aDepartmentofElectricalandElectronicsEngineering,BilkentUniversity,06800Ankara,Turkey
bUNAMInstituteofMaterialsScienceandNanotechnology,BilkentUniversity,06800Ankara,Turkey
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Articlehistory:
Received18July2013
Receivedinrevisedform
16September2013
Accepted25September2013
Available online 18 October 2013 Keywords:
Surfaceplasmonresonance
Biomolecularsensing
Localizedplasmonresonancesensors
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Plasmonresonancesarewidelyusedinbiomolecularsensingandcontinuetobeanactiveresearch fieldduetotherichvarietyofsurfaceandmeasurementconfigurations,someofwhichexhibitdown tosinglemoleculelevelsensitivity.Theresonancewavelengthshiftoftheplasmonicstructureupon bindingofmolecules,stronglydepends,amongotherparameters,onhowwellthefieldoftheresonant modeisconfinedtothebindingsite.Hereitisshownthat,byusingproperlydesigned metal-insulator-metaltyperesonators,improvedwavelengthresponsecanbeachievedwithlocalizedsurfaceplasmon resonators(LSPRs)comparedtothatofthecommonlyusedKretschmanngeometry.Usingcomputational toolsweinvestigatetheoreticallytherefractiveindexresponseofseveralLSPRstructurestoa2nmthin filmofbindingmolecules.LSPRresonatorsareshowntofeatureimprovedsensitivityoverconventional Kretschmanngeometryinthewavelengthinterrogationschemeforsuchathinfilm.Moreover,someof theLSPRmodesarequasi-omnidirectionalandsuchangularindependence(upto30◦angleofincidence)
allowshighernumericalaperturestobeusedincolorimetricimaging.Resultshighlightthepotentialof LSPRsforbiomolecularsensingwithhighsensitivityandhighspatialresolution.
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
ResonanceexcitationanddetectionofSurfacePlasmon Polari-tons (SPPs) provide a versatile platform for affinity based refractometric biomolecular sensing [1,2]. Typically, the refrac-tiveindex profilenear a metal surface is modified due to the refractiveindexcontrastbetweenabuffersolutionandattached biomoleculesandtheresonanceoftheplasmonicmodesareshifted frequency,whichcanbemonitoredusingreflectionortransmission measurements.Duetotheeaseoffarfieldmeasurementandthe simplicityoffabricatingsurfacesexhibitingplasmonicresonances, surfaceplasmonresonance(SPR)techniquehasbeenextensively usedoverthelastfewdecadesforbiomolecularsensing[3–5]. Com-monly,prismandgratingcouplinghavebeenusedtoexciteSPPson planarorquasi-planarsurfaces,andprismcouplingisacceptedto exhibithighersensitivity[6].Theadventofnanofabricationtools allowedmoreandmoretop-downstructurestobeemployedin plasmonicsensing.Therichphysicsofsingleorcoupledplasmonic structuresallows highperformance biomolecular sensing using orderedperiodiclocalizedplasmonmoderesonances(LSPR)[7–9]. Typically,SPRimagingallowssimultaneousdetectionfrom multi-pleindividuallyfunctionalizedspots.AsprismcoupledSPRhasa
∗ Correspondingauthor.Tel.:+903122903502.
E-mailaddress:aykutlu@unam.bilkent.edu.tr(A.Dana).
strongangulardependenceoftheresonantexcitation,alow numer-icalaperturehastobeused,andSPRimaginghasbeeninherentlya lowspatialresolutiontechnique[10,11].Inordertoperform plas-monresonanceimagingwithhighspatialresolution,itisdesirable tohaveplasmonicsurfacesthatarenotsensitivetoangleof excita-tion(omnidirectionality),whilemaintainingahighrefractiveindex sensitivity,therebyallowingsensingofmonolayerbiomolecular coatings.Periodiclocalizedplasmonstructurescan bedesigned withopticalbandstructureswhichexhibitquasi-omnidirectional response[12].
In this article, we explore theoretically the refractive index sensitivityofseveralplasmonicdesigns.Typically,asimple figure-of-merit (FOM),defined asthe ratio of sensitivity (shift of the resonance wavelength in nmper change in thebulk refractive indexofthesensingmedium)tothefull-width-at-half-maximum (FWHM)oftheresonantlineshape,isusedtocomparethe perfor-mance[13,14].Wenotethat,inbiomolecularsensingapplications, theperformancedependsnotonbulkrefractiveindexchangebut onmolecularbinding.Therefore,insteadofusingthebulkrefractive indexchangeforthecomparisonofsensitivityofstructureswith thatofaprism,athin(2nmthick)organicfilm(representingthe adsorbedmolecularlayer)isused.Hereweobservethat,whena thinfilmisconsideredinsteadofthebulkrefractiveindex,properly designedlocalizedplasmonresonancesensorsexhibitgreater sen-sitivitycomparedtoaprismcoupler.Previously,variousplasmonic resonatorsdesignedor fabricatedonplanarsurfaceshavebeen
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Fig.1.Schematiccrosssectionofplasmonicstructuresanalyzedinthiswork.(a)Ametal-insulator-metal(MIM)metasurfacewithathinorganiclayercoatedontop.(b)The
MIMsurfaceisfurtherprocessedtopartiallyeliminatetheinsulatorandopenupspacethatwillallowbindingofmoleculesinbetweenmetallayers(referredtoasT-shaped
MIMstructureor,MIM-T).(c)All-metalversionoftheMIM-Tstructure(referredtoasMMM-T).(d)Metallicnanocavity(MNC)structures.(e)Metal-insulatornano-islands
(MI-NI).(f)Commonlyusedprism-coupledplasmonresonancegeometry.
consideredasplasmonicinterfacesforLSPR sensing[15,16].We alsoconsiderplasmonicstructuresonplanarsurfacesthatcanbe fabricatedbytop-downmethods.Wechoosegeometricstructures thatarestraightforwardtoimplementusingstandard nanofabrica-tiontechniquessuchaselectronbeamlithography.Particularly,we investigatethestructuresshowninFig.1.Thestructuresincludea metal-insulator-metal(MIM)metasurfacewithathinorganiclayer coatedontop(Fig.1a).TheMIMsurfaceisfurtherprocessedto partiallyeliminatetheinsulatorandopenupspacethatwillallow bindingofmoleculesinbetweenmetallayers (referredtoas T-shapedMIMstructureor,MIM-T,Fig.1b).Anallmetalversionof theMIM-Tstructure,referredtoasMMM-TisshowninFig.1c.In additiontoMIMstructures,ametallicnanocavity(MNC)arrayis studied(Fig.1d).TheMNCissimilartoalamellargrating, how-evertherectangulargroovesaregeometricallydesignedtofeature broadlocalizedplasmonmodesconfinedprimarilyintothegroove, thereforeservingasplasmonicnanocavities.TheMNClayer struc-tureallowsfabricationbyasimplenanoimprintingprocess.Alsoa metal-insulatornano-island(MI-NI)arrayisconsidered(Fig.1e). Suchstructurescanbefabricatedbytop-downfabrication tech-niquessuchaselectronbeamlithographyorthegeometrycanbe usedtoapproximateself-organizedmetalnanoislands, typically formedbydewettingofthinmetalfilmsonglasssubstrates[21]. Wecomparethesurfaceswiththecommonlyusedprism-coupled plasmonresonancegeometry(Fig.1f).
2. Sensingbywavelengthinterrogationofplasmon resonance
ThepropagationconstantkSPPofthesurfaceplasmawave
prop-agatingattheinterfacebetweenasemi-infinitedielectricmedium andametalisgivenby[1]
kspp=ω c
εmεdo εm+εdo≥k0= ω c εdo (1)where εd=n2do and εm=n2m arethe dielectric functionsof the
dielectricandthemetal,ωisthefrequencyandko=2/0isthefree
spacewavevector.SincekSPPislargerthank0,freespaceresonant
couplingtosuchamodeisnotpossibleandaprismofrefractive indexnp>1isgenerallyusedtocouplefreespacelightintotheSPP
mode.Resonantcouplingtakesplaceatanangleofincidence, when
kSPP∼= npkosin() (2)
Intheprismcoupledscheme(Kretschmanngeometry),athin dielectric film,ofrefractiveindexnd andthicknessh,shifts the
resonanceaccordingto[1] ıkSPP∼= K 3 SPP K2 0n3do [1−exp(−2dh)]ınd (3) where d=ikoεdo/(
(εm+εdo)) and h1/Re{d} and
ınd=nd−ndo,ndobeingthebulkrefractiveindexofthedielectric
medium.In LSPRsensingusinganisolatedplasmonicresonator wherethemodefieldcanbeassumedtoisotropicallydecayinto thesurroundingmedium, thewavelengthshiftupon bindingis given similarly by ı=Sınd(1−exp(−2h/1d)), where sis the
sensitivityfactor(shiftinresonanceperRIUchangeinenvironment refractiveindex)andldiselectromagneticdecaylengthoftheLSPR
mode[8–17].However,foranisotropicresonators,suchasprism ordiskshaped,themodefieldisnon-uniformlydistributedonthe resonatorandsensitivityofbindingassumesalocationdependent form.Thesameistrueforresonatorsboundtoasubstrate,where onlyonesideoftheresonatorisavailableformolecularbinding. For ageneralplasmonicresonatorincludingcoupledresonators andcavitiesinbufferoronasurface,asimpleequationincluding asingledecaylengthcannotbeusedtoexpressthewavelength shift.Instead, perturbation theorycan be used toestimate the resonancewavelengthshiftuponachangeintherefractiveindex profilethrough[18] ıres res =− 1 2
|E(r)|2(n2d(r)−n2do)dv
|E(r)|2dv
(4)whereE(r)istheelectricfielddistribution(modeprofile),ndoisthe
refractiveindexprofilebeforeperturbation,nd(r)istherefractive
indexprofileaftertheperturbationandintegrationisperformed overallspace.
Sensitivityforwavelengthinterrogateddetectionis[6] S= ıres
ınd
structuresofcoupledLSPRresonators,wedonotattempttomodel thebandstructuresusingsuchmodels.Instead,thedimensions ofthestructuresareoptimizedbyRCWAcalculationssothatthe bandstructuresofthemetasurfacesfeaturearesonancearound 600–700nmwavelengthfornormalincidence.Suchachoiceallows resistivelossestobesimilar,resultinginstructureswithsimilar resonanceFWHM.Wealsochoosedimensionsthatexhibithigh sensitivitytorefractiveindexchangewhilekeepingthestructures largesothatfabricationismorepracticalusingtop-down tech-niques.Structuresizesareh1=2nm(film),h2=10nm,h3=50nm,
gratingheighth5=50nm,periodof=200,gapofg=35forthe
MIM(seeFig.1fordimensionlabels).FortheMIM-Tand MMM-T,b=62.5nm,w=200nm,topmetalh5=10nm,h4=10nm,period
=250nm,gapg=50nm.ForMNC,h6=40nm,period=200nm,
gapg=35nm.ForMI-NI,topmetalthicknessh5=100nm,period
=200nm,gapg=25nmandfortheKretschmannconfiguration h7=50nm.
Theelectricfielddistributionsofthestructuresexcitedon res-onance at normal incidence are shown in Fig. 2. For the MIM structure,onresonancethefieldsareprimarilyconfinedtothe insu-latorandthespacebetweentopmetalislands(Fig.2a).According toEq.(4),MIMstructuresareexpectedtohavealowersensitivity tochangesoftherefractiveindexofthebackground,sincefieldsare partiallyconfinedtothedielectricinsidetheMIM.Incontrast,the MIM-Tstructureshavealargersurfacetowherethemoleculescan bebound(Fig.2b).TheMMM-Tconfigurationdisplaysfieldsthat aresimilartotheMIM-T,however,fieldpenetrationintothe inter-mediatemetalpostregionissmallerandgreaterfractionofthefield isconfinedtotheexposedareas(Fig.2c).TheMNCstructurehas allthefieldsconfinedwithinthenanocavity,whichisacompletely exposedsurface(Fig.2d).TheMI-NIstructuresaresimilartothe MNC,howeverfieldpenetrationintothedielectricreducesthe frac-tionofmodewithintheexposedareas(Fig.2e).IntheKretschmann configuration,theSPPmodeisconfinedtothesurface,however themodevolumeislargeduetothelargedecaylengthintothe dielectric(Fig.2f).
TheabsorptionspectraofthestructuresarecalculatedbyRCWA andareshowninFig.3.TheLSPRstructuresmaypossessfurther resonancesoutsidethewavelengthrangeshown,howeverweare onlyconcernedabouttheresonancenear650–850nm.The absorp-tioniscalculatedfor normalincidenceforchanges inrefractive indexofa2nmthickfilmonan=1.33background.Ascanbeseen inFig.3,anincreaseintherefractiveindexofthefilmtypically resultsinaredshiftoftheresonancelinesduetodielectricloading oftheresonators.
Thefieldscalculatedbynumericalcalculationsareusedfor esti-mationof theresonance shiftthrough Eq.(5) and are given in Table1.Theresultsareacquiredforresonanceexcitationandfora shiftofthebackgroundindex.Theshiftsarecomparedwithdirect calculationofresonancefrequenciesusingRCWAandEq.(5)(Eq.
approach.Therefore,intuitivelywecanexpectthestructureswith greaterexposedareaandsmallermodevolumetoexhibitimproved sensitivitytothepresenceofthinmolecularfilms.Itmustbenoted that,inordertoachievecorrectresultsusingEq.(5),the dielec-tricfunction ofthemetalfilmand fieldspenetratinginsidethe metalstructuresmustbeincludedinthecalculation.Otherwise, forexamplebyincorrectlyassumingzerofieldpenetrationintothe metalandassumingthedielectricportionsoftheresonatoraccount forallofthemodevolume,theperturbationapproachproduces erroneousresonanceshiftsthatare100–300%largerthanthose predictedbydirectnumericalcalculation.Therefore,modeenergy insidethemetalsurfacescontributesasignificantportionofthe actualmodevolume.
Anorganicmolecularfilmisexpectedtohavearefractiveindex of1.47around600–800nmandtheindexcontrastwithbuffer solu-tion(nd=1.33–1.34)resultsintheperturbationofresonances.We
calculatetheabsorptionspectraofsurfacesusingRCWAasshown inFig.3,andextractpeakpositionsoftheabsorptionpeaksaround 650–800nmtocalculatesensitivitytotherefractiveindexofa2nm thickfilmaswellastothebackgroundrefractiveindex(Table2).Itis observedthat,KretschmannconfigurationsurpassesallLSPR struc-turesinsensitivityifthebackgroundrefractiveindexischanged. However,forathinorganicfilm,duetothesmallmodevolume of theLSPR resonators, severalLSPR structuresshow improved responseovertheprism.Forexample,acomparisonofthe spec-tralshiftsofMIM-TandKretschmannconfigurationsisgivenin Fig.4forthinfilmandbackgroundrefractiveindexchange.Inthe presenceofathinfilm,alargershiftofresonancefrequencyofthe LSPRstructurecomparedtoaprismisinteresting(Fig.4aandb), sinceprismcouplingistypicallyacceptedtobesuperiortoother SPPstructures(suchasgratingcoupledplasmonresonance[6]) intermsofwavelengthsensitivity.Athin organicfilmisalsoof greaterpractical interestthan bulkbackgroundrefractiveindex change,asthisconfigurationmoreaccuratelydescribesanactual molecularbindingexperiment.Forthethinfilmcase,itisseenthat allLSPRstructures excepttheMIM exhibitbetterabsolute sen-sitivitycomparedtotheprism.TheMIM isanexception,being lesssensitiveduetothefactthatgreaterportionofthemodeis Table2
Wavelengthsensitivityupon%1changeofrefractiveindexof2nmthinfilm,around
n=1.47,comparedwith%1changeofbackgroundindexnearn=1.33,calculated
usingfiniteelementanalysis.
Structure 2nmfilm(nm/RIU) Background(nm/RIU)
MIM 18.9 60 MIM-T 97.3 330 MMM-T 78.4 330 MI-NI 59.5 260 MNC 56.8 390 Prism 48.7 4500
Fig.2. Relativeelectricfieldintensitydistributionsascalculatedbynumericalanalysisfornormalincidenceon(a)MIMmetasurfaceat=800nm,(b)MIM-Tstructuresat =750nm,(c)MMM-Tstructuresat=625nm,(d)MNCsurfaceat=650nm,(e)MI-NIstructuresat=650nm.Scalebarsare50nmwide.
confinedintheunexposedinsulatorregion.TheMIM-Tand MMM-Tfeatureabouttwicethesensitivityofa prism,partiallydueto thefactthatbothtopandbottomsurfaceshave2nmfilm cover-ingthemandthereisabouttwicemorematerialperunitareathat contributetoresonancewavelengthshift.IfwecalculatetheFOM givenbyEq.(6),fora2nmfilm,wegetvaluesof0.34fortheMIM, 0.62fortheMI-NI,0.44fortheMNC,1.35theMIM-T,0.91forthe MMM-Tand0.87fortheKretschmann.IntermsoftheFOM,the MIM-TandMMM-Texhibitimprovedperformancecomparedto theprismcoupledSPR.Itmustbenotedthat,forLSPRsensingby resonatorsinfluid,arefractiveindexsensitivityof∼500nm/RIU isconsidereddesirable[20]Itisobservedthatforthestructures excepttheMIM,sensitivitiesof300–400nm/RIUcanbeachieved withsurfaceboundstructuresconsideredhere.Thesensitivityfor LSPRstructuresonsurfacesisinherentlylowascomparedtofree
structuresinfluid,duetosinglesidedmolecularbinding.For sur-faceboundstructures,sensitivityvaluesrangingfrom80nm/RIU havebeenpreviouslyreported[16].Thesensitivitycanbeincreased to750nm/RIUforverticalcavitystructuressimilartotheMIM-T, designedinthenear-infraredportionofthespectrum.Thisincrease wasattheexpenseofincreasedfabricationtolerancesanda poten-tiallycomplicatedfabricationprocedure[15].
Anotherissueweaddressincomparingtheperformanceofthe LSPRsensorsistheangularsensitivityoftheresonances.Inthecase ofafocusedbeamwithahalf-coneangle,thebroadeninginthe wavelengthresponseofaplasmonicresonancewillbegiventoa firstorderapproximationby∼= 2(∂res/∂).Theresonance
willfeatureapeakgivenbytheconvolutionoftheangularspread withtheintrinsicresonancelineshape,resultinginanoverall res-onancewidthoftotal∼=
2
res+2.Rewritingtheangular
Fig.3. Absorptionspectraat30◦angleofincidenceisshownforthe(a)MIM,(b)MIM-T,(c)MMM-T,(d)MNC,(e)MI-NIand(f)Kretschmannconfigurationsforchangesin
Fig.4. RIresponsescomparedforbulkand2nmfilm.(a)TheMIM-Tstructureexhibitsasuperiorsensitivityascomparedtotheprismcoupledcaseshownin(b)forathin
film.Refractiveindexofthefilmisassumedtobe1.33,1.5and1.7forthesolid,dottedanddashedcurves.(c)ResponseoftheMIM-Tstructuretoabackgroundrefractiveindex
change.(d)Responseofprismtoabackgroundrefractiveindexchange.Forbackgroundrefractiveindexchange,theprismcoupledconfigurationissuperiorinsensitivity.
Legendshowsrefractiveindicesofthebackgroundforeachgraph.
Fig.5.(a)RIresponsescomparedfor2nmfilmforthestructuresshowninFig.1.(b)TheangularsensitivityisplottedfortheLSPRstructures.TheMIM-T,MMM-Tand
MNC,MI-NIstructuresexhibitweakangulardependenceupto30◦ofangle-of-incidenceduetotheirbandstructure,whereastheMIMstructuresshownon-zeroangular
sensitivity.TheKretschmannconfigurationhasinherentlyaveryhighangularsensitivity.
spreadintermsofthenumericalapertureofthefocusinglens(N.A.), forsmallN.A.thefigureofmeritwillbegivenapproximatelyby FOM ∼= FOMo
res2
res+[2(∂res/∂)(N.A./n)
(7) whereFOMois thefigureofmeritmeasuredusingacollimated
beam.Inthecase ofanomni-directionalresonance,∂res/∂≈0
andalargeN.A.wouldnotdegradetheFOM,allowinghigh spa-tialresolution imaging.Theangular sensitivityistypicallylarge foraprismcoupledconfiguration,howeverLSPRsurfacestypically exhibitangleindependent spectralresponse. Angularresponses oftheLSPR resonanceareshowninFig.5b,alongwithspectral responseto2nmthickfilmsofvaryingrefractiveindex(Fig.5a). It isseen that, fortheMIM-T, MMM-Tand MI-NIsurfaces,the ∂res/∂res(i.e.∂res/∂≈0)issatisfiedforanglesupto30◦.
ThiswouldallowaN.A.=0.66tobeused(inbufferwithn=1.33) forcolorimetricimagingofLSPRresonances,resultingina diffrac-tionlimitedresolutionof490nmassumingaresonancewithpeak around650nm.TheactualbandstructureofcoupledLSPR struc-turesdeterminethevalueofangularsensitivityres/∂andMIM-T,
MMM-TandMI-NIsurfaceshavesuperioromni-directionalityas
comparedtotheMIMsurfaceforthesetofgeometricparameters giveninTable1.
4. Conclusion
WehavecomparedtherefractiveindexresponseofseveralLSPR structuresandprismcoupledSPRinthepresenceofathinorganic film.Itisseenthatforabulkbackgroundrefractiveindexchange, theprismpossessessuperiorrefractiveindexsensitivitywhileLSPR structurescanexhibitimprovedsensitivityfora2nmthickfilm. Themainreasonbehindthisimprovedsensitivityisthefactthat themodesareconfinedtosmallvolumeswith10–30nm character-isticdimensionsfortheLSPRcase,whereagreaterfractionofthe modeisperturbedbythepresenceofthethinfilm.Itisalsoshown that,iftheLSPRmodesareconfinedtoregionswhere biomolec-ularadhesionis inhibited bythepresence of a dielectric,as in theMIMcase,sensitivityisdegraded,sinceasmallerportionof themodevolumeisperturbedbybiomolecularadhesion. Calcu-lationsvalidatetheapplicabilityoftheperturbationapproachto calculatingtherefractiveindexresponse,asseenbycomparingEq. (5)withdirectnumericalcalculations.Inthecalculations,itwas
seenthatgeometricvolumeofthemetal-freeregionofthe res-onatorsdoesnotcorrespondtotheactualmodevolume,which needstobe calculated taking intoaccount themetal dielectric constantsandfieldsconfinedinsidethemetalregions.Therefore, althoughtheperturbationapproachprovidesintuition,thefields thatpenetrateintothemetallayermustbeconsidered,asthemetal dielectricfunctionmayhave alargemagnitudeascomparedto organiclayers.TheangularresponsesoftheLSPRstructuresare alsocalculated, demonstratingtherelative insensitivity of LSPR structurestochangesintheangleofincidence.Theresultsshow that,properlydesignedLSPRstructuresretainomni-directionality whilemaintainingahighersensitivityascomparedtoaprismfor athinorganicfilm.Particularly,theMIM-Ttyperesonators exhib-itedabout2timesimprovedsensitivityoveraprism.Therefore,the calculationsshowthathighresolutioncolorimetricplasmon reso-nanceimagingispossibleusingoptimizedLSPRsurfaceswithout compromisingsensitivityforbiomoleculardetection.
Acknowledgment
Thiswork waspartially supportedby TUBITAK under Grant 111M344,EUFP7:People-IAPPNanoBacterPhageSERS.
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Biographies
YasinKayaisaresearchassistantattheUNAMMaterialsScienceand
Nanotech-nologyProgramatBilkentUniversity.Hisresearchinterestsare inthefieldof
computationalelectromagneticsanddesignofplasmonicnanostructures.
SencerAyasisaPh.D.studentattheUNAMMaterialsScienceand
Nanotechnol-ogyProgramatBilkentUniversity.Hisresearchisonthedesignandrealization
ofplasmonicnanostructuresandmetasurfaces,withapplicationstoimagingand
spectroscopy.
AhmetEminTopalisaPh.D.studentattheUNAMMaterialsScienceand
Nano-technologyProgramatBilkentUniversity.Hisresearchinterestsareinthefieldof
colorimetricbiomolecularsensingandimagingusingplasmonresonances.
HasanGünerisaPh.D.studentattheUNAMMaterialsScienceandNanotechnology
ProgramatBilkentUniversity.Hisresearchinterestsareinthefieldofdesignand
implementationofplasmonresonancesensingsystems.
AykutluDanaisaAssociateProfessorattheUNAMMaterialsScienceand
Nanotech-nologyProgramatBilkentUniversity.HereceivedhisPh.D.fromStanfordUniversity
and continuesto work inthe fieldsof nanoscale optoelectronic devicesand
