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Dual wavelength sensing based on interacting gold nanodisk trimers

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2010 Nanotechnology 21 305501

(http://iopscience.iop.org/0957-4484/21/30/305501)

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IOP PUBLISHING NANOTECHNOLOGY

Nanotechnology 21 (2010) 305501 (7pp) doi:10.1088/0957-4484/21/30/305501

Dual wavelength sensing based oninteracting gold nanodisk trimersVivian Kaixin Lin1, Siew Lang Teo1, Renaud Marty2,Arnaud Arbouet2, Christian Girard2, Esther Alarcon-Llado1,Shu Hua Liu1, Ming Yong Han1, Sudhiranjan Tripathy1 andAdnen Mlayah2

1 Institute for Material Research and Engineering, A*STAR, 3 Research Link, 117602,Singapore2 Centre d’Elaboration de Materiaux et d’Etudes Structurales, CNRS-Universite de Toulouse,29 Rue Jeanne Marvig Toulouse, 31055, France

E-mail: tripathy-sudhiranjan@imre.a-star.edu.sg and Adnen.Mlayah@cemes.fr

Received 25 March 2010, in final form 18 May 2010Published 6 July 2010Online at stacks.iop.org/Nano/21/305501

AbstractFabrication and surface plasmon properties of gold nanostructures consisting of periodic arraysof disk trimers are reported. Using electron beam lithography, disk diameters as small as 96 nmand gaps between disks as narrow as 10 nm have been achieved with an unprecedented degreeof control and reproducibility. The disk trimers exhibit intense visible and infrared surfaceplasmon resonances which are studied as a function of the disk diameter and of the pitchbetween trimers. Based on simulations of the optical extinction spectra and of the electricnear-field intensity maps, the resonances are assigned to a single trimer response and tocollective surface plasmon excitations involving electromagnetic interaction between thetrimers. The sensing properties of the disk trimers are investigated using various coating media.The reported results demonstrate the possible use of gold disk trimers for dual wavelengthchemical sensing.

S Online supplementary data available from stacks.iop.org/Nano/21/305501/mmedia

(Some figures in this article are in colour only in the electronic version)

1. Introduction

Metallic nanoresonators sustain collective charge densityoscillations giving rise to strongly localized electromagneticfields associated with sharp spectral resonances. The possibleuse of these localized surface plasmon (LSP) excitationsfor scattering, guiding and concentrating the electromagneticenergy is currently giving rise to new applications: lightscattering based bio-imaging, enhanced optical spectroscopies,enhanced spatial resolution for near-field optical microscopy,medical applications such as hyperthermia for cancer therapyand assisted surgery. The closely spaced nanoshells, nanorods,or nanoparticles arrays are suitable templates for surface-enhanced Raman spectroscopy (SERS) as well as for surface-enhanced infrared absorption (SEIRA) spectroscopy, wherelarge electromagnetic field enhancements at selective spatiallocations can be tuned by controlling the small interparticledistances [1–10]. In such interacting noble metal nano-objects,

the plasmon resonances of individual objects hybridize andform red-shifted bands in the near-infrared and mid-infraredspectral regions. In the case of closely spaced nanoparticledimers or trimers, the hybridized surface plasmon resonancesare expected to depend on the geometries and separationsbetween the individual nanoparticles [11, 12].

Moreover, tunable plasmonic nanostructures consistingof periodic arrays of disks, rings and concentric ring–diskparticles have been investigated theoretically and were pointedout as potential candidates for efficient chemical- and bio-sensing [13–17]. However, comparison with experimentsaddressing the validity of the theoretical approaches andthe importance of the various physical parameters (size,shape, spacing, and surrounding medium) that influence theplasmonic properties is still lacking. This is because thefabrication of nanoresonators with precise control of shape,size and position within large ensembles remains a challenge.Top-down approaches such as focused ion beam milling,

0957-4484/10/305501+07$30.00 © 2010 IOP Publishing Ltd Printed in the UK & the USA1

Nanotechnology 21 (2010) 305501 V K Lin et al

colloidal lithography, and electron beam lithography (EBL) aswell as chemistry-based bottom-up approaches are used. EBLcan produce arbitrary 2D nanostructures with various particleshapes and with varying interparticle spacing. For instance, ithas been used to create periodic arrays of disks, disk dimers,rings and bowtie nanoantennas [18–21]. The limitation ofEBL in terms of nanoparticle size and features is due to thescattering of the electrons inside the photoresist. The typicalspatial resolution of nanostructures achievable with EBL isaround 10 nm.

In this study we report on the fabrication of largearea plasmonic nanostructures consisting of periodic arraysof gold disk trimers using EBL. The EBL technique ispushed to the limits in terms of separation between disksand reproducibility of the particle shape over a large area.In particular, gaps between disks of about 10 nm with diskouter diameter in the 90–130 nm range have been achieved inthis work. The optical properties of the nanostructures wereinvestigated using linear transmission measurements. Twosharp plasmonic resonances, reflecting the ordered characterof the nanostructures and the small gap fluctuations betweendisks, are observed in the visible and in the near-infraredspectral range. Our choice to investigate trimer structureswas motivated by their specific ability to yield infraredlocalized surface plasmon resonance (LSPR) while preservinga high localization of the electromagnetic (EM) near-field.Similar red-shifts could be obtained with elongated objectssuch as nanorods [22] but the concomitant delocalizationof the EM intensity over the whole structure would yieldmuch lower detection levels. Moreover, the intra-trimerarrangement impacts the surface plasmon resonance in thevisible spectral range and thus provides an additional degreeof freedom. Using Green formalism based calculations of theoptical extinction spectra and of the electromagnetic near-fielddistributions, the surface plasmon resonances are interpreted interms of single trimer properties and in terms of interactionsbetween trimers. We show that the intra- and inter-trimerinteractions allow the generation of visible and near-infraredsurface plasmon resonances associated with highly localizedelectric near-fields. Sensing experiments were carried outwith various coating media. The suitability of such plasmonicnanostructures as templates offering double plasmon resonancefor sensing applications is demonstrated in this work.

2. Experimental section

The gold disk trimers are prepared using electron beamlithography (EBL) and thermal evaporation of gold on quartzsubstrates. The square lattice array of gold nanodisk trimersis prepared over an area of 100 μm × 100 μm. To preparesamples with variable disk geometries, we have first designedthe pattern masks using e-beam resist. In our case, the e-beamresist is ZEP5A (1:3), where ZEP consists of a copolymer ofchloromethacrylate and methylstyrene. The resist is spun ontothe quartz substrates followed by a soft baking at 180 ◦C for2 min. The EBL is performed using an Elinoix ELS 7000set up at 100 kV with a 60 pA beam current with a dose of360 μC cm−2. The resist development is done in oxylene.

The patterned templates are then loaded into the evaporationchamber for deposition of a very thin layer of chromiumfollowed by a desired thickness of gold (Cr/Au:3/26 nm).The RDEC thermal evaporator was used for metal deposition.The purity of the metal targets used is Cr (99.99%) and Au(99.99%). The base pressure during thermal evaporation wasabout 3.29 × 10−6 Pa. For the 3 nm Cr layer, the beamcurrent was 135 A for an expected deposition rate of 0.2 A s−1.For 26 nm Au, the beam current was 115 A for an expecteddeposition rate of 0.5 A s−1. The lift-off process to realize goldnanopatterns is performed using dimethyl sulfoxide. Seriesof scanning electron microscopy experiments are performedto obtain optimized thickness and geometry of square latticepatterned trimers. The processing conditions are tuned toobtain fully touching disks or closely-spaced disks within thetrimer geometry.

The optical properties of these 2D arrays of goldnanodisk trimers are studied with microscopic transmissionmeasurements. The spectra are collected using a CRAICmicro-spectrophotometer where light transmitted by thetrimers is directed to the spectrometer through the microscopeobjective lens. The transmission data are normalizedwith respect to reference spectra acquired from the quartzsubstrate. For LSPR sensing experiments, we have carriedout transmission measurements on such gold nanostructures byvarying the refractive index of the environment. The sensitivityof the surface plasmon resonances to the refractive index isstudied in the cases of both isolated and interacting trimers. Weused drop coating of solvents on top of the trimers and repeatedspectral measurements were carried out in liquid for 30 s. Thetrimers were fully covered with about 100 μl of solvent. Theintegration time used for each spectrum is about 1300 ms (withmaximum 1.5 s for trimers with a large pitch). The repeatableoptical signal recorded by far-field long distance microscopeobjectives for each solvent coating determines the LSPR peakshift associated with solvent refractive indices.

3. Results and discussion

We have optimized the fabrication of trimers with differentperiodicity in a 100 μm × 100 μm square lattice format.Figure 1 shows the typical SEM and AFM images representingthe fabrication process transitioning from fully touching disktrimers to trimers with spacing between disks around 10 nm.The average outer diameter of the disks in this case is about96 nm. The pitch between trimer axes is about 400 nm.Figure 1 also shows the gold disk trimers with differentgeometries. In particular, we have also fabricated patterns witha disk diameter of about 127 ± 2 nm. The gold disk height inall cases is about 25 ± 2 nm. For the case of fully touchingdisks within the trimers, the disk diameters are almost same.

The corresponding optical density spectra recorded fromthese trimers are shown in figure 2. Optical resonances, due tosurface plasmon excitations of the disk trimers, are observedin the visible (600–800 nm) and in the near-infrared (1300–1400 nm) spectral regions. The wavelength, linewidth andrelative intensities of the visible and infrared bands dependon the characteristic dimensions of the trimer nanostructures

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Figure 1. SEM images of gold nanodisk trimers fabricated by EBL. (a), (b) Interacting (400 nm pitch) and isolated (1 μm pitch) disk trimers,respectively, with a disk diameter of 127 nm. (c), (d) Two extreme cases: trimers consisting of fully touching disks (left), and trimers with adisk separation gap of 10 nm (right); the disk diameter is around 96 nm. (e), (f) Typical 2D and 3D AFM micrographs of these disk trimers.

(disk diameter, gap between disks and separation betweentrimers). Optical transmission spectra from different points ofthe 100 μm×100 μm areas showed a good homogeneity of thedisk trimer characteristics (see supporting documents availableat stacks.iop.org/Nano/21/305501/mmedia).

In order to understand the optical properties of the disktrimer we have performed extensive numerical simulationsusing the Green dyadic tensor formalism. Among the variousMaxwell equation resolution schemes, the electrodynamicaltheory of the field susceptibility is particularly useful in thecase of nano-objects with low symmetry and complex shape.

From the susceptibility theory, the electric field E(r, ω0)

at the point r is written as [23]

E(r, ω0) = E0(r, ω0) +∫

VS0(r, r′, ω0) · P(r′, ω0) dr′ (1)

in which V is the metal volume, E0(r, ω0) is the incidentelectric field at a frequency ω0, S0(r, r′, ω0) labels the fieldsusceptibility tensor of the environment, and P(r, ω0) is theinduced electric polarization. According to the linear responsetheory, the relationship between the local electric field and theelectric polarization is given (in CGS units) by

P(r′, ω0) = χ(r′, ω0) · E(r′, ω0). (2)

The susceptibility χ(r′, ω0) can be expressed in terms of themetal and environment dielectric functions εm(ω0) and εenv,respectively:

χ(r, ω0) = εm(r, ω0) − εenv

4π. (3)

From equations (1) to (3), one can derive the Lippmann–Schwinger equation:

E(r, ω0) = E0(r, ω0)

+∫

VS0(r, r′, ω0) ·χ(r′, ω0) · E(r′, ω0) dr′. (4)

The quartz substrate is taken into account by replacing the free-space dyadic S0(r, r′, ω0) by S(r, r′, ω0) = S0(r, r′, ω0) +Ssub(r, r′, ω0) where the latter term accounts for the (infinite)substrate contribution to the field susceptibility. The disktrimer nanostructure is meshed regularly on a hexagonal latticeof N points ri , leading to 3N linear equations that can be self-consistently solved so providing the electric field E(ri , ω0).From these calculations, the extinction cross-section Cext andthe (normalized) near-field intensity I are computed.

Let us first discuss the situation where the disk trimers canbe considered as isolated, which corresponds to samples with1 μm pitch between the trimers. Figures 2(a) and (b) show the

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Nanotechnology 21 (2010) 305501 V K Lin et al

Figure 2. Measured optical density (black color plots) and simulated extinction spectra (blue and red color plots) of the samples consisting oftrimers with disk diameter of 96 nm ((a), (c)) and 127 nm ((b), (d)). The pitch between trimers is about 1 μm in (a) and (b), and about 400 nmin (c) and (d). In plot (b), the simulations were performed for 10 and 11.5 nm gaps between the disks within trimer geometry. All simulatedextinction spectra were rescaled to allow for comparison with the measured spectra.

(a) (b) (c)

Figure 3. Normalized near-field intensity maps computed at 25 nm above the disks and associated with the optical extinction peaks at(a) 633 nm, (b) 767 nm and (c) 833 nm. The disk diameter is about 127 nm and the gap between disks is 10 nm. The arrows indicate thepolarization of the electromagnetic excitation field.

simulated extinction spectra of a single trimer with 10 nm gapbetween the disks. The disk diameters, 96 and 127 nm, arethose measured from the SEM (figure 1). For the small disks(figure 2(a)), the calculations predict a single optical resonancepeaking at 650 nm, in close agreement with the measurements(maximum extinction around 620 nm).

For the large disks (figure 2(b)), the optical resonanceband around 700 nm is composed of three peaks located at633, 767, and 833 nm (figure 2(b)). These lines correspond tothe excitation of different surface plasmon modes of individualtrimers and arise from the electromagnetic coupling betweenthe three disks forming the trimer as shown in figure 3. Fromthe near-field intensity maps, it clearly appears that mainly

the top disk of the trimer is excited at 633 nm (figure 3(a)),whereas the near-field distributions associated with the peaksat 767 and 833 nm are localized on the two bottom disksof the trimer and are connected with the emergence of ahot spot in the gap between the disks (figures 3(b) and (c)).Despite their similar spatial distributions, the near-fields at 767and 833 nm differ significantly in intensity (see figures 3(b)and (c)): the maximum intensity at 833 nm is twice thatat 767 nm, thus leading to a stronger contribution to thesimulated extinction spectrum (figure 2(b), trimer with 10 nmgap between the disks). Moreover, because of their partiallocalization in the gap region between the disks (figure 3), theoptical resonances at 767 and 833 nm are very sensitive to the

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Nanotechnology 21 (2010) 305501 V K Lin et al

(a ) (b)

Figure 4. Near-field intensity distributions computed at 25 nm above 16 trimers of gold disks of diameter 127 nm. Excitation wavelengths are(a) 760 nm and (b) 1050 nm. Iso-intensity is plotted in black. The arrows indicate the polarization of the electromagnetic excitation field.

gap value. As shown in figure 2(b), increasing the gap betweendisks by only 1.5 nm strongly affects the spectral featuresassociated with these resonances and hence the lineshape ofthe extinction band. As expected from the associated near-field intensity map (figure 3(a)), the resonance at 633 nm isless sensitive to gap change (see figure 2(b)). By comparingthe simulated and measured extinction spectra of the largedisks (figure 2(b)), one can see that inhomogeneous broadeningdue to small gap fluctuations between the disks could explainthe experimental bandwidth. In the case of small disks,the interaction between the disks is weaker for a 10 nmgap, and disk diameter and shape fluctuations are mainlyresponsible for the inhomogeneous broadening of the opticalresonance (figure 2(a)). From figure 3, it clearly appears thatthe intra-trimer arrangement impacts the electromagnetic fielddistributions associated with the surface plasmon resonanceobserved in the visible spectral range. Thus, in addition tothe disk diameter and thickness, the interaction between diskswithin the trimer provides an additional degree of freedom thatcan be exploited for tailoring the optical properties of surfaceplasmon nanoresonators.

We now address the issue of the interaction betweentrimers and periodicity effects which are expected to manifestin samples with a high density of trimers, i.e. samples with400 nm pitch in our case (figures 2(c) and (d)). The extinctionspectra simulated for an array of 16 disk trimers with 96 nmand 127 nm disk diameters are shown in figures 2(c) and (d),respectively. In addition to the characteristic resonances ofisolated trimers (around 650 nm in figure 2(c) and 760 nmin figure 2(d)), infrared bands come out in the simulatedextinction spectra around 1250 nm in figure 2(c) and 1050 nmin figure 2(d), respectively. These bands appear only whenconsidering several trimers and are therefore associated withelectromagnetic interactions between the trimers. This issupported by the fact that the infrared band is much morepronounced for the larger trimers, i.e. for the smallest inter-trimer separation (compare figure 2(d) with (c)). Thetheoretical results are in a rather good agreement with the

measurements: in both simulated and measured spectra theinfrared interaction band comes out when decreasing the pitchfrom 1 μm to 400 nm (compare figures 2(a), (c), (b) and (d)).

In order to ascertain the origin of the infrared band, near-field intensity maps were generated for 16 trimers consistingof disks of diameter 127 nm and separated by 10 nm gap;the pitch between trimers is 400 nm. They are shownin figure 4. For excitation at the single trimer resonance,i.e. at 760 nm, the electric field intensity is localized aroundindividual trimers similar to the case of the single trimerconsidered in figure 3. Whereas for infrared excitation at1050 nm (i.e. at the calculated resonance wavelength infigure 2(d)), the electric field is maximum in the space betweenthe trimers. This corroborates the assignment of the measuredand simulated infrared extinction bands to electromagneticinteraction between trimers. By comparing the simulated andmeasured extinction spectra, some discrepancies between theobserved and calculated wavelengths and linewidths of theinfrared interaction bands can be noticed. These differencesare due, on one hand, to the small number of trimers inthe simulations (only 16) and, on the other hand, to theinhomogeneous broadening induced by disk diameter, shapeand gap fluctuations which are not taken into account in thecalculations.

In order to showcase the double resonance sensingproperties of the disk trimers, the dependency of the visibleand near-infrared surface plasmon resonances on the refractiveindex of the surrounding medium has been investigated.Several organic solvents of different refractive indices wereused in this study: water n = 1.33, acetone n = 1.359,ethanol n = 1.361, hexane n = 1.374, isopropanol n = 1.377,tetrahydrofuran n = 1.407, ethylene glycol n = 1.431, andtoluene n = 1.49. Figure 5 shows typical optical densityspectra of the large and small trimers when coated with asolvent. As expected, the surface plasmon resonances fromthe coated trimers are red-shifted with respect to the case oftrimers surrounded by air. It is worthwhile noting that theshift of the infrared surface plasmon resonance, due to the

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Nanotechnology 21 (2010) 305501 V K Lin et al

Figure 5. Measured optical density spectra of the samples consisting of trimers with a small disk diameter (left panel: 96 nm) and a largerdisk diameter (right panel: 127 nm). The pitch between trimers is about 400 nm. Spectra plotted in blue (blue-shifted peaks) were recordedwith trimers surrounded by air. The spectra recorded with trimers immersed in ethanol (right panel) and toluene (left panel) solutions areshown in red (red-shifted peaks). The dotted lines and arrows highlight the red-shift of the surface plasmon resonances.

interaction between trimers, is larger than that of the visibleresonance associated with single trimers (figure 5). Figure 6shows the red-shift of the visible and infrared resonances asa function of the optical index of the surrounding medium.The resonant wavelengths scale linearly with the surroundingrefractive index unit (RIU). Moreover, for the interactingtrimers, the change of the infrared resonance wavelength withoptical index (373.9 nm/RIU) is nearly three times larger thanthat of the visible resonance (130.3 nm/RIU). This allows forsensitive dual sensing based on both visible and near-infraredresonances.

The refractive index sensitivity of the visible wavelengthLSPR obtained from these disk trimers is comparable to othertypes of gold nanostructures [24, 25]. Although the RIUsensitivity is not very high compared to some high aspect ratiogold nanostructures [1, 14], the visible wavelength resonancesare relatively narrow with a linewidth variation of about 125–130 nm in water. Using the discrete dipole approximationmodel, Lee and El-Sayed [26] have quantitatively predictedthe sensitivity of the surface plasmon resonance of noble metalnanospheres and nanorods. In the case of gold nanospheres ofdiameter D, the sensitivity of the surface plasmon resonancered-shifts quasi-linearly from 153.92 nm/RIU for D = 40 nmto 331.35 nm/RIU for D = 120 nm. Based on thesecalculations, we deduce3 that the sensitivity of our gold disktrimers is comparable to that of spherical nanoparticles withdiameters in the 30–50 nm range. More surprisingly, thevariation of the sensitivity with the disks size is oppositeto the one predicted in nanospheres; the smallest particlesyield the best sensitivity: 170 nm/RIU for the small trimersand 130.3 nm/RIU for the large ones. This counterintuitive

3 The data for the refractive index of quartz have been taken from Handbookof Optics [27] and the permittivity of gold from Johnson and Christy [28].

Figure 6. Measured red-shift of the surface plasmon resonances ofgold nanodisk trimers as a function of the optical index of thesurrounding medium. The experimental data in the 600–750 nmspectral range (blue and black solid circles) are obtained from thevisible surface plasmon resonance peak of the trimers with a smallerdisk diameter (black circles: 96 nm disk) and a larger disk diameter(blue circles: 127 nm disk). The data in the 1100–1400 nm spectralrange (solid circles) show the shift of the infrared surface plasmonresonance of the trimers with a larger disk diameter.

behavior could possibly be due to the peculiar topology ofthe field in our disk trimers. Indeed, the near-field intensitydistribution of figure 3 evidences a strong localization of the

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Nanotechnology 21 (2010) 305501 V K Lin et al

electric field in the gap region between the disks. This hot spotprovides an active sensing zone; its extension is fixed by thetrimer arrangement. As a consequence, a change in refractiveindex is expected to have a stronger impact in the case of smalldisks for which the active zone is relatively more important.This is an important point for the investigated structure, asthe hot spot regions will allow for the detection of very lowconcentration of molecules.

The sensitivity of the near-infrared surface plasmonresonance (i.e. associated with the interactions betweentrimers) is 373.9 nm/RIU and is comparable for instance tothat of the longitudinal surface plasmon resonance of a nanorodwith an aspect ratio R = 2.53 and with an equivalent radius(i.e. radius of the sphere of equivalent volume) of 20 nm [26](see footnote 3). Since the near-infrared surface plasmonresonance is connected with the interaction between trimers,and the associated electric near-field is localized between thetrimers (figure 5), its sensitivity to changes in refractive indexof the surrounding medium cannot actually be compared tothat observed for single nano-objects. Besides the fact thatthe nanodisk trimer structures provide double surface plasmonabsorption peaks with similar resonance strength, their mainadvantage is to allow for a near-infrared resonance whilepreserving a high localization of the electromagnetic near-field (figure 3). Indeed, longitudinal surface plasmons ofnanorods are associated with electromagnetic near-fields thatare delocalized over the whole nanorod [22]. That limits thesensitivity in terms of minimum number of adsorbed moleculesyielding a detectable shift of the surface plasmon resonance.The nanodisk trimer structures are thus expected to improvethe detection limits and to allow for the design of high-sensitivity plasmonic nanosensors. Moreover, the near-infraredsurface plasmon resonance provide a means for chemicalsensing which is not based on size or shape effects but on theinteraction between nano-objects.

4. Summary

In summary, the fabrication of gold disk trimers, usingelectron beam lithography, and the study of their opticalproperties have been reported in this work. Owing to theprecise control of the disk diameter and separation betweendisks, and to the good reproducibility of the trimers withinthe patterned array, intense visible and near-infrared surfaceplasmon resonances are observed in the optical extinctionspectra. Based on simulations of the optical extinction and ofthe electric near-field intensity maps, these resonances wereassigned to single trimer surface plasmons and to surfaceplasmon excitations arising from electromagnetic interactionbetween the trimers. We have also studied the disk trimersfor chemical sensing experiments and demonstrated the useof such plasmonic nanostructures offering double plasmonresonance wavelengths for sensing. The double resonancebeing provided here is not by means of a shape effect, asin nanorods, but using single and collective properties ofnanodisk trimers. The main advantage of the trimer structureis to provide a near-infrared surface plasmon resonance whilepreserving a high localization of the electromagnetic near-field.

That property is important for the design of high sensitivityplasmonic sensors.

Acknowledgments

The authors are grateful for funding support from the Agencyfor Science, Technology, and Research (A*STAR) Singaporeand the French Embassy—Merlion Project Program. Thiswork was also supported by the computing facility centerCALMIP of Paul Sabatier University.

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