plasmonic nanoparticles and metasurfaces

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Plasmonic nanoparticles and metasurfaces to realize Fano spectra atultraviolet wavelengths

Christos Argyropoulos, Francesco Monticone, Giuseppe D'Aguanno, and Andrea Alù

Citation: Appl. Phys. Lett. 103, 143113 (2013); doi: 10.1063/1.4823575

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APPLIEDPHYSICSLETTERS103,143113

(2013)

PlasmonicnanoparticlesandmetasurfacestorealizeFanospectraatultravioletwavelengths

??1,a)ChristosArgyropoulos,1FrancescoMonticone,1GiuseppeD’Aguanno,2andAndreaAlu

DepartmentofElectricalandComputerEngineering,TheUniversityofTexasatAustin,Austin,

Texas78712,USA2

AEgisTechnologies,NanogenesisDivision,410JanDavisDr.,Huntsville,Alabama35806,USA

1

(Received3May2013;accepted13September2013;publishedonline1October2013)

WediscussthepossibilitytorealizesharpFanoscatteringsignaturesintheultraviolet(UV)range,basedondipolarscatteringofnanoparticles.Atthesefrequencies,materiallossesusuallydonotallowsharpresonanteffects,hinderingplasmonicapplicationsbasedonhigher-ordermultipolarmodes,likeconventionalFanoresonances.Weproposetoexcitedegeneratescatteringstatessupportedbycore-shellnanoparticlesmadeofasapphirecoreandanaluminumshell.Wepredictenhanced,highlycon?ned?elds,supportingsharpfar-?eldscatteringsignaturesfromsinglenanoparticlesandplanararraysofthem.TheseresultsmayleadtothedesignofUV

C2013AIPPublishingLLC.?lters,photodetectors,sensors,andenergy-harvestingdevices.V

[http://wendang.chazidian.com/10.1063/1.4823575]

Signi?cantresearcheffortshavebeenrecentlydedi-catedtoenhancingandengineeringtheinteractionoflightandmatteratsubwavelengthdimensions.Thisinteresthasfosteredthegrowthoftherecentresearch?eldofplas-monics,demonstratinggreatpotentialtowardscombiningtheminiaturizeddimensionsofsemiconductorelectronictechnologywiththelargedataratesandenhancedband-widthperformancesachievedbyphotonicdevices.1Excitingapplicationshavebeenenvisionedbasedonthestrongcon?nementoflightindeeplysubwavelengthdimen-sions.2Oneofthemostinterestingeffectsenabledbyplas-monicsistheultra-sharpFanoresonantscatteringresponsesupportedbymetallicnanostructuresatopticalandinfrared(IR)frequencies.3–7Theseasymmetricresonancesareusu-allybasedontheinteractionbetweendipolarbrightmodeswithhigher-order,morenarrowbanddarkmodes.3Theircharacteristicshaveshowngreatbene?tsinvariousscenar-ios,inparticular,foroptical?ltering,sensing,lasing,andslow-lightapplications.

Theinvolvementofhigher-ordermodes(quadrupoles,octupoles,etc.)leadstoenhancedstoredenergyintheplas-monicdevice,whichtranslatesintohighersensitivitytotheOhmiclossestypicalofmetalsatopticalfrequencies.Furthermore,conventionalplasmonicFanoresonances,aris-ingfromtheinterferencebetweentwodifferentscatteringorders,canbeusuallyobservedonlyoveraportionoftheangularspectrumduetointerference.Recently,itwaspro-posedthatsimilarlysharpFano-likeresonantsignaturescanbeobtainedbytheinteractionofpurelydipolarmodes.8–13Inourrecentworks,weshowedthatthispossibilitymaybecontrolledbycloselycouplinginthefrequencyspectrumcloakingandresonantstatesofasymmetricandisotropicnanoparticle.Inthiscase,adipole-dipoleFano-likeresponsecanbeobservedforallanglesofincidenceandinthetotalscatteringcrosssection(SCS),evenforasinglenanoparticle.Moreover,enhanced?eldsareobtainedatthissharpdipolar

a)

Authortowhomcorrespondenceshouldbeaddressed.Electronicmail:alu@mail.utexas.edu

resonance,stronglycon?nedatthecoreofthenanoparticle,allacrosstheFanospectrum.Dependingonhowthesestatesareexcited,theseresonantphenomenamaybemademoreorlesssensitivetothebackgroundmediumandthecoremate-rial,12providinganunprecedented?exibilitytorealizedif-ferentfunctionalities.Aplethoraofapplicationshasbeenenvisionedfromtheseplasmonicstructures,suchasrobustsensors,11nano-biomarkers12andef?cientnonlinearall-opticalswitches.10

Untilnow,themostcommonmaterialsconsideredforplasmonicapplicationshavebeensilver,copper,andgold,becausetheycansustainef?cientexcitationofbothpropa-gatingandlocalizedsurfaceplasmonsatopticalfrequencies.However,theysufferfrominherentdisadvantages,suchasincreasedlosses,highcost,andlowabundanceonEarth,13whichsetseverelimitationstowardsthefuturecommercialapplicationsandmass-productionofplasmonicdevicesbasedonthesematerials.Alternatively,itwasdemonstratedthatef?cientplasmonexcitationcombinedwithrelativelylowlossescanalsobesustainedatmid-infraredfrequenciesusingdopedsemiconductormaterials.12,14,15

Comparatively,lessresearchattentionhasbeendedi-catedtoplasmoniceffectsoccurringatveryhighfrequen-cies,suchasintheultraviolet(UV)spectrum.Thisismainlybecausetheaforementionednoblemetalsandsemiconduc-torshavelowerplasmafrequenciescomparedtotheUVfre-quencyrangeand,asaresult,theyaremostlytransparentanddonotexhibitplasmonicbehaviorinsuchregimes.Aluminum(Al)istheidealmaterialtosustainsurfaceplas-monsatUVwavelengths,duetoitshighplasmafrequency($15eVork¼85nm).Inaddition,AlisthethirdmostabundantelementonEarth,whichmakesitcheapandidealformass-production.InterestinganddiverseapplicationshavebeenrecentlyenvisionedatUVfrequenciesbasedonAlplasmonicdevices,suchasboostednonlinearopera-tions,16ef?cientsurface-enhancedRamanscattering(SERS)substrates,17,18plasmonicnanoantennas,19enhanced?uores-cence,20,21extraordinaryopticaltransmissiongratings,22,23andenergyharvestingdevices.24

C2013AIPPublishingLLCV

0003-6951/2013/103(14)/143113/5/$30.00103,143113-1

143113-2Argyropoulosetal.Appl.Phys.Lett.103,143113(2013)

LessattentionhasbeenpaidtoplasmonicstructuresexhibitingFanoresonancesatUVfrequencies,whichmaybeused,forinstance,assharpsolarblindstoisolateportionsoftheUVfrequencyspectrum,whiletransmittingopticalradiation,orassensitivenanosensorsandphotodetectors.ThereasonforthecomparativelylessamountofresearchonthistopicisthelargerlossfactorofAlintheUVcomparedtoothernoblemetalsatlowerfrequencies,whichhindershigher-orderresonancesonwhichconventionalFanoresponsesarebased.Inthisletter,weexplorethepossibilitytoinducedipole-dipoleplasmonicFanoresonancesatUVfrequenciessupportedbyproperlyengineeredcore-shellsub-wavelengthnanoparticles,similartotheconceptproposedinRef.10foroperationinthevisible.Theproposedgeometry,shownintheinsetofFig.1(b),iscomposedofasapphire(Al2O3)dielectriccorewithradiusacsurroundedbyametal-licshellmadeofaluminumwithradiusa.AccordingtothetheorypresentedinRefs.10–12forthick,subwavelengthplasmonicshells,theelectricdipolarresponseofthisnano-particlecanpresentanultrasharpdegenerateresonantstatewheneAl¼ÀeAl2O3=2.Underthiscondition,thefrequencydispersionofacloaking(dark)statemergeswiththeoneofaresonant(bright)state,producinganasymmetricFano-typeresonanceinthescatteringspectrum.Sincethetwointerfer-ingstatesareassociatedwiththesamedipolarscatteringmode,weareabletorealizeanasymmetric,sharpFano-resonantscatteringsignaturewithasingle,isotropic,andcenter-symmetricsubwavelengthscatterer.

Thescatteringperformanceoftheproposednanoparticleisquantitativelyanalyzedusingthewell-establishedMiescatteringtheory.25RealisticlossesareconsideredforbothAlandAl2O3,theirpermittivityvaluesareobtainedfromex-perimentaldata26andthenanoparticleisembeddedinfreespace.ThenormalizedSCSofasubwavelengthcore-shellnanospherewithinnerlayerradiusac¼3nmandouterra-diusa¼8:5nmiscomputedandplottedinFig.1(a).Interestingly,thenormalizedSCScangofromÀ13dB[pointIinFig.1(a)]toalmostÀ30dB[pointIIinFig.1(a)]inanarrowwavelengthrange.NotethatthissharpFanores-onantlinewidthisobtainedatdeep-UVfrequencies,wheremostmetalsareactuallysemi-transparent,whiledielectricnanoparticlesdonotgenerallyshowpeculiarscatteringsig-naturesbecausedielectricmaterialsareopaqueinthiswavelengthrange,duetotheirlargelyincreasedabsorptionaroundtheinterbandtransitions.Realisticlossesofplas-monicanddielectricmaterialshavebeenfullytakenintoaccountinourcalculations,accuratelyre?ectingtheresponseofarealisticstructureimplementablewithincurrentnanofabricationtechniques.

TheinsetofFig.1(a)showstwosnapshotsintimeoftheEx?eldcomponentatthehighscatteringpointI(resonance)andlowscatteringpointII(cloaking).Enhancedandstronglycon?ned?eldsareobtainedinsidethecompositenanopar-ticleforbothresonantandcloakingstates.Incontrast,out-sidethecore-shellparticle,the?eldsaredramaticallydifferentacrossthelinewidthoftheUVFanoresonance.Intheresonantstate(pointI),largescatteringef?ciencyisobserved,similartoordinarydipolarplasmonicresonancessupportedbysilverorgoldnanoparticlesatopticalfrequen-cies.2Conversely,inthecloakingstate(pointII),theimping-ing?eldstravelaroundthescattereralmostunperturbed,similartoscatteringcancellationdesignsproposedinRefs.27and28.Itisinterestingthatsuchstrongvariationofthe?elddistributioncanbeachievedoveranarrowfrequencyrange,whichindicatesastrongandpeculiarinteractionofthesubwavelengthnanoparticlewithUVradiation.InFig.1(b)across-sectionofthe?eldenhancementðjEj=jEincjÞisplottedalongthex-axiscrossingthecenterofthenanopar-ticleattheresonant(pointI/redline)andcloaking(pointII/blueline)wavelengths.Enhanced?eldsareobtainedinsidetheplasmonicnanoparticleacrosstheentireFanospectrumshowninFig.1(a)andarestronglycon?nedattheinterfacebetweentheAlshellandthesapphirecore.Inthiscase,astronglylocalizedplasmonisformed,andthehigher?eldenhancementatthemetallicshellsideobservedinthe?gurecanbeexplainedbyapplyingtheboundaryconditionontheradialcomponentoftheelectricdisplacementvectoratthisinterface.Evenatthecloakingfrequency,signi?cantlyenhanced?eldsareobservedinthecore,whichmaybeofin-teresttoboostnonlineareffectsatUVfrequencies,asdem-onstratedforopticalantennas29,30andepsilon-near-zeroplasmonicwaveguides31atopticalfrequencies.Thesefea-turesmayleadtothedesignoflow-intensitymemories,switches,andsensitivetunablesensorsatUVwavelengths.

Next,wechecktheeffectofdifferentdielectricback-groundmaterialsinthescatteringresponseofthe

proposed

FIG.1.(a)Normalizedscatteringcrosssectionofthecompositenanoparticlewithgeometryshownintheinsetofpanel(b).Theradiusofthecoreisac¼3nmandtheouterlayerradiusisa¼8:5nm.Theinsetof(a)showssnapshotsintimeoftheEx?eldcomponentattheresonanceandcloakingwavelengths,denotedbyIandII,respectively.(b)Fieldenhancementacrossthecompositenanoparticleasafunctionofthenormalizedtransversedirectionðx=aÞattheres-onantI(redline)andcloakingII(blueline)wavelengths.

143113-3Argyropoulosetal.Appl.Phys.Lett.103,143113(2013)

FIG.2.SensitivityoftheUVFanoresonancetothepermittivityebofthebackgroundmedium.ThenanoparticlehasthesamedimensionsasinFig.1.Thenarrowresonance,cloaking,andbroaddipolarresonancewavelengthsaredenotedbyI,II,andIII,

respectively.

plasmonicnanoparticle.TheUVFanodispersionisplottedinFig.2asafunctionofthesurroundingmediumpermittiv-ity.ItisevidentthattheasymmetricFanoresponseisonlyslightlyaffectedbyvariationsofthebackgroundpermittiv-ity,whichisconsistentwiththeresultspresentedinRef.12.Thephysicalreasonforthisbehavioristhatthecloaking-resonancepairsupportingthedipole-dipoleUVFanoreso-nanceissustainedbythelocalizedplasmonresidingattheinnersphericalinterfacebetweentheplasmonic(Al)shellanddielectric(Al2O3)core.Hence,thepeak(pointI)anddip(pointII)inthescatteringresponsearealmostinsensitivetothebackgroundenvironment.Onthecontrary,thebroaddipolarresonanceatlowerfrequencies(pointIIIinFig.2)isstronglyaffectedbythebackgroundmaterial.12Asaresult,thenanoparticlescanbeembeddedinanyhostmediumwith-outsigni?cantlyaffectingthemainfeaturesandrobustnessofthis“built-in”UVFanoresponse,whichmayleadto,e.g.,UVtaggingapplications.

Inarealisticscenario,theidealcore-shellgeometrythatwehavedescribedwouldbemodi?eduponexposuretoambi-entconditions,sincethealuminumouterlayermayoxidizeandathinnativeoxideAl2O3layerwouldformaroundtheAlinterface.32,33Here,weexplorethisissueandquantitativelyassesstheperformanceofthemodi?edgeometry,consideringanadditionalthinlayer(1:5nm)ofAl2O3aroundtheplas-monicouterlayerofthecore-shellgeometryofFig.1,as

shownintheinsetofFig.3(b).Thenanoparticlehasnowthreelayersandthedimensionsarechosentobe:coreradiusac1¼3nm,middleplasmoniclayerradiusac2¼8:5nm(samedimensionsasinFig.1forafaircomparison),andouterlayerradiusa¼10nm.ThecalculatednormalizedSCSforthismultilayerednanoparticleisshowninFig.3(a),demon-stratingsimilarlysharpfeaturesintheUVrange.Asexpected,thescatteringexcursionisslightlysmaller,andtheresonancebandwidthabitbroadercomparedtothepreviouscase[seeFig.1(a)],duemainlytotheOhmiclossesofthesapphireouterlayer,whichdissipatespartoftheimpingingUVradia-tion,hencereducingtheef?ciencyofplasmonexcitation.However,afairlysharpFanoresponseisstillpreserved.Alsointhiscaseweassumedafree-spacebackground,butweexpecttheUVFanoresonancetobealmosttotallyunaffectedbyachangeinhostmedium,asshowninFig.2.

The?eldsarestillcon?nedinsidethenanoparticleandstronglyenhancedacrosstheFanoresonancelinewidth[Fig.3(b)]whencomparedtotheincidentradiation,asshowninthe?elddistributionsintheinsetofFig.3(a)atthewave-lengthsoftheresonant(pointI)andcloakingstates(pointII).The?eldenhancementisslightlylowercomparedtothetwo-layercaseshowninFig.1(b),duetotheadditionalthinouterlayerofAl2O3,whichstronglyaffectsthesurfaceplas-monlocalizedattheoutersphericalinterface,asevidentbyacomparisonofthe?elddistributionsinFigs.1and3.However,highenhancementoftheelectric?eldisstillobtained,especiallyattheinterfacebetweentheAlshellandtheAl2O3core,andthe?eldlevelinsidethecoreisalmostthesameasbefore.Notethattheabruptdropin?eldenhancementintheoutersapphirelayerismainlyduetothehighvalueofpermittivityofAl2O3inthisfrequencyregion26andthecontinuityofthenormalcomponentoftheelectricdisplacementvector.TheseresultsdemonstratethatnarrowdipolarFanoresonantfeaturesandhighlyenhanced?eldsarepreservedevenafteroxidizationoftheAlshells,whichmaketheproposedcompositeplasmonicstructureappealingforpracticalrealizationsandrealisticapplications.

Thescatteringsignalobtainedfromanindividualplas-monicnanoparticleissmallinthefar-?eld,duetoitsextremelysubwavelengthdimensions,andforpracticalpur-posescollectionsofsuchparticleswouldbeofmoreinteresttoformmorecomplexnanodevices.Forthisreason,

we

FIG.3.(a)Normalizedscatteringcrosssectionofthethree-layercompositenanoparticlewithgeometryshownintheinsetofpanel(b).Theradiusofthecoreisac1¼3nm,themiddlelayerradiusac2¼8:5nm,andtheouterlayerradiusa¼10nm.Theinsetof(a)showstwosnapshotsintimeoftheEx?eldcompo-nentattheresonanceandcloakingwavelengths,denotedbyIandII,respectively.(b)Fieldenhancementacrossthethree-layercompositenanoparticleasafunctionofthenormalizedtransversedirectionðx=aÞattheresonantI(redline)andcloakingII(blueline)wavelengths.

143113-4Argyropoulosetal.Appl.Phys.Lett.103,143113(2013)

FIG.4.(a)Anin?niteperiodicarrayofcompositenanoparticleswithgeometry,parametersanddimensionsasinFig.1.Theperiodicityischosentobedx¼dy¼2:5a¼21:25nm.Amplitudeof(b)re?ection,(c)transmission,and(d)absorptioncoef?cientsofthemeta-surfaceshowninpanel(a).

considernextanarrayofcompositenanoparticles,inwhichtheeffectsofinterferencebetweenradiated?eldsmaybetai-loredatwill.InFig.4(a),weshowaplanararrayofthepro-posedplasmonicnanoparticles,whicheffectivelyrealizesaquasi-homogeneousplanar“metasurface.”29,30Thenanopar-ticleshaveparameters,geometryanddimensionsasinFig.1andarearrangedinasquarelatticewithperiodicitydx¼dy¼2:5a,whereaistheouterradiusofeachnanopar-ticle.Transmissionandre?ectioncoef?cientsofthisplanar-izedin?nitearrayarecalculatedwithafastconvergingsummation,34whichtakesintoaccountthefulldynamiccou-plingamongnanoparticles.Theamplitudeoftheresultedre?ectionandtransmissioncoef?cientsareplottedinFigs.4(b)and4(c),respectively.Theproposed“UV-metasurface”providessharpFanotransmissionandre?ectionfeaturesatdeep-UVfrequencies,similartothescatteringresponseofthesinglenanoparticleshowninFig.1(a).Itisinterestingthatthetransmissionsignatureresembleselectromagnetic-induced-transparency(EIT)scatteringpatterns,35,36withtherelevantdifferencesthatitisbasedhereonpurelydipolarresonances,providingmorerobustnesstoloss,anditarisesatdeep-UVfrequencies.Wehaveveri?edthatinthelimitofweakcoupling,theFanoresonanceofthe?nitearraytendstotheoneofanisolatednanoparticle,aslongastheinterpar-ticledistanceissubwavelength,andgratingeffectsarenotpresent.

Finally,theabsorptionoftheproposedUVnanodeviceiscalculatedfromthetransmissionandre?ectioncoef?cientsusingpowerconservation,anditisplottedinFig.4(d).Almost50%ofthetotalabsorptionisinterestinglyachievedinadeep-UVfrequencywindow.Thisbehaviorisidealtowardstherealizationofdeep-UVphotodetectorsandsolarblinds,forwhichweneedtoef?cientlyisolatetheUVfre-quenciesfromtheimpingingsolarradiationspectrum.Similarperformancewillbeobtainedincasethemetasurfaceisembeddedindifferentbackgroundmedia,asdemonstratedinFig.2.NumerousotherapplicationsmaybeenvisionedbasedontheproposedUV-metasurfaces,suchasnon-line-

of-sightcommunications37withsuchshort-wavelengthradi-ation,andef?cientUVoptical?ltersindependentofback-groundvariationsmaybedesignedbasedontheproposedconcepts.

Toconclude,wehaveexploredthedesignandperform-anceofacore-shellplasmonicnanoparticlethatstronglyinteractswiththeimpingingradiationatdeep-UVfrequen-cies,allowingstrongmanipulationofthescatteringresponse.DipolarUVFanoresonanceshavebeendemonstratedbasedonasinglesubwavelengthcompositenanoparticleconsistingofanaluminumshellsurroundingasapphirecore.RealisticOhmiclossesandthenaturaloxidizationofAlhavebeentakenintofullaccountinourcalculations.ThedescribedUVscatteringresponsemaybetunedtoalargedegreebyalter-ingthesizeandshapeofthestructure,providingadditional?exibilityinthedesignoftheproposedUVoptical?lters.ThecollectiveresponseofaplanarmetasurfacecomposedofanarrayoftheproposedplasmonicnanoparticleshasbeenshowntoexhibitsharpUVFanoandEITfeaturesinthere?ectionandtransmissioncoef?cients,whichmayleadtotherealizationofintegratedplanarizedUVnanodeviceswith?lteringeffects.Moreover,themetasurfacehasbeenfoundtoachievealmost50%ofabsorptioninadeep-UVfrequencywindow,aninterestingfeature,inparticular,forenergyhar-vestinganddefenseapplications.TheproposedFanoscatter-ingfeatureshavebeenfoundtobeveryrobusttodifferentdielectricbackgroundmedia.Inconclusion,webelievethatour?ndingsmayextendthereachofplasmonicFanoresonancesandotheranomalouseffectstodeep-UVwave-lengths.OurresultsprovethataluminumisanidealmaterialtosustainlocalizedplasmonsatUVfrequenciesandmayleadtotherealizationofef?cientUVplasmonicdevicesandnanocircuits.

ThisworkhasbeenpartiallysupportedbytheAROSTTRproject“DynamicallyTunableMetamaterials,”AFOSRwiththeYIPAwardNo.FA9550-11-1-0009andtheONRMURIGrantNo.

N00014-10-1-0942.