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1、Chapter6Near-fieldopticalprobesOpticalnear-fieldsarecreatednearmaterialboundaries.Moregeneral,thecon-centrationofopticalfieldstodimensionssmallerthanthediffractionlimitreliesoninhomogeneitiesinanotherwiseemptyspace.Theunderstanding,construction,andoptimizationoflightconfiningstructuresisoneofthekeytopics
2、innano-optics.Itisdesirablethattheactuallightconfiningstructure,theopticalprobe,canbepo-sitionedandmanipulated.Thisrequiressomekindofmechanicalsupport.Becauseoftheevanescentnatureofopticalnear-fields,stronglightconfinementcanonlybeguaranteednearthematerialboundariesoftheprobe.Withincreasingdistancefrom
3、theprobethefieldsspreadoutandthefieldstrengthdecaysrapidly.Near-fieldop-ticalprobes,suchaslaser-irradiatedmetaltips,arethekeycomponentsofnear-fieldopticalmicroscopesdiscussedinthepreviouschapter.Nomatterwhethertheprobeisusedasalocalilluminator,alocalcollector,orboth,theopticalspatialresolutionsolelydepe
4、ndsontheconfinementoftheopticalenergyattheapexoftheprobe.Thischapterdiscusseslightpropagationandlightconfinementindifferentprobesusedinnear-fieldopticalmicroscopy.Whereapplicablewestudyfundamentalprop-ertiesusingelectromagneticsimulations(seechapter??)andprovideanoverviewofcurrentmethodsusedforthefabric
5、ationofopticalprobes.Wehopetoprovidetheba-sicknowledgetodevelopaclearsenseofthepotentialsandthetechnicallimitationsofsuchprobes.Themostcommonopticalprobesare1)uncoatedfiberprobes,2)aper-tureprobes,3)pointedmetalandsemiconductorprobes,and4)nanoemitters,suchassinglemoleculesornanocrystals.Thereciprocit
6、ytheoremofelectromagnetismstatesthatasignalremainsunchangeduponexchangeofsourceanddetector[1].Therefore,itsufficestoinvestigateagivenprobeonlyinonemodeofoperation.Inthemajorityofapplicationsitisundesirabletoexposethesamplesurfaceonalargescaleduetotheriskofphotodamageorlong-rangeinterferenceeffectscompl
7、icatingimage-reconstruction.Therefore,wewillpreferentiallyconsiderthelocalilluminationconfiguration.12CHAPTER6.NEAR-FIELDOPTICALPROBES6.1DielectricprobesDielectric,i.e.transparenttipsareanimportantclas
