Plasmonic mode mixing in nanoparticle dimers with nm ... · Bottom-up fabrication techniques such...

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



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




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]

| www.editorialmanager.com/nare/default.asp

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

www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research

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

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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)


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

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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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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

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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


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.


8 Nano Res.

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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)


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




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]


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.

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