Chapter 12_ Surface-plasmons .pdf

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1、Chapter12Surfaceplasmons12.1IntroductionTheinteractionofmetalswithelectromagneticradiationislargelydictatedbythefreeconductionelectronsinthemetal.AccordingtothesimpleDrudemodel,thefreeelectronsoscillate180◦outofphaserelativetothedrivingelectricfield.Asaconsequence,mostmetalspossessanegativedie

2、lectricconstantatopticalfrequencieswhichcausese.g.averyhighreflectivity.Furthermore,atopticalfrequenciesthemetal’sfreeelectrongascansustainsurfaceandvolumechargedensityoscillations,calledplasmonpolaritonsorplasmonswithdistinctresonancefrequencies.Theex-istenceofplasmonsischaracteristicforthein

3、teractionofmetalnanostructureswithlight.Similarbehaviorcannotbesimplyreproducedinotherspectralrangesus-ingthescaleinvarianceofMaxwell’sequationssincethematerialparameterschangeconsiderablywithfrequency.Specifically,thismeansthatmodelexperimentswithe.g.microwavesandcorrespondinglylargermetalstr

4、ucturescannotreplaceexperi-mentswithmetalnanostructuresatopticalfrequencies.Thesurfacechargedensityoscillationsassociatedwithsurfaceplasmonsattheinterfacebetweenametalandadielectriccangiverisetostronglyenhancedopticalnear-fieldswhicharespatiallyconfinedtotheinterface.Similarly,iftheelectrongasi

5、sconfinedinthreedimensions,asinthecaseofasmallsubwavelengthparticle,theoveralldisplacementoftheelec-tronswithrespecttothepositivelychargedlatticeleadstoarestoringforcewhichinturngivesrisetospecificparticleplasmonresonancesdependingonthegeometryoftheparticle.Inparticlesofsuitable(usuallypointed)

6、shape,extremelocalchargeaccumulationscanoccurthatareaccompaniedbystronglyenhancedopticalfields.Thestudyofopticalphenomenarelatedtotheelectromagneticresponseofmetalshasbeenrecentlytermedasplasmonicsornanoplasmonics.Thisrapidlygrowingfieldofnanoscienceismostlyconcernedwiththecontrolofopticalradia

7、tiononthesubwavelengthscale.Manyinnovativeconceptsandapplicationsofmetalopticshave407408CHAPTER12.SURFACEPLASMONSbeendevelopedoverthepastfewyearsandinthischapterwewilldiscussafewex-amples.Wewillfirstreviewtheopticalpropertiesofnoblemetalstruct