Revealing the Hidden Plasmonic Modes of a Gold Nanocylinder - The

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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

Revealing the Hidden Plasmonic Modes of a Gold Nanocylinder Artur Movsesyan, Anne-Laure Baudrion, and Pierre-Michel Adam J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b05705 • Publication Date (Web): 20 Sep 2018 Downloaded from http://pubs.acs.org on September 21, 2018

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The Journal of Physical Chemistry

Revealing the Hidden Plasmonic Modes of a Gold Nanocylinder Artur Movsesyan,

∗

Anne-Laure Baudrion, and Pierre-Michel Adam

Light, nanomaterials and nanotechnologies (L2n),Charles Delaunay Institute, CNRS FRE 2019, University of Technology of Troyes, Troyes, France

E-mail: [email protected]

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Abstract The metallic nanocylinder is one of the most studied simple plasmonic nanostructures. Due to its internal symmetry, it can sustain dierent modes, but the majority of the studies focus on the horizontal dipolar resonance. Studying the whole variety of the excited modes in an individual nanocylinder is crucial to the development of further applications. Herein we analyze the modes excited in a single gold nanocylinder on a glass substrate with standard optical characterization techniques and numerical simulations. The analysis of the scattering radiation patterns of the dierent modes shows that specic collection geometries are required to distinguish them. We propose a new method to record the hidden modes using a conventional dark-eld scattering microscope and for the rst time we show how the vertical dipolar mode of a single nanocylinder can be revealed optically.

Introduction Localized surface plasmon resonances (LSPRs) are the collective oscillations of conduction band electrons at the surface of metal nanoparticles (NP).

13

The LSPRs are relaxed by

non-radiative (absorption) and radiative (scattering) losses. The LSPR leads to a strong enhancement and a high connement of the electric eld around the NP.

4,5

A single nanoparticle

may exhibit many tunable plasmonic resonances (modes). Each plasmonic mode features a special spectral position and spatial distribution of the electric eld, which is fundamental for applications such as surface enhanced spectroscopy, nonlinear optics, near eld microscopy and biosensing.

612

These applications benet from the tunability of the resonances due to

the high sensitivity to shape, size, refractive index of the surrounding medium and plasmonic material.

1315

A metallic nano-object may sustain a dipolar mode and higher order modes known as quadrupole, hexapole, octupole and etc.

1618

Even plasmonic modes such as quadrupolar,

octupolar have net zero dipolar moment and they are considered as dark modes.

2


1921

It

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means these modes do not radiate, and they are relaxed only by absorption.

Hence, the

scattering measurements do not record these modes even though they are excited. For nanoparticles, whose sizes are greater than the quarter of the illumination wavelength, multipolar modes may be excited due to non-uniform electric eld inside the nanoparticle caused by phase or amplitude variation.

17

For the nanoparticles much smaller than the illumi-

nation wavelength, when the electric eld is uniform across the nanoparticle, the multipolar even modes do not couple to a planewave and are not usually excited.

22

However, near the

substrate, a symmetry breaking creates nonuniform electric eld across a small nanoparticle and leads to hybridization between dipolar and multipolar plasmonic modes, which allow the excitation of the hybridized multipolar modes with a plane wave.

15,20,23

For nanoparticles

like nanocylinders, the multipolar modes can be excited using an oblique illumination.

18,24

The excited dark modes can be revealed with a use of extinction spectroscopy, because this technique gives a sum of the scattering and absorption. As an excited dark mode contributes on the absorption spectrum, it contributes on the extinction spectrum too. Besides the higher order modes, a nanocylinder may exhibit dipolar modes of two symmetries by building-up a dipolar oscillation along the diameter (in-plane) and along the height (out-of-plane). Former (in-plane) dipolar mode is a main resonance discussed for the majority of the plasmonic systems. Latter dipolar mode (vertical), with oscillations along the height, is rarely observed because of the special excitation requirements like an electric eld with a strong vertical component. Vertical dipolar mode position and charge distribution for the single nanocylinder were predicted by numerical simulations,

25,26

but it was not sup-

ported by experimental proof. The observation of this mode has been mentioned for colloidal solution of chemically synthesized silver nano-prisms of dierent sizes, it was described as a weak, broad and barely discernible mode.

27

Csete et al. also reported the observation of

the transverse (out-of-plane) plasmonic mode of the aggregates of silver nanoparticles.

28

The

experimental optical observation of the vertical dipolar mode for a single nanoparticle was never reported before.

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In this paper we analyze plasmonic modes appearing in a single nanoparticle spectrum, regarding radiative and non-radiative losses. We study the dark-eld scattering response of a single gold nanocylinder and compare with angle resolved extinction measurements. The oblique polarized illumination couples to the usually hidden plasmonic modes and allows to detect them under specic collection conditions. Hence, we report on the rst experimental observation of the vertical (out-of-plane) dipolar mode in a single nanocylinder.

The

thorough analysis of the broad spectrum far-eld radiation patterns for the wide spectral range explains the plasmonic modes directionality and the demanded collection specications. These specications are fundamental for the applications using nanoparticles as nanoantennas. The characteristics of a nanocylinder present it as a multiresonant system which was previously observed in complex structures.

The understanding of the new plasmonic

modes appearing and behavior are completed by the calculations of the surfaces charges distributions. Finally, we demonstrate a novel trial approach to recover experimentally the hidden plasmonic modes via dark-eld spectroscopy.

Method Gold nanocylinders (GNCs) were produced with the help of electron beam lithography. The GNCs diameter is 170 nm and the height is 50 nm. We study single nanoparticles and lowdensity array. It has been shown that an array of GNC with interparticle distance of 1µm shows the characteristics of a single nanoparticle plasmonic resonances.

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Dark-Field (DF)

commercial microscope (Axio Imager Z2 from Zeiss) is used for single particle measurements. The illumination numerical aperture (NA) is 0.8-0.95, which correspond to incidence angles of 53-71 degrees. The illumination and the collection scheme is shown in Fig 1a. The angle resolved extinction spectra are taken using a home-made optical setup and its simplied illustration is depicted on the Fig. 1b. We use a broad band white light source with a ber output. The light is collimated with a set of lenses and goes through a polarizer placed before

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the illumination objective (NA=0.28) presenting a long focal distance objective. The sample can be rotated by a controllable angle up to 60 degrees with regards to the illumination (objective) axis.

The collection objective (NA=0.42) is coupled to a 200µm-core ber in

order to spatially lter the collection zone. The diameter of the collection area is about 8

µm.

The signal is then sent via the ber to a spectrometer (Ocean Optics QE65000). The

numerical simulations are performed with the help of a commercial software from Lumerical, based on the Finite Dierence Time Domain method.

The calculated extinction spectra

present the sum of the scattering spectrum and the absorption spectrum. We calculated the surface charges distribution using the following formula (Gauss' theorem):

ρ(r) = 0 ∇ · E, where

ρ(r)

space, and

∇

is the charge density at the specic point,

(1)

0

is the permittivity of the free

is the divergence of the electric eld.

Results and discussion The Fig.

1c and 1d show an experimental DF scattering spectrum of a single GNC and

an extinction spectrum recorded on a GNC array respectively.

The optical response of

plasmonic nanoparticles can depend on the illumination and the collection conditions.

30,31

Nevertheless, both spectra show a similar single peak, which we attribute to the dipolar mode excitation. The extinction spectrum of Fig. 1d is obtained using a close to normal incidence excitation. The incoming eld of normal incidence induces a phase variation along the height of GNC. 50 nm height is small compared to the illumination wavelengths in order to create an important phase variation inside the GNC. Therefore, we can consider that the eld inside the GNC is homogeneous and the retardation eect is not signicant, then the higher modes are not excited. Also, the higher order modes may be strongly blue shifted compared to the dipolar mode (below 500 nm wavelengths) and totally damped by

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the inter-band transitions of gold. In dark-eld microscopy the conic illumination induces a phase variation approximately along the diagonal of GNC. In this case the electric eld inside the GNC is not uniform anymore and the other modes besides the dipolar one can be excited due to retardation eect. However, it may not be observed in Fig. 1c because of the non-radiative character of the high-order modes or the limitation of the numerical aperture of the objective. We perform the angle resolved extinction measurements for P and S polarized illuminations.

The Fig.

2a shows the extinction spectra of the GNC array measured at dierent

◦ ◦ ◦ ◦ angles (15 , 30 , 40 , and 50 ) relative to the objective optical axis. P-polarized.

The illumination is

The spectrum for the illumination at 30 degrees (red curve) shows a rise of

the plasmonic band around 605 nm. Moreover, with the increase of the illumination angle

collection

a)

b)

illumination

Extinction

Scattering 0.14 0.12

0.07

c)

0.06

0.10

Intensity (a.u)

Intensity (a.u)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.08 0.06

d)

0.05 0.04 0.03

0.04 0.02

0.02 500

550

600

650

700

750

800

850

900

0.01 500

550

600

650

700

750

Wavelength (nm)

Wavelength(nm)

Figure 1: (a) The scheme of DF illumination and collection.

800

850

900

.

The illumination is linearly

polarized before the objective. At the focus of the objective there is a mixture of S and P polarizations. (b) The simplied scheme of the angle resolved extinction spectroscopy. (c) Experimental DF scattering spectrum of a 170 nm-diameter and 50 nm-height GNC. (d) Experimental extinction spectrum of 170 nm-diameter and 50 nm-height GNCs at normal incidence.

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besides the well pronounced peak (i) at 761 nm, one may see the appearance of two peaks, around 605 nm (ii) and around 525 nm (iii). The peak positions are determined by tting the curve with Gaussian functions. It is important to note that new peaks were not observed for the 15 degrees or normal incidence illumination (Fig. 1d). Depending on the angle of the illumination the electromagnetic wave can couple to the plasmonic modes which are not excited for normal incidence.

18,25,26

The Fig. 2b shows the simulation of the extinction

spectra of a GNC for the same conditions. In Fig.

2c we show the experimental extinction spectra for a S-polarized wave at the

oblique illumination from 15 to 50 degrees.

There are two plasmonic modes expressed at

738 nm (mode 1) and at 601 nm (mode 2) for the illumination at 50 degrees.

Mode 2

becomes more evident for 40 and 50 degrees illuminations, while for the normal and the 15 degrees incidences it is imperceptible. The simulated extinction spectra of the GNC for the S-polarization are shown in Fig.

2d.

One may note a good agreement between the

experimental and calculated spectra for both polarizations. To understand the nature of the non-dipolar modes we show the calculated scattering and absorption spectra of GNC for dierent polarizations in Fig.

3.

In this manner we

separate the radiative and non-radiative contributions of the plasmonic modes. The calculated absorption and the scattering spectra for P-polarization illumination are shown in the Fig. 3a (respectively black and red curves). The absorption spectrum shows three plasmonic modes labeled (i),(ii) and (iii), while the scattering spectrum shows a small shoulder between the position of mode (ii) and mode (iii). The radiative losses cause a strong broadening of the plasmonic resonances and the mode (ii) and (iii) are overlapped and not separable in the scattering spectrum.

We assume that the non-radiative losses cause smaller spectral

broadening, which leads to the separation of modes in the absorption spectrum. In Fig. 3c we show the absorption spectrum of GNC for S-polarized, 50-degrees illumination. The absorption spectrum shows two plasmonic modes labeled (1) and (2) like in the extinction spectrum (Fig. 2d), while the scattering spectrum (red curve in Fig. 3c) shows

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Simulation

Experiment 0.06

0.45 Ex k

a)

Ez E

y

b)

x

P-polarized

Extinction

z

0.04

0.30 i

i

15o

15o

30o

o

30

0.02

o 0.15 40 iii

o

40

ii

iii

ii

50o

50o

0.00 500

600

700

0.00 900 500

800

600

700

800

900

Wavelength (nm)

Wavelength (nm) 0.15

0.45 E

c)

k

y

d)

x

S-polarized

z

0.10

Extinction

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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15o

15o

30o

30o

0.05 40o

0.15

2

40o

50o

0.00 500

1

0.30

1

2

50o

600

700

800

0.00 900 500

Wavelength (nm)

600

700

800

Wavelength (nm)

900 .

Figure 2: (a) Experimental P-polarized angle resolved extinction spectra of GNCs for different angles of illumination, (b) corresponding numerical simulation. (c) Experimental Spolarized angle resolved extinction spectra dierent angles of illumination, (d) corresponding numerical simulation. The dashed lines are the ts of the spectra by Gaussian proles. The spectra are oset for better visualization.

only one mode and it does not show any genuine radiative mode between 500 nm to 600 nm. Then we consider "mode 2" as a dark mode. In order to identify the excited plasmonic modes we perform the surface charges distribution (SChD) calculations.

The calculated SChDs of 750nm (mode i), 594nm (mode ii)

and 523nm (mode i) for P-polarized, 50 degrees, plane wave excitation are shown in the

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Intensity (a.u.)

Ex

а) k

0.12

0.08

d)

Ez E(P)

iii

ii 0.04

500

i 550

600

650

700

z 750

800

850

900

Wavelength (nm)

0.20

Intensity (a.u.)

b)

S-polarized

Scattering Absorption

0.16

P-polarized

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


y

c)

Scattering Absorption

x

E(S)

0.15

k Mode 2

0.10

2 0.05

0.00 500

1 550

600

650 700 750 Wavelength (nm)

800

850

900

.

Figure 3: (a) Calculated absorption and scattering spectra (collection - 4π ) of the GNC, when the illumination is at 50 degrees and P-polarized. (b) Calculated charge distributions for the wavelength of 523 nm (mode (iii)), 594 nm (mode(ii)) and 750 nm (mode(i)). The dashed line on the charges distribution map of mode (i) shows that the dipole moment is tilted with respect to the normal. (c) Calculated absorption and scattering spectra of the GNC for 50 degrees and S-polarized illumination. (d) Calculated charges distributions for the wavelengths 588 nm (mode 2) and 744 nm (mode 1).

Fig. 3b. The excitation wave vector and the electric eld directions are depicted also. Here, the mode (i) shows a dened dipolar mode excitation, where the negative and the positive charges are placed on the counter sides and create an eective net dipole moment. The mode (ii) SChD on XZ (out-of-plane) prole is revealed to be diagonal quadrupole-like mode. The excitation of a quadrupole mode takes place when there is an overlap between the eld distribution of the incident planewave and the eld distribution of the mode itself. Herein, we demonstrate it is possible to excite a quadrupole-like mode with an oblique illumination, since the opposite charges are counter localized compared to the illumination axis. On the one hand, using the oblique incidence we increase optical path of the incident eld along the GNC and the phase variations inside GNC, which results in a new plasmonic mode (mode

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Page 10 of 21

ii). On the other hand, we obtain a complex and asymmetric charge distribution caused by the substrate, the asymmetry of the GNC along the illumination and the direction of the incoming eld (polarization). This asymmetric charge distribution of mode (ii) for in-plane (XY) and out-of-plane (XZ) proles, results in the radiative character of the mode due to the nonzero net dipole moment. The mode (iii) is associated with a vertical dipole mode (Fig. 3b). Unlike the horizontal dipole (mode (i)), the vertical dipole oscillates along the height axis of the GNC. This mode is only visible under P-polarized illumination, since it requires an out-of-plane component of electric eld (Ez ) to be excited.

The angle resolved extinction spectra show that the

vertical dipolar mode can be observed for a tilted illumination at 40 or higher degrees. For the illumination at smaller angles, the out-of-plane component of electric eld has not enough magnitude to couple eciently with the vertical dipolar mode. To summarize, the P-polarized tilted illumination excites three plasmonic modes such as the horizontal dipolar mode (mode i), a diagonal quadrupolar mode (mode ii) and the vertical dipolar mode (mode iii) and they radiate. In Fig.

3d we present the calculated SChD of the GNC XY prole, for 744nm, S-

polarized, 50 degrees excitation.

In this case the incoming electric eld is parallel to the

substrate. Herein, we clearly see that the "mode 1" is a dipolar mode. The calculated SChD for the "mode 2" (588nm) features the in-plane quadrupolar mode, when the two counter dipole moments results in almost zero dipole moment, and so a non radiative or dark mode. The extinction spectra on Fig. other modes than dipolar.

2a and 2c show that the oblique incidence can excite

However, the scattering spectrum on Fig.

1c shows only a

dipolar mode although the conditions to excite higher order modes like the illumination at oblique incidence, are there.

Indeed, the main dierence between the extinction and the

DF scattering measurements is the collection process. We conrm the assumption that an oblique incidence may excite other modes than dipolar but they are not detected by the DF microscopy. Further to understand why we do not observe experimentally the higher order

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modes in the DF spectrum we perform the calculation of the scattering radiation patterns for two collection geometries (reection and transmission) and two polarizations in air (Fig. 4). The Fig. 4a and 4b show the illumination geometry respectively for the P and the S polarizations. The illumination takes place at an angle of 50 degrees compared to the normal of the

◦ sample (marked as 90 on the schemes). The red dotted semicircle corresponds to the transmission geometry and the green one corresponds to the reection geometry. The calculations are done for an azimuthal angle equal to zero and the polar collection angle varies from 0 to 180 degrees. The Fig. 4c pictures the radiation pattern for the reection geometry projected on a semicircle. The horizontal axis shows the wavelengths and the vertical one shows the polar angle. The prominent dipolar mode peak is at the wavelength of 750 nm and at the

◦ angle of 100 degrees. While the emission of a horizontal dipole should be centered at 90 , the tilted illumination creates a tilted dipole moment, as shown on the charge distribution of mode (i) (Fig. 3b). This 10

◦

shift is then a consequence of the tilted illumination. Fur-

thermore, besides the dipolar mode, there are one or several modes in form of two lobes in dierent directions (indicated by white arrows on the gure). One is mostly back scattered (120-160 degrees) at the wavelengths 500-560 nm and the second is forward reected (20-50 degrees) at the wavelength 570-610 nm. On the far eld radiation pattern for the transmission geometry (Fig. 4d), one can observe a lobe near the dipolar mode. This lobe is located around 500-600 nm and is mainly forward scattered (20-60 degrees). Although these lobes for transmission and reection geometries show the radiative nature of mode (i) and mode (ii), we do not see clearly these modes on the scattering spectrum (Fig. 1c). The scattering spectrum on Fig. 3a shows that even if we could collect the scattering for full 4

π

angle, we do not see other mode than dipolar one. Indeed, the strong dipolar mode

intensity covers the other modes and makes them invisible.

Moreover, the experimental

scattering spectrum is collected by an objective with a numerical aperture of 0.8 represented by the horizontal dashed lines on Fig 4c and 4e. Therefore it does not collect completely the

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10 -13

0

c)

4 3.5

Angle (degree)

90

Ex 50 o

k

0

20 40

a)

O

4.5

O

180

x

3

80 2.5

100

O

120

180 500

1 0.5

550

600


800

850 10 -13 6

20 40

90

2

1.5

0

d)

z

E

2

160

Ez E(P)

y

60

140

Angle (degree)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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5

60 4

80 3

100 120

E

2

2

O

140 1

160 180 500

550

600


800

850

e) 90

b)

O

E

2

E(S) 50 o

k

0

O

y

180

x

O

f)

z

E 90

2

O

. Figure 4:

(a) and (b), schemes of the P-polarized and S-polarized 50 degrees excitation

respectively.

The reection and transmission collection geometries are given by the green

and the red dotted semi-circles respectively. - polarization,

Ex

and

Ez

k - wave vector, E - electric eld, P and S

components of the electric eld.

(c) and (d), angle dependent

collection far-eld map for reection and transmission geometry respectively for P-polarized, 50 degrees excitation. (e) and (f ), angle dependent collection far-eld map for reection and transmission geometry respectively for S-polarized, 50 degrees excitation. The white dashed lines show the collection boundaries for a numerical aperture 0.8, when the collection is centered at 90 degrees.

radiated power of the lobes as their radiation is out of the collection zone. That is why we do not observe experimentally these new modes on the DF scattering spectrum.

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The radiation patterns of the reection and the transmission collection geometry for a S-polarized illumination are shown respectively in the Fig.

4e and 4f.

These maps show

a mode (560-620nm range) which radiates from 20 to 40 degrees. However, the scattering spectrum (Fig. 3c, red curve) does not show any evidence of the quadrupole mode ("mode 2"), even though there is a small radiative part besides the absorption.

Herein, also the

dipolar mode covers other excited modes due to the ratio between the radiative weights of each mode.

Therefore, the only way to observe these new modes using the scattering

technique is to minimize the horizontal dipolar mode contribution on the recorded scattering spectrum. Although it has been reported multiple studies of plasmonic nanoparticles using scattering microscopies, these hidden modes were not observed due to their low scattering cross-sections compared to the main dipolar mode. revealed by electron energy loss spectroscopy.

3236

21,37

In some works, the hidden modes are

This technique with huge potential has

also some important limitations such as the substrate type (TEM specimen) and it requires quite expensive experimental setup. We propose to use the dark-eld distance translation spectroscopy on a single GNC to retrieve these hidden modes. Fig. 5 shows the dark-eld scattering spectra for a GNC when the collection zone is either centered on it or apart from it. These spectra have been recorded by using a piezo stage. In the case of D = 0, the GNC is centered compared to the collection zone as shown by the dark-blue dashed line on the inset of Fig. 5. If we move laterally the sample to a distance D, the collection area is moved to the same distance. It is important to note that a micro-metric motion cannot change signicantly the excitation process as the illumination with a broadband source has a large (hundreds of microns) spot. The red dash line on the inset of the Fig. 5 (gray colored zone) corresponds to the new collection angles. In this manner we change the collection geometry and lter some radiation coming from the dipolar mode. In that case, the ratio between the radiations collected from the dipolar mode and from the other modes is changed. The translation below 500 nm does not lead to any signicant changes except the reducing

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O

90

O

O

0

y

180

D x z

i'

ii' iii' . Figure 5: DF scattering spectra of a 170 nm-diameter GNC when the collection is centered on it and when it is moved from the center of the GNC by D distance. The inset illustrates the collection zones when the collection is centered on the GNC (blue dash line) and when the collection is moved from it (red dash-line) by D distance.

The spectra are oset for

better visualization.

of the dipolar mode intensity. The main changes on the scattering spectra are observed when the collection zone translation is 500 nm and more. The spectrum for D = 500 nm shows a small plasmonic band around 525 nm and a symmetry breaking of the main peak (left side around 550 nm). For the furthest translation of the collection zone, one may observe that another plasmonic band arises around 585 nm. This mode is observed for longer translation because the long distances provide a strong reduction of the dipolar mode contribution, which enables one to observe the hidden modes. Indeed, the experimental dark-eld scattering spectrum recorded with this new collection geometries (Fig. 5, D = 1000 nm) conrms the detection of new peaks (mode (iii') and mode (ii')). The mode (i') is the prominent dipolar mode, whereas the mode (iii') is vertical dipolar mode and the mode (ii') matches spectrally with the quadrupolar mode. Note, the dark-eld illumination contains in-plane and out-of-plane components of the electric eld. They may

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excite two quadrupole modes such as in-plane (mode 2) and diagonal (mode ii), which are spectrally close. Therefore, the scattering spectrum represents the contributions of in-plane and out-of-plane excited modes.

Conclusion In this article, we showed that the standard use of dark-eld spectroscopy on single metallic nanocylinders is mostly suitable to record the prominent (horizontal) dipolar plasmonic resonance.

We used an angle resolved extinction spectroscopy and numerical simulations

to investigate the dierent plasmonic modes of a single GNC of 170 nm-diameter.

The

calculations of the far-eld maps show that hidden modes radiates mostly in directions, which are out of the collection geometry of the objective.

We propose a new method to reveal

the hidden modes with a standard dark-eld microscope by changing easily its collection geometry. During this study, we found that the quadrupolar mode is spectrally close to the horizontal dipolar mode.

The spectral position of the quadrupolar mode depends on many

parameters, while the vertical dipolar mode is strongly correlated with the height of the GNC. The height and diameter control of the GNC can lead to a plasmonic system possessing two easily controllable resonances. These two tunable LSPRs can be used in surface enhanced spectroscopies and bio-sensing. As both modes have dierent spatial distribution of their electric eld, they can also be used in sensing applications based on the sensitivity of the plasmonic resonances to the local refractive index.

Acknowledgement This work was prepared in the context of the European COST Action MP1302 NanoSpectroscopy.

We are grateful to Loïc Le Cun, Alina Muravitskaya and Frédéric Laux for

productive discussions. We express our gratitude to Aurélien Bruyant for loaning the piezo-

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stage. The authors acknowledge the Nano'Mat platform for nanofabrication facilities. The numerical simulations were supported by the HPC Center of Champagne-Ardenne ROMEO. We acknowledge gratefully Ministère de l'Education Nationale, de l'Enseignement Supérieur et de la Recherche for providing the Doctoral fellowship.

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Revealing the Hidden Plasmonic Modes of a Gold Nanocylinder - The

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