structures using FDTD simulations, which
provide more detailed information about the charge
and electric field distributions associated with
specific features in their far-field scattering spectra.
Gold nanoparticles are fabricated using a two-step
angle evaporation technique, as shown in Figure 1a
[43]. Electron beam lithography is used to pattern
40-120 nm diameter holes in a thin bi-layer of
MMA-MAA and PMMA 950K resist. MMA-MAA is
used to produce a large undercut necessary for the
angled second deposition, while the top layer of
PMMA serves as the masking layer. A thin layer of
metal (e.g. Au or Ag) is first deposited at normal
incidence. The sample is then tilted by a small angle
(10-20°) and a second layer of metal is deposited. For
overlapping depositions, the size of the nanogap is
estimated by the relative angle between the two
evaporations, θ, and the thickness of the first
evaporation, t1 , and is given by t1 tanθ . For
non-overlapping depositions with larger particle size
or tilt angles, the gap size is a function of the mask
layer height, t, the relative angle θ, and the hole
diameter, d, and is given by t tanθ − d. The second
nanoparticle is inevitably smaller than the first due to
undesired deposition on the sidewalls of the hole in
the lithographic mask and a smaller effective cross
section to evaporate through when the mask is tilted.
The nanoparticles are fabricated on commercially
available non-porous silicon nitride membrane
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3 Nano Res.
windows (SiMPore, Inc.) to enable high-resolution
transmission electron microscope imaging using a
JEOL JEM-2100F advanced field emission TEM.
In order to obtain scattering spectra from
individual nanoparticle dimers, a confocal dark-field
micro-spectroscopy setup was built similar to that of
Fan et al. [44], as illustrated schematically in Figure
1b. This setup uses a Fianium SC450 supercontinuum
light source to provide collimated light from 450 nm
to 2 μm. The output is polarized and sent into a small
pinhole aperture to spatially filter the light so that
only a small point source enters through the back
aperture of the objective lens (Mitutoyo NIR 50X,
NA=0.42). The objective serves as both the condenser
and collection objective lenses, as shown in Figure 1b.
The position of the pinhole can be adjusted so that
the light is aligned parallel to the objective but off
center. A beam blocker is used just before the
objective to remove the reflected light from the
sample surface entering on the opposite side of the
objective lens. The incident angle is governed by the
numerical aperture of the objective lens and the
off-axis distance of the spatially filtered light (~12-15°
in practical use). The scattered light is analyzed using
a grating spectrometer with a thermoelectrically
cooled CCD camera. The collected scattered light is
reimaged on another pinhole aperture (150 μm)
before it is sent into the spectrometer to limit
collection to a very small spatial area (~3 μm),
enabling the measurement of isolated scattering from
single nanoparticle pairs.
The experimental instrumentation is critical to this
work as it overcomes several problems encountered
when trying to measure optical scattering from
membrane substrates with typical dark-field
spectroscopy setups. Standard commercial dark field
condenser objective lenses are designed to illuminate
a large sample area (~20-100 m), which induces a
large amount of background scattering from the
membrane edges where the nitride meets the
supporting silicon substrate. While our
implementation uses a pinhole filter to limit the
collection area of the spectrometer, it also uses a
spatially filtered source to illuminate a much smaller
area (~5 m) of the membrane (100 m x 100 m) to
prevent undesired scattering. The low angle of
illumination in this configuration also provides
near-normal excitation, which minimizes retardation
effects and improves the signal-to-background ratio
[44]. Finally, this technique also employs a dry
objective, which alleviates difficulties associated with
using small, fragile TEM membranes with oil
immersion objectives.
Electromagnetic simulations are performed with
the Lumerical FDTD Solutions package running on
USC’s 0.53 petaflop HPCC supercomputer cluster,
Fianium
white light
source
Pinhole
aperture
Beam
splitter
Beam
blocker
Scattering to
spectrometer
Microscope
objective
Scattered
light
Figure 1 (a) Schematic diagram of the angle evaporation
technique used to create nanogap heterodimers. For
overlapping depositions, the gap size is a function of first
evaporation thickness (t1) and the relative angle between the
two evaporations (θ). (b) Schematic diagram of dark-field
spectroscopy setup. The microscope objective is used as both
the focusing condenser and collection optic. Incoming
polarized white light is spatially filtered through a pinhole
aperture and reflected off a beam splitter into the objective off
the center axis. A beam blocker is used to eliminate reflected
light, while the remaining scattered light is sent to the
spectrometer.
a)
b)
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4 Nano Res.
where we typically make use of 256 or 512 processors
running in parallel over a high performance, low
latency Myrinet network. A grid spacing of 2 Å is
used in the immediate vicinity of the small 1-2 nm
separations between particles, while a larger grid
spacing of 4 Å covers the remaining space of the
metallic nanoparticles. A temporal grid spacing of
0.001 fs is used with a total of 100,000 time steps,
where an initial plane wave source irradiates the
metal nanoparticles with a Gaussian pulse in the
frequency domain of wavelengths ranging from 350
nm to 1000 nm. The frequency response of the system
is recovered by taking a Fourier transform of the time
response. Perfectly matched layer boundary
conditions are used with 12 layers to decrease the
size of the simulation space. The dielectric function of
gold is based on optical data obtained by Johnson
and Christy that is fit to a Lorentz-Drude formula
[45]. The dielectric function of the 10 nm thick silicon
nitride membrane is based on a fit to optical data
with an approximately constant relative permittivity
of 7.5.
Figure 2a shows a high resolution transmission
electron microscope image of a gold nanoparticle
heterodimer fabricated using the angle evaporation
technique. A heterodimer is a nanoparticle pair with
asymmetry in the size, shape, and/or material of the
constituent NPs. Our angle evaporation fabrication
process produces a large asymmetry between the two
particles in all spatial dimensions while yielding a
gap size of less than 2 nm running an 80 nm length
between the nanoparticles. This process also
produces a large number of smaller residual gold
nanoparticles surrounding the two intended
nanoparticles, as shown in Figure 2a. This is likely
due to scattering of metal atoms from the sides of the
PMMA mask opening as well as a slightly
omni-directional flux of metal source vapor coupled
with the high surface mobility of metal atoms and
island-like growth formation of thin metallic films on
oxide and nitride surfaces. While the far-field
scattering from individual particles of such size is
negligible, they can play a strong role in the
near-field and far-field electromagnetic behavior
depending on the particular geometry. The
normalized scattering spectrum for this dimer,
heterodimer A, is shown in Figure 2b. Two resonant
peaks are visible at 660 nm and 755 nm with
polarization aligned parallel to the common dimer
axis, and perpendicular scattering shows a single
peak around 655 nm with a slightly asymmetric
profile. For the case of perpendicular polarization,
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Figure 2 (a) TEM image of an asymmetric nanoparticle dimer, heterodimer A. (b) Measured scattering spectra of dimer
with polarization aligned parallel (black) and perpendicular (red) to the common NP axis. (c) Normalized simulated
scattering spectra based on dimer geometry in (a).
a) b)
c)
Perpendicular
Parallel
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5 Nano Res.
there is weak interaction between the modes of the
two particles, and scattering is dominated by the
perpendicular mode of the largest nanoparticle. The
two peak scattering spectra observed in the parallel
polarization results from the interaction of the broad
dipole-dipole coupled plasmon mode that radiates
efficiently to the far-field and a narrower
quadrupolar plasmon mode that radiates less
efficiently, which causes a dip in the broad
dipole-dipole scattering. The long crescent-shaped
particle and its conformation to the edge of the larger
particle also offer a path for excitation of the gap
modes through perpendicular polarization, which
can explain the asymmetry on the low energy side of
the perpendicular polarization resonance. Deviations
from the ideal circular shape of the first particle and
the position and orientation of the second particle
can also permit weak excitation of the modes
dominant in the opposite polarization. Figure 2c
shows the scattering spectra simulated by the FDTD
method for the nanoparticle dimer based on the
HRTEM image in Figure 2a taken with 2 Å resolution.
In the simulation, the two polarizations of incident
light show two peaks at 678 nm and 744 nm for
parallel polarization and one peak at 671 nm for
perpendicular polarization, respectively. The results
exhibit slightly narrower resonances than those
measured experimentally, which may arise from the
vertical sidewalls and sharp corners defined in the
simulation. Further details about the definition of the
nanoparticle geometries in FDTD are available in the
Electronic Supplementary Material (ESM). The
fabricated nanostructures are polycrystalline in
nature as evident from TEM imaging. Roughness
from such grains increases the surface plasmon losses
and broadens the LSPR resonances [46, 47]. We find
that annealing these structures helps to decrease the
polycrystallinity and may reduce such losses but can
also fuse the heterodimer into a single asymmetric
particle. Despite our idealization of semi-cylindrical
nanoparticles, excellent agreement between
simulation and experiment is observed.
Figure 3 shows heterodimer B, with a smaller size
than the first dimer and with a smaller number of
surrounding residual nanoparticles. This
heterodimer also shows two nanogap “nodes” at the
top and bottom of the gap region, with particle
spacing of 1.2 and 2.0 nm, respectively. The observed
scattering resonances are blueshifted with respect to
heterodimer A, due to size-dependent retardation
effects that lower the plasmon resonance energy of
the larger nanoparticles. The experimental and
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a) b)
c)
Figure 3 (a) TEM image of an asymmetric nanoparticle dimer, heterodimer B. (b) Measured scattering spectra of dimer
with polarization aligned parallel (black) and perpendicular (red) to the common NP axis. (c) Normalized simulated
scattering spectra based on dimer geometry in (a).
Perpendicular
Parallel
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6 Nano Res.
simulated scattering spectra again show good
agreement with two peaks in the parallel polarization
with the same relative intensity. The experimental
spectrum shows a less exaggerated dip in the center
of the two measured peaks and a general blueshift
with respect to the simulation by about 40 nm. The
sharply defined cylindrical geometry used for the
simulated nanoparticles and a local change in index
of refraction due to multiple surface treatments with
oxygen plasma to remove amorphous carbon,
respectively, are two possible reasons for this
discrepancy. The oxygen plasma treatment also acts
to thermally anneal the gold particles, reducing the
polycrystallinity of the lithographically patterned
structures. Fewer grains are observed in these
metallic structures after annealing (Figure 3a) than in
structures before annealing (Figure 2a). Thermal
annealing also reduces the number of residual
nanoparticles surrounding the dimers, where the
residuals either coalesce with one another or fuse
into the two larger heterodimer particles.
The heterodimer asymmetry and gap size defines
the plasmon mode coupling and spectral scattering
behavior. In symmetric nanoparticle dimers, the
lowest energy dipolar plasmon modes of each
particle (and higher order modes) couple together
forming “hybridized” or coupled dimer modes.
Decreasing the separation between two nanoparticles
causes significant interparticle coupling between the
individual particle modes with similar energy [48,
49]. Such dimers are characterized by a single peak in
their far-field scattering spectrum from an in-phase
(bonding) dipole-dipole coupled mode, which
redshifts as the separation between the two particles
is decreased. The out-of-phase (anti-bonding)
dipole-dipole mode conversely blueshifts with
decreasing separation, but the out-of-phase dipole
moments of the induced charge oscillation effectively
cancel far-field scattering. Likewise, the symmetry of
higher order modes prevents excitation from and
radiation to the far-field. It should be noted that the
formalism of plasmon hybridization is strictly only
valid in the quasistatic limit where there is no
radiative damping or phase retardation, but we will
use the term “hybridized mode” to describe a
coupled mode in a heterodimer where the larger
particle may slightly exceed this limit [50]. Symmetry
breaking can allow formerly dark modes to couple
with bright modes and add peaks to radiative
scattering [42, 50-54]. Asymmetry in the nanoparticle
size and shape can result in significantly different
plasmon mode energy profiles for the two particles,
which can separate the low energy modes but also
overlap higher order modes of one particle with
lower order modes of the other particle [50].
The underlying nature of the modes responsible for
the heterodimer scattering can be understood by
looking at the frequency dependent charge
distribution, as shown in Figure 4. The two peak
scattering observed in these heterodimers is the
result of interference between a broad dipolar
hybridized plasmon mode and a dark quadrupolar
hybridized mode, often referred to as a Fano
interference [55]. These modes are shown in the
charge profiles of heterodimer B in Figure 4 at two
different wavelengths, λ = 650 nm and 715 nm. The
charge map at 715 nm provides a clear visualization
of the bonding dipole-dipole coupled resonance of
the heterodimer. This mode forms a broad
superradiant envelope for scattering as the
summation of the individual electric dipole moments
of the two nanoparticles. The significant overlap of a
subradiant “dark” mode alters the charge
distribution where the superradiant mode peaks, so a
wavelength is chosen in the tail of the superradiant
mode to demonstrate the dipolar nature of this
resonance without interference. The surface charge
map at 650 nm shows distinctly different coupling
between the particles, where the lower 5 nm portion
of the two facing metal surfaces of the heterodimer
have an opposite charge polarity than the upper
portion of the faces. Since the charge density is
concentrated in the nanogap region of the
heterodimer, these charges create two strong but
opposing electric dipole moments spanning the top
and bottom of the nanogap. The magnitudes of the
dipole moments are not the same due to differences
in charge concentration near the substrate and the
top of the particles. The net result is a reduced dipole
moment that produces a significant decrease in
far-field radiation. The charge map at the scattering
dip shows a mixture of dipolar and quadrupolar
character, disproportioned by the localization of the
charges at the nanogap. From the charge
distributions, we can see that the quadrupolar-like
modes involved in the subradiant mode are not
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7 Nano Res.
quadrupolar in the xy-plane, but predominantly
“xz-modes” where the sign of the charge near the
substrate is flipped with respect to the charge at the
top of the particles. The interaction of the bright and
dark modes and the dimer asymmetry result in
nonuniform charge distributions at the measured
scattering peaks with parallel polarization, as shown
in Figure S1 in the ESM. For perpendicular
polarization, there is predominantly dipolar
character in both nanoparticles at the scattering peak
of 622 nm. However, the nanoparticle asymmetry
also allows weak excitation of the modes
predominantly excited by parallel polarization. We
see evidence for excitation of these other modes in
the charge distribution of Figure 4, where the charges
in the nanogap region switch polarity near the
substrate and differ from the simple dipolar
distribution.
Due to the significant asymmetry in the
three-dimensional size of the two NPs, the energetic
overlap of these modes on the two particles is
generally unfavorable yet enabled by strong coupling
induced when the particles are spaced by just a
couple of nanometers on a dielectric substrate. Figure
Figure 4 Scattering spectrum for parallel (red) and perpendicular (black) polarized light incident on heterodimer B, and
three-dimensional charge distributions corresponding to λ = 622 nm, 650 nm, and 715 nm. The upper charge distribution
mapping shows the surface charge of the full heterodimer. The lower charge distributions show the same distribution but on
the individual nanoparticles, rotated for clarity. A mixed dipole-quadrupole coupled mode is visible on both particles at 650
nm (left). A bonding dipole-dipole mode is visible at 715 nm (right), representing the broad superradiant scattering
envelope. For perpendicular polarization, a hybridized mode featuring two in-phase dipole modes is visible at 622 nm (top),
with evidence for other mode coupling visible in the charge distribution in the nanogap region.
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8 Nano Res.
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+20 nm
+15 nm
+8 nm
+6 nm
+4 nm
+2 nm
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Heterodimer
Heterodimer vacuum
Large NP only
Small NP only
5a shows the simulated scattering from heterodimer
B as the spacing between the first and second
nanoparticle is increased incrementally. As the
separation between the particles is increased from 1.2
nm to around 5-6 nm, the strong dip of the
subradiant mode moderately blueshifts and quickly
loses intensity. The interference is lost for separations
larger than 6 nm, and we observe a single scattering
peak of bonding dipole-dipole modal character that
slowly blueshifts as the gap size is increased. We note
that the superradiant mode exhibits a very gradual
redshift with decreasing gap size, which we attribute
to weak coupling from the large energy difference in
the lowest energy dipole plasmons of the two
nanoparticles (due to the large size asymmetry). The
gap-dependent frequency shift observed in bonding
dipole-dipole plasmons of homodimers is much
more rapid due to the stronger coupling of two
equally energetic modes.
The dielectric substrate is critical to the prevalence
of the dark mode interference, as shown in Figure 5b.
Here, we have plotted the absorption spectra of
heterodimer B on a silicon nitride membrane,
heterodimer B in vacuum (without a silicon nitride
membrane), and the larger and smaller NPs by
themselves. We observe a notable change in the
absorption (and scattering) spectra of the
heterodimer with removal of the dielectric membrane.
There is a general blueshift in the absorption
spectrum due to the removal of dielectric screening
charges that effectively lowers the plasmon energies,
but most importantly, the dark plasmon now has an
almost negligible impact on the bright mode. The
charge profiles at the absorption peak have the same
general distribution in vacuum and on substrate,
governed by the asymmetric NP geometry, but the
latter is more localized near the dielectric interface.
While the image charges induced in a dielectric
substrate act back on each particle plasmon to simply
lower that same plasmon’s energy, the induced
charges can further mediate coupling between that
plasmon mode and other plasmon modes that induce
the same image charge. These effects are strongest for
planar structures where the plasmon surface charges
are in very close proximity to dielectric screening
charges in the substrate and are further strengthened
by the nm-separation between the nanoparticles. This
densely localizes the plasmon and image charges to
the nanogap region and enables stronger coupling
between the hybridized dimer modes (i.e, dipole -
dipole, dipole - quadrupole, quadrupole -
quadrupole). The local absorption spectra for the
individual nanoparticles that compose heterodimer B
show relatively negligible interaction with dark
modes; although, a slight dip is present on the high
energy side of the smaller particle’s absorption peak.
Experimental and theoretical work has previously
shown that a dielectric can induce interference even
Figure 5 (a) Simulated scattering spectra as gap size is
incrementally increased from measured value of 1.2 nm for
heterodimer B. The dashed line indicates the scattering dip in
the spectra. (b) Simulated absorption spectra for heterodimer
B (black), the heterodimer in vacuum (magenta), the larger
nanoparticle only (red dashed), and the smaller nanoparticle
only (blue dashed). Limited or no Fano interference effects
are visible in the isolated NP absorption spectra or the
heterodimer in vacuum in the absence of substrate-mediated
mode coupling.
a)
b)
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9 Nano Res.
in a single isolated plasmonic nanoparticle, such as
an isolated metallic nanocube, through
substrate-mediated interaction between its own
bright dipole and dark quadrupole modes [56-58].
Depending on the nanoparticle geometry and
dielectric constant of the substrate, this interaction
may not yield a Fano interference if there is little
spectral overlap between the bright and dark modes,
as seen in this heterodimer’s individual
nanoparticles.
We note that the planar heterodimers used in this
work are not the only nanoparticle structures to
exhibit Fano interference, but the origin of the Fano
interference observed here is different than in
previously studied cases. The most commonly used
planar configurations of dolmen slabs and
non-concentric ring/disc arrangements produce Fano
resonances, but the typical separations between
nanoparticles are larger than 5-10 nanometers, where
our coupling effects are minimal. These charge
distributions show little variation in the xy-cross
sections at different heights above the substrate, and
the coupling is not due to the intense localization of
charge near the substrate surface. Fano interferences
are also observed in asymmetric combinations of
nanospheres, nanoshells, and nanorods [50, 59].
While very small gap sizes can be created between
these structures, the nm-separation that is formed
between these particles is usually localized tens of
nanometers above the substrate, which reduces the
influence of substrate image charges.
Our study has combined fabrication, microscopy,
and simulation to provide a complete understanding
of plasmonic effects and is one of the first to analyze
these effects optically in top-down
lithographically-patterned asymmetric dimers with
nm-separations. Nm-separation between
nanoparticles is extremely important for intensifying
the local electric field. Such small separations induce
very strong interparticle plasmon coupling that can
lead to very pronounced and sensitive effects in the
electromagnetic near- and far-field regions. We
employed a unique dark-field microscopy system to
measure the spectral scattering from individual
metallic heterodimers fabricated on TEM-compatible
membranes. Together with simulation, we found that
these nm-separated heterodimers interact strongly
with both each other and the dielectric substrate and
can exhibit a Fano interference in the far-field
scattering spectrum. Our simulations show that this
Fano interference is unique compared to previous
planar heterodimer studies, where opposing dipole
moments at different heights above the substrate
lower the effective radiative scattering. The
simulations show that the prominence of this
interference is governed by the physical dimensions
of the two nanoparticles, which determines the
energy of the plasmon modes on the individual
particles, and the amount of separation between the
particles, which determines not only how strongly
these modes couple to each other but also how
strongly the substrate mediates coupling between
these interparticle/hybridized modes.
Acknowledgements
We would like to thank P. James Schuck and Dan
Gargas for their valuable help and discussions about
the experimental measurements. This research was
supported by ONR Award No. N00014-12-1-0570 and
NSF Award No. CBET-0854118.
Electronic Supplementary Material: Supplementary
material (further details about structural modeling of
nanoparticles and charge distributions at additional
wavelengths) is available in the online version of this
article at http://dx.doi.org/10.1007/s12274-***-****-*
(automatically inserted by the publisher). References
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Nano Res.
Electronic Supplementary Material
Plasmonic mode mixing in nanoparticle dimers with
nm-separations via substrate-mediated coupling
Jesse Theiss, Mehmet Aykol, Prathamesh Pavaskar, and Stephen B. Cronin ()
Supporting information to DOI 10.1007/s12274-****-****-* (automatically inserted by the publisher)
STRUCTURAL DEFINITION OF NANOPARTICLE GEOMETRIES IN SIMULATION
The spatial x and y dimensions of each metallic nanoparticle are defined by transmission electron microscope
images at magnifications between 60-100kX. We use threshold algorithm to convert the high contrast grayscale
TEM images into a binary black and white image. This image is imported into the Lumerical FDTD Solutions
package to define the x and y coordinates of a metallic structure. The z dimensions of the two heterodimer
nanoparticles are defined to be the evaporation thicknesses of 30 nm and 15 nm. The smaller residual particles
are also defined by the binary image, where the height of these particles is estimated from their individual size.
For circular cross sections in the xy image, the radius of this circular area is used for the height of the residual
particle. For elliptical particles, the height is set to be the minor radius of the particle. For more complicated
cross-sectional shapes, the particles are subdivided into major sections where approximations of the previous
two techniques are combined.
Address correspondence to Stephen B. Cronin, [email protected]
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Nano Res.
Figure S1 Simulated scattering spectrum for heterodimer B with parallel (black) and perpendicular (red) polarization. Charge
distributions depicted for the calculated scattering peaks instead of bright envelope and dark dip shown in Figure 4. Insets show
three-dimensional charge distributions for parallel polarization at λ = 608 nm (lower left) and 672 nm (lower right) and perpendicular
polarization at λ = 622 nm (upper right). Blue represents positive charge and red represents negative charge. Lower charge distributions
show the same charge distribution but on the individual nanoparticles, rotated for clarity. Complex charge distributions occur in the
nanogap between the nanoparticles for both polarizations.