A-fiber-optic-chemical-sensor-based-on-surface-pla

SensorsandAcruofors8,12(1993)213-220213Afiber-opticchemicalsensorbasedonsurfaceplasmonresonanceR.C.JorgensonandS.S.YeeSensorTechnologyResearchGroup,DepartmentofElectricalEngineering,UniversityofWashington,Seattle,WA98195(USA)(ReceivedSeptember23,1992;inrevisedformDecember15,1992;acceptedf&ember20,1992)Afiber-opticchemicalsensorispresentedwhichutilizessurfaceplasmonresonanceexcitation.ThesensingelementofthefiberhasbeenfabricatedbyremovingasectionofthefibercladdingandsymmetricaBydepositingathinlayerofhighlyrefleotingmetalontothefibercore.Awhite-hgbtsourceisusedtointroducearangeofwavelengthsintothefiberoptic.Changesinthesensedparameters(e.g.,bulkrefractiveindex,filmthicknessandf&arefractiveindex)aredeterminedbymeasuringthetransmittedspectral-intensitydistribution,Experimentalresultsofthesensitivityandthedynamicrangeinthemeasurementoftherefractiveindicesofaqueoussolutionsareinagreementwiththetheoreticalmodelofthesensor.1.introductionManyopticalsensingsystemshavebeendevelopedutilizingthesensitivityofsurfaceplasmonresonance(SPR)totherefractiveindicesofbulkandthin-tihndielectricsandtothethicknessesofthinfilms.ThesesensingsystemsinconjunctionwiththeappropriatechemicallysensitivelayershaveallowedresearcherstodevelopavarietyofSPR-basedchemicalsensors,in-cludingimmunoassay[l-3],gas[1,4],andliquidsen-sors[5].Onesuchchemical-sensingsystemhasbeencommercializedbyPharmacia(Uppsala,Sweden).However,allthesesystemsutilizebulk-opticconfigura-tionsthatarelimitedbytheuseofacouplingprism.Theresultingsystemsarerelativelylargeandexpensiveandarealsoinapplicableforremotesensingapplica-tions.Traditionally,SPRismeasuredusingtheKretsch-mannconfiguration,illustratedinFig.1,withaprismandathinhighlyreflectingmetaliayer(e.g.,silverorgold)depositedupontheprismbase[6].TheSPRreflectionspectrum(reflectedlightintensityversusangleofincidencewithrespecttothenormalofthemetal/dielectricinterface)ismeasuredbycouplingtransversemagnetically(TM)polarizedmonochromaticlightintotheprismandmeasuringthereflectedlightintensityoftherayexitingtheprismversustheangleofincidence.AnexampleoftheresultanttheoreticalspectrumisillustratedinFig.2.Theangleatwhichtheminimumreflectionintensityoccurs,t?,,,istheresonanceangleatwhichcouplingofenergyoccursbetweentheincidentlightandthesurfaceplasmonwaves.IncidentLightThlPolarizedReflectedLightetalBalkDielectricFig.I.TheKretschmannconfiguration.Fig.2.TheoreticaFSPRretkctioaspect+forabulkdielectricofwater,awavelength~of620nm,a550Athicksilverfilm,andafusedsilicaprism.’.,!Anovelfiber-optic.SPRsensingconfigurationwith-outtherequiredlight-couplingprismispresented.Thissensingcon&uratiunallowsforasmallsensingelementandsamplevolme,simplifiedopticaldesign,potentialusefordisposablefiber-opticsensorsandthecapabilityforremotesensing.0925-4C'J5/93/$[e-mail]214Thesensingelementisasegmentoffiberinwhichthecladdinghasbeenremovedanda5508,thicksilverfilmhasbeensymmetricallydepositedonthefibercore,viaelectron-beamevaporation.Thelengthofthecladdingremovedwascalculatedtooptimizetheaveragenumberofreflectionsforahthepropagatingraysoflightinthefiber.UnliketraditionalSPRmeasurements,whichem-ployadiscreteexcitationwavelengthwhilemodulatingtheangleofincidence,theSPRfiber-opticsensorsys-temusesawhite-lightsource,resultinginalargerangeofexcitationwavelengths.Therangeofincidentanglesislimitedtoonlythoseanglesthatpropagateintheopticalfiber.Thesensedparameterscanthenbedeter-minedfromthemeasuredresonancespectruminthetransmittedspectralintensitydistribution.2.Theory2.1.BackgroundSPRisaresonancephenomenoninwhichsurfaceplasmonwaves(SPWs)areexcitedatametal/dielectricinterface.Theelectricfield,E,oftheSPWpropagatinginthez-direction(Fig.I)isexpressedas[7]E,(x,2,1)=Et(x)exp(iwl-ik,z)(1)whereo=angularfrequencyandk,=k:+ik:istheSPRpropagationconstantalongthez-direction.TheSPRpropagationconstant,k,,isdependentuponthefree-spacewavenumber,k,,andthecomplexpermittivitiesofthemetal,a,,,,andthedielectric,ad.Thepropagationconstantisexpressedasl/2(2)Attheresonanceangle0,,thepropagationconstantofthe/ncidentbeamparalleltotheprismbase(i.e.,alongthez-direction,konpsin,&,)isequaltotherealpartoftheSPRpropagationconstant.Thisequalityconditionisexpressedbyki=k,,npsint?,,(3)wherenpistherefractiveindexoftheprism.Equations(2)and(3)provideforthetheoreticaltransductionmechanismforSPRasasensor.Specifi-cally,anincreaseinrefractiveindexofthesenseddielectricwillcauseashiftintheresonancespectrumtowardlargerSPRcouplingangles.BymeasuringtheSPRresonanceparameters(couplingangle,Q,,fullwidthathalfmaximum,A@,andthereflectedlightintensityattheresonancecouplingangle,Z&,)onecandeterminethecomplexrefractiveindexofthedielectricusingthecalculatedSPRpropagationconstant,k,[8].TheSPRreflectionspectruminFig.2isforadiscretewavelength.ForotherexcitationwavelengthstheSPRpropagationconstantwillchangeaccordingly,sincetheprismrefractiveindex,bulkdielectricrefractiveindexandthemetalpermittivityareallafunctionofwave-length[9].Atheoreticalthree-dimensionalSPRreflec-tionspectrumforarangeofw