Selective Excitation of Plasmon Resonances of Single Au Triangles byPolarization-Dependent Light ExcitationChawki Awada,† Traian Popescu,† Ludovic Douillard,*,† Fabrice Charra,† Antoine Perron,‡

Helene Yockell-Lelievre,‡ Anne-Laure Baudrion,*,‡ Pierre-Michel Adam,‡ and Renaud Bachelot‡

†CEA, IRAMIS, Service de Physique et Chimie des Surfaces et Interfaces, F-91191 Gif sur Yvette, France‡Laboratoire de Nanotechnologie et d’Instrumentation Optique, ICD UMR CNRS 6279, Universite de Technologie de Troyes, 12rue Marie-Curie, BP 2060, F-10010 Troyes, France

*W Web-Enhanced Feature *S Supporting Information

ABSTRACT: The plasmonic properties of single Au triangularnanoprisms are investigated using photoemission electronmicroscopy with particular emphasis on polarization depend-ence. Two localized surface plasmon resonances (LSPRs) arestudied, namely, the in-plane dipolar and quadrupolar plasmonexcitations. Experimental maps of the near-field spatialdistribution upon polarization and wavelength of the excitingfield are interpreted in the framework of a group theorydescription and finite difference time domain simulations. Thiswork demonstrates the selective excitations, by lifting of degeneracy, of the different LSPR eigenmodes at the single object leveland opens ways for the active control of the angular radiation patterns of optical nanoantennas. This approach is general andapplies to any nano-object, whatever its initial shape symmetry.

■ INTRODUCTIONMetallic nanoparticles (NPs) exhibit remarkable and uniqueoptical properties in connection to plasmon resonances.1 Theselocalized surface plasmon resonances are coherent collectiveoscillations of the charge carriers inside the particle, resulting inthe confinement of the electromagnetic field below thediffraction limit of traditional optics. LSPRs offer a way toconcentrate and manipulate light on the nanoscale, opening awide range of applications from biomedical2 (chemical/biological sensors,3−5 molecule labeling,6,7 imaging contrastagents,8−10 medical therapy,10−12 and catalysis13,14) toinformation technology15 (light guiding16−18 and energyapplications19−21) and so on.In the past recent years, investigations have demonstrated

that LSPRs are extremely sensitive to NP composition,22

size,5,23 shape,5,23 and dielectric environment.5,24−26 Beyondmaterial and geometrical factors, research today focuses onways to finely tune the light matter interaction. Amongpromising tracks, the control of the directional aspect is ofparticular interest.27,28 Recent pioneering works along this linedemonstrated full control of the emission direction of quantumsystems (angular radiation pattern), such as fluorescentmolecules29,30 or quantum dots31,32 through the use ofdirectional nanoantennas.33 These works open new perspec-tives for nano-optical microscopy,34,35 single-molecule fluo-rescence (plasmophore/fluoron),36,37 antenna-based photo-detectors,38 and so on.To that respect, knowing the different polarization radiation

patterns of a plasmonic subwavelength structure as a functionof wavelength or polarization of the exciting field is of prior

importance. So far, the reported experimental investigationsdealt mainly with object ensembles39,40 conducted with far-fieldspectrometries or simple longitudinal objects.41−43 Worksfocusing on the polarization states of more complex plasmonicobjects on the nanoscale level are still few.44−49

Here we present an experimental and theoretical polarizationstudy of the LSPRs of metallic flat nanoprisms, (equilateraltriangles) by photoemission electron microscopy. Both dipolarand quadrupolar resonance eigenmodes are investigated.Beyond a simple energy level description, access to individualspatial polarization patterns acquired at the single object levelreveal high directional contrast of the charge oscillations uponexcitation. Experimental results are interpreted within acomprehensive group theory description complemented byfinite difference time domain (FDTD) simulations.

■ MATERIALS AND METHODSSample Preparation. Regular triangles with height

(altitude) sizes sampling the interval [100 nm, 300 nm], thatis, {100, 140, 200, 250, 300 nm}, were nanofabricated byelectron beam lithography (EBL). The nanoprisms aredistributed according to 4 × 4 square matrices of 16 identicalobjects 5 μm distance apart measured center-to-center (0.5 μmfor extinction measurements) to avoid any significant fieldcoupling. Objects are made out of a 50 nm thick Au filmdeposited on a thin 5 nm titanium oxide (TiO2) layer acting as

Received: April 11, 2012Revised: June 8, 2012Published: June 11, 2012

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an adhesion layer. Some previous works show the TiO2 layer isinactive onto the metallic resonance.50 The substrate consists ofa commercial polished high-quality float glass coated with a 5nm indium tin oxide (ITO) layer.51

Experiment. Photoelectron emission microscopy measure-ments were carried out on a previously described setup.52 Inshort, the photoemission process is strongly enhanced uponexcitation of LSPR, and the multiphoton photoemissionelectron maps reflect the actual distribution of the opticalnear-field of the metallic object under investigation.52−55 In thecurrent experiment, Au nanoprisms, held at low temperature(∼200 K), are excited under grazing incidence (angle of 75°between k-vector and surface normal) with the pulsed output ofa mode-locked Ti:sapphire-oscillator (Chameleon Ultra II,Coherent, repetition rate 80 MHz, pulse width 140 fs)delivering infrared (IR) photons in the λ = 690−1000 nm(1.80 to 1.24 eV) wavelength range. Peak power densities at thesample surface are in the range 50−150 MW/cm2. Thepolarization of the laser beam can be adjusted with a half-waveplate from p- to s-polarization. Experimental conditionsare optimized to maintain fundamental parameters of theilluminating beam during data acquisition, in particular,intensity, pulse width, and focus position constant. Note thatwithin the investigated wavelength window the multiphotonphotoemission process corresponds to a three-photon photo-emission regime; see the Supporting Information for moredetails. The photoemission microscope (Elmitec PEEM/LEEMIII) operating under ultrahigh vacuum conditions routinelyachieves spatial resolutions down to 20 nm in PEEM imagingmode. In addition to PEEM imaging, the instrument can beoperated in low-energy electron microscopy (LEEM) mode,where backscattered electrons are used to create an imagereflecting the topography of the sample. In LEEM mode,routine spatial resolution is close to 10 nm, and topographyimages can be spatially correlated to the PEEM signature. Forimage processing, LEEM and PEEM images are background-corrected, and intensities are determined via signal integrationover regions of interest.Note that in contrast with electron-based techniques56

PEEM makes use of a true optical excitation scheme (photonin, electron out) and the polarization state of the excitationlight can be easily controlled.Extinction spectrometry (ES) was used to study the NP

optical properties.57−59 In our experiment, we use a straightmicroscope connected to a white-light halogen lamp as asource. The transmitted light is collected with a ×20/0.4objective and sent on a 80% reflection, 20% transmissionmirror. The transmitted beam reaches a CCD camera aimed atthe particles studied. The reflected light is focused through alens in a 50 μm core multimode optical fiber leading to aspectrometer, providing a confocal filter of the collection area(∼13.3 μm in diameter). Polarization can be changed with apolarizer in front of the illumination source.Figure 1a shows a scanning electron microscopy (SEM)

image of several Au triangles presenting an altitude (in-planeheight) size of 200 nm. The inhomogenously broadened ESspectra recorded on this pattern are shown in Figure 1b. Thered dashed curve corresponds to the spectra recorded with thein-plane polarization aligned along one side of the triangles,whereas the black solid curve has been recorded along oneheight of the nanoprisms. The ES spectra display two opticalresonances. The main peak located at 875 nm stands for the in-plane dipolar LSPR of the nanoprisms. A second peak visible at

∼600 nm corresponds to the in-plane quadrupolar nanoprism.For both resonances, whatever the polarization, the peak widthand intensity are similar, indicating a degeneracy of the mode,which is consistent with the group theory approach presentedhereafter. The latter conclusion has been also checked at thesingle object level by dark-field spectrometry60 (not shown).Note that extinction and photoemission spectra may differ by

a rigid shift whose origin can be traced back to (i) thenanoprism geometry (inhomogeneous vs homogeneous inves-tigations) and (ii) differences in the physics involved. Indeedphotoemission exploits a multiphoton absorption process whileES is the sum of absorption and scattering.47

■ GROUP THEORYThe investigated regular nanoprisms exhibit structuralsymmetries corresponding to the D3h point group (notconsidering the substrate). The different resonance modescan be determined from the examination of the correspondingcharacter table 1. Following standard conventions, the principalorder rotation axis is taken as the z axis of the Cartesianreference coordinate system (Ox, Oy, Oz).

The irreducible representations (irrep) of the dipolarresonance modes are A″2 and E′.61 The 1D irrep A″2corresponds to an electric dipole along the principal axis zand can be interpreted as an in-phase oscillation of the chargecarriers perpendicular to the triangle plane. The 2D E′ irrepcorresponds to electric dipole modes in the xy plane, that is, in-plane collective oscillations of the charge carriers. Similarly, thequadrupolar modes of the triangle nanoprisms correspond tothe A′1, E′, and E″ irreducible representations with E′ renderingthe symmetry of the in-plane quadrupolar modes.Beyond symmetry consideration of the object itself, one has

to consider how it interacts with an external field. As a matter of

Figure 1. Extinction spectra. (a) SEM image of Au regular triangles ofheight (altitude) 200 nm. (b) Corresponding ensemble extinctionspectra. The red dashed and solid black curves correspond to theextinction spectrum recorded with the in-plane incident electric fieldpolarized along, respectively, one edge side and one height of thetriangles. Spectra show two optical resonances. The main peak locatedat 875 nm corresponds to the in-plane dipolar LSPR. The lesspronounced peak at ∼600 nm is the quadrupolar LSPR.

Table 1. Character Table for the Group D3h

D3h E 2C3(z) 3C′2 σh(xy) 2S3 3σvlinearfcts quad. fcts

A′1 +1 +1 +1 +1 +1 +1 x2 + y2, z2

A′2 +1 +1 −1 +1 +1 −1 Rz

E′ +2 −1 0 2 −1 0 (x,y) x2 − y2, xyA″1 +1 +1 +1 −1 −1 −1A″2 +1 +1 −1 −1 −1 +1 zE″ +2 −1 0 −2 +1 0 (Rx, Ry) (xz, yz)

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Figure 2. Dipolar modes. (a,b) Symmetry-adapted linear combinations (SALCs) corresponding to the two dipolar eigenmodes of a solid triangle ofD3h symmetry. The charge patterns within the horizontal σh plane exhibit two orthogonal polarization states. (c,d) Maps of the out-of-plane (Ez)

2

near-field computed by FDTD simulations of the dipolar eigenmodes for p and s light polarizations. In-plane height of the triangle 200 nm,excitation wavelength 800 nm, and light incidence 15°. (e,f) Top and side views of the illumination geometry.

Figure 3. Quadrupolar modes. (a,b) Symmetry-adapted linear combinations corresponding to the two quadrupolar eigenmodes of a solid triangle ofD3h symmetry. (c,d) Maps of the out-of-plane (Ez)

2 near-field computed by FDTD simulations for quadrupolar eigenmodes for p and s lightpolarizations. In-plane height of the triangle 300 nm, excitation wavelength 730 nm, and light incidence 15°. (e) Quadrupolar LSPR signature of a300 nm height flat triangle, PEEM picture, photon wavelength λ = 730 nm, and beam incidence angle α = 75 ± 2°. (f) 300 nm height flat triangleimaged by LEEM, electron kinetic energy 0.8 eV.

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fact, the interaction between the different LSPR modes En of anobject with an external field Eext depends on the symmetry ofboth the object under investigation and the external field.Recently, Zhang et al.62 extended the selection rules establishedfor a scalar field (a wave function) to the case of a vector field.The full selection rule states that: if the ith component of En,respectively Eext, transforms as the irreducible representationΓn,i, respectively Γext,i (i = x, y, z), then the excitation strength ofthe resonance mode n ⟨En|Eext⟩ vanishes unless there is aproduct Γn,i ⊗ Γext,i transforming as the totally symmetric irrepA. In the present case, the light illuminating the triangularnanoprisms under grazing incidence is vectorial in nature Eext =(Eext,x, Eext,y, Eext,z) and transforms as the translation vectors (x,y, z). So, the dipolar mode A″2 is optically active because the zcomponent of the electric field exhibits similar symmetry.However this out-of plane mode corresponds to a resonance ofhigh-energy level inaccessible to the wavelength window of thepresent laser exciting source. Similarly, the two-fold degen-erated E′ state is bright when excited with in-plane electric fieldcomponents (x,y). For the quadrupolar modes the selectionrules conclude that only the in-plane E′ state can be excited inthe present configuration. Details of the selection rules arepresented in the Supporting Information.In the following, the group theory is used to interpret the

charge distribution patterns associated with the dipolar andquadrupolar plasmon resonances of regular flat nanoprisms.Interestingly, upon light excitation, the dipolar plasmonresonances of a solid trigonal object exhibit strong similaritieswith the eigenmodes of a cluster of particles of equivalentsymmetry,63−70 with the difference being that at each of thesolid object’s corners only one type of charge (positive ornegative) resides.71 These resonances can thus be described aslinear combinations of the three corner modes by analogy tolinear combinations of atomic orbitals (LCAOs) and the use ofa symmetry-adapted linear combination (SALC) basis is likelyto provide further insight into the understanding of the LSPRmodes of the flat nanoprisms.The irreducible representation for the ( = 1) dipolar E′

mode is of degeneracy 2, so the physical charge distributions atresonance correspond to linear combinations of a pair of twoeigenvectors basis. The latter is obtained by application of theprojector operators to a basis function set.72,73 Followingorthogonalization, we obtain two orthogonal polarization states,which can be selectively excited via proper choice of thepolarization of the incident external field. In more detail,starting with a set of s-type orbitals mimicking charges withinthe σh(xy) plane attached to the corners of one triangle, theSALC basis for the E′ mode is constituted of the two linearcombinations (2, −1, −1) and (0, −1, 1), whose meanpolarizations are perpendicular. For the ( = 2) quadrupolarmode, the same route can be followed starting with an initialbasis of s-type functions attached to both corner and mid-edgeside positions, as suggested by experiments. Again, the two-folddegenerated E′ mode is spanned by two orthogonal vectorscorresponding to the (2, −1, −1) and (0, −1, 1) linearcombinations of the basis set. However, for the quadrupolarcase the charge patterns exhibit reduced mean polarizations.Figures 2a,b and 3a,b detail the SALC basis obtained for boththe = 1 and = 2 resonances of E′ symmetry, respectively.The group theory analysis has been completed by FDTD

simulations.74 The FDTD computational method is a well-established algorithm to model the spatial distribution of theelectromagnetic field in complex systems. The full set of

Maxwell’s equations is numerically solved on a grid. On eachgrid point, the electric field vector is solved at a given time,whereas the magnetic field vector is solved at the next moment.This process is iteratively repeated until a steady electro-magnetic state is achieved. The calculations have beenconducted on a cubic grid with a 2 nm discretization step.The modeled triangles are 200 and 300 nm in altitude (in-planeheights) and 50 nm in thickness, placed on a-SiO2 layer withrefractive index75 nSiO2

= 1.47. Ti adhesion layer and ITOconductive layer are not considered in the calculations. Therefractive index of the Au metal is taken from Johnson andChristy.76 Simulations are carried out in the illuminationgeometry of the experiment, that is, at 15° (75° off normal)grazing incidence with the beam wavevector aligned to onetriangle height direction. To conform to the sensitivity of thephotoemission microscopy, we report only the vertical out-of-plane Ez component on the metal side of the resonance in theFigures. Figure 2c,d shows the (Ez)

2field distributions expected

for the dipolar E′ eigenmodes in both p and s polarizations,respectively. In agreement with previous works,63,65 dipolarresonance manifests itself through field enhancements atcorners of the triangular shape, all three corners for ppolarization, and only the two corners delimiting the edgeparallel to the incident electric field in s polarization. Similarly,Figure 3c,d reports similar calculation results for thequadrupolar modes. At variance with dipolar signature,quadrupolar resonance modes exhibit enhanced field positionsalong the nanoprism’s mid-edges and corners with lesssignificant azimuthal dependence. All FDTD simulationsshow full agreement with the corresponding SALC eigenvectorsobtained from group theory describing charge patterns atresonance.

■ RESULTS AND DISCUSSIONDipolar Mode. The illumination geometry takes place at

grazing incidence with the light wavevector k aligned along onetriangle height. As a consequence, any rotation of the electricfield around the laser beam translates into an in-planepolarization component whose directions span continuouslyall three edge and height directions of any particular triangle(cf. illumination geometry Figure 2e,f). This peculiarillumination geometry allows for the selective excitation ofthe two orthogonal degenerated dipolar resonance modes of aregular triangle identified by symmetry, namely, E′(2, −1, −1)in p polarization and E′(0, −1, 1) in s polarization.To validate this approach, we carried out full polarization-

dependence studies of the electron emission of regular solid flatnanoprisms. The true exciting field to consider is a super-position of the incident and reflected beams at the samplesurface. Its strength can be formally calculated with the aid ofthe Fresnel coefficients yielding

α θ θ

α θ

= − · + +

·⊥r r r

(E , E , E )

E (( 1) cos cos , (1 ) sin , (1 )

sin cos )

x y zExc. Exc. Exc.

0 // //

(1)

In the above relation, the Fresnel coefficients r// and r⊥correspond to the relative amplitudes of the reflected andincident waves in two appropriate situations, one with thepolarization vector parallel to this incident plane (r//) and theother with the polarization vector perpendicular to it (r⊥).

77 αand θ are the incident and polarization angles, respectively. For

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the illumination geometry used (α = 75°, k // triangle in-planeheight, ITO/SiO2 substrate), the matching of the in-plane fieldcomponent with the edge and height directions of a regulartriangle takes place at polarization angle values θedge = (±90°,−33.5°, +33.5°) and θheight = (−63.3°, 0°, +63.3°) respectively;see the Supporting Information for illustration and calculationdetails.Figure 4 reports the PEEM images recorded at the

polarization angles θedge and θheight defined above andwavelength λ close to dipolar resonance of a 200 nm triangleparticle. The full polarization dependence is available as a videosequence in avi format in the HTML version of this paper.When excited by an in-plane electric field vector aligned alongone of its edges, a nanoprism displays a two-spot near-fieldresonance reminiscent of the SALC E′(0, −1, 1) state and infull agreement with the FDTD simulation. Similarly, forexcitation along triangle heights, dipolar resonances show onestrong spot in qualitative agreement with the expected E′(2, −1,−1) SALC state. Field signature in full agreement with the

expected three spots field distribution is seldom obtained.Missing of the two low-intensity spots is attributed to (i)departure from the expected D3h symmetry,70 and (ii) lowexperimental dynamic recording. According to FDTD, theintensity ratio between low- and high-level intensity spots islarge. Photoemission signature scales as (Ez),

6 and thus theexpected photoemission intensity ratio is frequently challengingthe dynamics of the detection line.

Quadrupolar Mode. In parallel with the dipolar mode, thequadrupolar mode has also been investigated. Note thatbecause of the limited range of available exciting wavelengthsthe quadrupolar resonance is investigated on larger 300 nmprisms to benefit from the red shift of a plasmon resonancewith object’s size. Figure 3e exhibits the quadrupolar resonanceof a 300 nm trigonal nanoprism recorded in p polarization.Owing to the high signal-to-noise ratio intrinsic to nonlinearphotoemission, a clear signature of the quadrupolar mode isobtained. For p polarization excitation, this = 2 signatureexhibits strong field intensities at mid-edge and bottom corner

Figure 4. Polarization dependence of the dipolar LSPR of a 200 nm in-plane height flat equilateral triangle. The top three images report the PEEMsequence recorded at the three successive polarization angles θedge corresponding to incident field polarized along triangle’s edges. The bottomPEEM sequence corresponds to polarization angles θheight, that is, incident field polarized along triangle’s heights. For each polarization angle, aschematic of the right excited eigenmode is displayed. Photon wavelength λ = 800 nm, beam incidence angle α = 75 ± 2°. White scale bar is 200 nm.

Figure 5. Comparison between experimental and (Ez)6 FDTD theoretical polarization dependencies of the quadrupolar LSPR of a 300 nm in-plane

height flat equilateral triangle. Across the sequence, the polarization angle varies from θedge, for which the in-plane incident field component is alignedto one triangle’s edge (s polarization) to (θheight − 15°) corresponding to incident field polarized close to one triangle height direction (ppolarization). For each polarization angle θ, two pictures are displayed, subset (1) corresponds to experiment and subset (2) corresponds totheoretical FDTD predictions. PEEM sequence recorded (respectively, FDTD simulation calculated) at λ = 730 nm photon wavelength, beamincidence angle α = 75 ± 2°, and step in polarization angle 15°.

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positions in full agreement with both the E′(2, −1, −1) chargepattern eigenvector and the theoretical near-field mappreviously determined. These experimental results also showagreement with previous theoretical and experimental inves-tigations.44,64,65,78 Note that at variance with experimentalworks carried out using scattering type near field microscopy s-SNOM,44 whose signal arose from the field outside the objects,the photoemission signature gives a map of the internal fieldclose to the surface (electron reservoir).Figure 5a−f compares the (Ez)

6 FDTD theoretical andexperimental polarization dependencies of the quadrupolarresonance of a 300 nm height triangle. Across this sequence,the direction of the in-plane component of the incident fieldvaries from an alignment parallel to one triangle’s edge (spolarization, θ = θedge = ± 90°) to one close to the triangleheight direction (p polarization, θ = θheight − 15° = 15°)). Foreach polarization angle θ two pictures are displayed: subset (1)corresponds to experiment and subset (2) corresponds totheoretical FDTD predictions.According to FDTD calculations, close to in-plane field

excitation (s polarization), a signature exhibiting first a fieldspot at the edge-midpoint on one side of the triangle, furthercomplemented by one corner, is predicted. When approachingp polarization, the field distribution becomes more symmetricwith a strong signal at the corner position belonging to theheight aligned on the field direction; see Figure 5a.2−f.2. Apartfrom s polarization for which the in-plane excitation translatesinto low electron collection along the optical axis of theinstrument (sample normal), the photoemission signal showsequivalent trends: (i) detection of a single spot at the edge-midpoint for grazing field (Figure 5c.1, −60°) furthercomplemented by one corner signature (Figure 5d.1, −45°)and (ii) asymmetric one-sided resonance evolving towardsymmetric signature on approach of the p polarization (Figure5f.1, −15°). For the sake of clarity, only half of the angularpolarization domain is reported in Figure 5; that is, −90° ≤ θ ≤0°. A symmetric behavior (right sided signature) is predictedand recorded in the complementary angular interval 0° ≤ θ ≤90°.Similarly, these observations can also be rationalized within

the group theory approach. Indeed, in relation to the linearcharacter of Maxwell’s theory, an arbitrarily resonance patternof the nanoprisms can be described as a linear superposition oftwo independent resonance eigenmodes. Starting from the E′(0,−1, 1) quadrupolar eigenvector (s polarization) lacking signalat corner position, a photoemission map resembling the (2, −1,−1) eigenvector signature is progressively obtained. Note thatfor a regular triangle both quadrupolar eigenmodes exhibitsmall azimuthal differences.The latter experiments demonstrate a way to lift the

degeneracy between the different eigenvectors defining theLSPR resonances of a D3h solid object. In terms of basicphysical understanding of LSPR phenomena, one obtainsdetailed field mapping information in addition to degenerateenergy loss signature.64 This approach is general and applies toany optical nanoantenna, whatever its initial shape symmetry.On the application level, taking into account the role of the

symmetry on an LSPR opens ways to finely tune the lightmatter interaction at the nanometer scale. The full control ofthe symmetry response of an optical antenna gives furthermastery over its angular radiation pattern, that is, the directionalproperties of the emitted light (through the reciprocitytheorem). For instance, one can envisage the design of hybrid

systems acting as single photon nanosources (or detectors) ofspecific angular patterns.

■ CONCLUSIONSThe conducted experiments demonstrate access to preciseplasmonics excitation states of solid nano-objects. Indeed, theuse of incident light of specific polarization opens a way to liftup the degeneracy of the LSPR resonances of an object ofspecific shape, that is, gives access to its pure resonanceeigenvectors. This general approach permits us to tune finelythe light matter interaction offering a better control over thefundamental modes of optical nanoantennas. In particular, fullcontrol of the symmetry response of an optical antenna allowsfor the tailoring of its directional properties (angular radiationpattern).

■ ASSOCIATED CONTENT*S Supporting InformationSelection rules for a D3h triangle excited by a plane-wave anddetailed analytical polarization dependence calculation. Thismaterial is available free of charge via the Internet at http://pubs.acs.org.

*W Web-Enhanced FeatureA video sequence of the full polarization dependence for thedipolar plasmon resonance of a 200 nm in-plane height regulartriangle is available in the HTML version of the paper.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: ludovic.douillard@cea.fr; anne_laure.baudrion@utt.fr.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSWe acknowledge financial support by the French NationalAgency (ANR) in the frame of its program in Nanosciencesand Nanotechnologies (PEEMPlasmon project no. ANR-08-NANO-034) and by the Region Champagne-Ardennes in theframe of the Ph.D. scholarship. Finally, this work has beensupported by NANO’MAT (www.nanomat.eu).

We are very grateful to G. Haran and L. Chuntonov fromWeizmann Institute of Science, Israel for fruitful exchangeregarding the use of group theory in plasmonics.

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The Journal of Physical Chemistry C Article

dx.doi.org/10.1021/jp303475c | J. Phys. Chem. C 2012, 116, 14591−1459814598