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SPECIAL TOPIC: NEAR-FIELD MICROSCOPY AND SPECTROSCOPY Scanning near-field optical microscopy with aperture probes: Fundamentals and applications Bert Hecht, a) Beate Sick, and Urs P. Wild Physical Chemistry Laboratory, Swiss Federal Institute of Technology, ETH-Z, Universita ¨tstr. 22, CH-8092 Zurich, Switzerland Volker Deckert and Renato Zenobi Organic Chemistry Laboratory, Swiss Federal Institute of Technology, ETH-Z, Universita ¨tstr. 16, CH-8092 Zurich, Switzerland Olivier J. F. Martin Electromagnetic Fields and Microwave Electronics Laboratory, Swiss Federal Institute of Technology, Gloriastrasse 35, ETZ ETH-Zentrum, CH-8092 Zurich, Switzerland Dieter W. Pohl Institute of Physics, University of Basel, Klingelbergstr. 82, CH-4056 Basel, Switzerland ~Received 13 October 1999; accepted 1 February 2000! In this review we describe fundamentals of scanning near-field optical microscopy with aperture probes. After the discussion of instrumentation and probe fabrication, aspects of light propagation in metal-coated, tapered optical fibers are considered. This includes transmission properties and field distributions in the vicinity of subwavelength apertures. Furthermore, the near-field optical image formation mechanism is analyzed with special emphasis on potential sources of artifacts. To underline the prospects of the technique, selected applications including amplitude and phase contrast imaging, fluorescence imaging, and Raman spectroscopy, as well as near-field optical desorption, are presented. These examples demonstrate that scanning near-field optical microscopy is no longer an exotic method but has matured into a valuable tool. © 2000 American Institute of Physics. @S0021-9606~00!70316-3# I. INTRODUCTION Scanning near-field optical microscopy ~SNOM or NSOM! is at the forefront of science and technology today because it combines the potentials of scanned probe technol- ogy with the power of optical microscopy. SNOM provides us with eyes for the nanoworld. Among the main parameters that might be of interest for a nano structure under investi- gation are, besides shape and size, its chemical composition, molecular structure, as well as its dynamic properties. In or- der to investigate such properties, microscopes with high spatial resolution as well as high spectral and temporal re- solving power are required. The classical optical microscope excels with respect to spectroscopic and temporal selectivity, although its resolution is restricted by diffraction to about half the wavelength, i.e., to 0.2–0.5 micrometer for visible light. Present-day science and technology, however, have an increasing need for tools that allow to characterize, generate, and manipulate structures as small as a few nanometers in size. Examples are readily found in biology, microelectron- ics, and medical sciences. Electron microscopes as well as scanning tunneling and atomic force microscopes easily achieve 10 nm spatial resolution and beyond, but they are relatively poor performers with respect to spectral and dy- namic properties. Electron microscopes have to be operated in vacuum, which limits their application in life sciences, requires special sample preparation, and complicates sample manipulation. SNOM combines the excellent spectroscopic and temporal selectivity of classical optical microscopy with a lateral resolution reaching well into the sub-100 nm re- gime. However, for a long time technical problems jeopar- dized the widespread application of SNOM. Near-field optics ~NFO! therefore became a focus of research and develop- ment in the field of optical microscopy in recent years. 1–9 Today, we have reached the point where SNOM represents a powerful tool for surface analysis that is technically and theoretically well understood. It is ready to be applied to a large variety of problems in physics, chemistry, and biology. In a simplified view of classical far-field optical micros- copy the object is illuminated by a monochromatic plane wave. The transmitted or reflected light, scattered by the ob- ject in a characteristic way, is collected by a lens and imaged onto a detector. For practical reasons, the lens is placed at least several wavelengths l of the illuminating light away from the object surface, i.e., in the far field. Since high spa- tial frequencies corresponding to the fine details of the object generate Fourier components of the light field that decay exponentially along the object normal, 10 they cannot be col- a! Electronic mail: [email protected] JOURNAL OF CHEMICAL PHYSICS VOLUME 112, NUMBER 18 8 MAY 2000 7761 0021-9606/2000/112(18)/7761/14/$17.00 © 2000 American Institute of Physics Downloaded 24 May 2011 to 200.19.190.5. Redistribution subject to AIP license or copyright; see http://jcp.aip.org/about/rights_and_permissions

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SPECIAL TOPIC: NEAR-FIELD MICROSCOPY AND SPECTROSCOPYScanning near-eld optical microscopy with aperture probes: Fundamentalsand applicationsBert Hecht,a)Beate Sick, and Urs P. WildPhysical Chemistry Laboratory, Swiss Federal Institute of Technology, ETH-Z, Universitatstr. 22,CH-8092 Zurich, SwitzerlandVolker Deckert and Renato ZenobiOrganic Chemistry Laboratory, Swiss Federal Institute of Technology, ETH-Z, Universitatstr. 16,CH-8092 Zurich, SwitzerlandOlivier J. F. MartinElectromagnetic Fields and Microwave Electronics Laboratory, Swiss Federal Institute of Technology,Gloriastrasse 35, ETZ ETH-Zentrum, CH-8092 Zurich, SwitzerlandDieter W. PohlInstitute of Physics, University of Basel, Klingelbergstr. 82, CH-4056 Basel, SwitzerlandReceived 13 October 1999; accepted 1 February 2000Inthisreviewwedescribefundamentalsofscanningnear-eldopticalmicroscopywithapertureprobes. After the discussion of instrumentation and probe fabrication, aspects of light propagationin metal-coated, tapered optical bers are considered. This includes transmission properties and elddistributions in the vicinity of subwavelength apertures. Furthermore, the near-eld optical imageformationmechanismis analyzedwithspecial emphasis onpotential sources of artifacts. Tounderline the prospects of the technique, selectedapplications includingamplitude andphasecontrast imaging, uorescenceimaging, andRamanspectroscopy, as well as near-eldopticaldesorption, are presented. These examples demonstrate that scanning near-eld optical microscopyis no longer an exotic method but has matured into a valuable tool. 2000 American Institute ofPhysics.S0021-96060070316-3I. INTRODUCTIONScanning near-eld optical microscopy SNOM orNSOM is at the forefront of science and technology todaybecause it combines the potentials of scanned probe technol-ogy with the power of optical microscopy. SNOM providesus with eyes for the nanoworld. Among the main parametersthat might be of interest for a nano structure under investi-gation are, besides shape and size, its chemical composition,molecular structure, as well as its dynamic properties. In or-der toinvestigate suchproperties, microscopes withhighspatialresolutionaswellashighspectralandtemporalre-solving power are required. The classical optical microscopeexcels with respect to spectroscopic and temporal selectivity,althoughitsresolutionisrestrictedbydiffractiontoabouthalfthewavelength, i.e., to0.20.5micrometerforvisiblelight. Present-day science and technology, however, have anincreasing need for tools that allow to characterize, generate,andmanipulatestructuresassmall asafewnanometersinsize. Examples are readily found in biology, microelectron-ics, andmedical sciences. Electronmicroscopesaswell asscanning tunneling and atomic force microscopes easilyachieve10nmspatial resolutionandbeyond, but theyarerelativelypoorperformerswithrespect tospectral anddy-namic properties. Electron microscopes have to be operatedinvacuum, whichlimitstheir applicationinlifesciences,requires special sample preparation, and complicates samplemanipulation. SNOMcombinestheexcellent spectroscopicand temporal selectivity of classical optical microscopy withalateral resolutionreachingwell intothesub-100nmre-gime. However, foralongtimetechnicalproblemsjeopar-dized the widespread application of SNOM. Near-eld opticsNFO thereforebecameafocusof researchanddevelop-ment intheeldofoptical microscopyinrecent years.19Today, we have reached the point where SNOM represents apowerful tool for surface analysis that is technicallyandtheoreticallywellunderstood.Itisreadytobeappliedtoalarge variety of problems in physics, chemistry, and biology.In a simplied view of classical far-eld optical micros-copytheobject is illuminatedbyamonochromaticplanewave. The transmitted or reected light, scattered by the ob-ject in a characteristic way, is collected by a lens and imagedontoadetector. Forpracticalreasons, thelensisplacedatleast several wavelengthsoftheilluminatinglight awayfrom the object surface, i.e., in the far eld. Since high spa-tial frequencies corresponding to the ne details of the objectgenerateFourier components of thelight eldthat decayexponentially along the object normal,10they cannot be col-aElectronic mail: [email protected] OF CHEMICAL PHYSICS VOLUME 112, NUMBER 18 8 MAY 20007761 0021-9606/2000/112(18)/7761/14/$17.00 2000 American Institute of PhysicsDownloaded 24 May 2011 to 200.19.190.5. Redistribution subject to AIP license or copyright; see http://jcp.aip.org/about/rights_and_permissionslected by the lens. This effect leads to the well known Abbediffractionlimit x/(2 NA),11whereNAis thenu-merical aperture of the lens.This resolution limit can be slightly pushed by scanningconfocal optical microscopy where a sharply focused spot oflightreplacesthewide-eldillumination.12,13Usingspecialgeometries of illumination and detection, sometimes in com-bination with nonlinear effects such as multiphotonexcitation,13further progress in this eld is still being made;however, steps are small and no major breakthrough is to beexpected.In1928, Synge, anIrishscientist, describedanexperi-mental scheme that would allow optical resolution to extendintothenanometer regime:14Heproposedtouseastronglight source behind a thin, opaque metal lm with a 100 nmdiameter hole in it as a very small light source. The tiny spotof light created this way should be used to locally illuminatea thin biological section. In order to guarantee the local illu-mination,heimposedtheconditionthattheapertureinthemetal lm be no further away from the section than the ap-erturediameter, i.e., lessthan100nm. Imagesweretoberecorded point by point detecting the light transmitted by thebiological section by means of a sensitive photo detector.Withhisproposal, Syngewaswell aheadofhistime.Unfortunately, he never tried to realize his idea. In 1932, heactually proposed an alternative, discarding his originalscheme because of the difcult sample approach to a planarscreen.15He therefore suggested to use the image of a pointlight source as an optical probe instead of an aperture. Theimagewastobegeneratedbyanellipsoidal mirrorwhichwouldprovide, inmodernwords, thelargest possiblenu-merical aperture. Ironically, this second scheme never wouldhaveachievedSNOM-typeresolutionsinceit ignoredtherole of near elds which cannot be recovered by any conven-tionalimagingscheme,nomatterwhatthenumericalaper-ture is.In 1984, shortly after the invention of the scanning tun-neling microscope,16nanometer-scale positioning technologywas available and an optical microscope similar to Syngesproposed and forgotten scheme was re-invented by Pohl anddemonstratedtogether withDenkandDuerigat theIBMRuschlikon Research Laboratory.1719Independently, a simi-larschemewasproposedanddevelopedbyLewisandhisgroup at Cornell University.2022The key innovation was thefabrication of a subwavelength optical aperture at the apex ofa sharply pointed transparent probe tip that was coated witha metal. In addition, a feedback loop was implemented main-taining a constant gapwidth of only a few nanometers whileraster scanningthesampleincloseproximitytothexedprobe.The promise to extend the power of optical microscopybeyondthediffractionlimit triggeredthedevelopment ofseveral experimental congurations that are able to generateoptical images with nanometer resolution. Figure 1a showstheclassicalapertureSNOMcongurationinwhichanap-erture probe is illuminating a small area of a samplesurface.4,5,1723Inthemost general congurationwithre-spect tolight detection, thesampleisplacedontopof ahemisphericalsubstratewhichallowsthecaptureofalltheradiationemergingfromtheprobesampleinteractionzoneinto the far eld.24,25Figure1b shows theintriguingapertureless NFOtechniques.2634In this conguration a strongly conned op-tical eld is created at the apex of a sharply pointed probe tipby external far-eld illumination. The relevant NFO signaltherefore often has to be extracted from a huge backgroundof far-eld scattered radiation. Resolutions ranging from120 nm have been reported in laboratory experiments. Thegeneral applicabilityof thetechniquetoawider rangeofsamples is currently being investigated.The third NFO microscopy congurationsee Fig. 1creliesonthedirectdetectionoflocalizedeldsintheneareldaboveasamplebyuncoateddielectrictips. Theso-called scanning tunneling optical microscope STOM,35alsocalledphotonscanningtunnelingmicroscopePSTM,36isschematically depicted in Fig. 1c. In selected experimentsthe STOM led to results that could not have been achievedby other techniques.3740II. THE APERTURE SNOMA. General setupIn this article we will concentrate on the apertureSNOM. Reachingaresolutionof50100nmonaroutinebase and having a potential down to 1030 nm, SNOM is atleast a factor of 5 to 10 better in resolution than a standardscanning confocal optical microscope with 1.4 NA. This rep-resents an enormous progress in view of the relevance of thesub-100 nm regime today, and certainly, aperture SNOM ispresentlythemost widelyusedandmost developedNFOtechnique. This is alsoillustratedbythefact that all thecommercial instruments currently available employ the aper-ture SNOM technique.In our opinion the reasons for this situation are obvious.Fromasimpliedpoint of view, imagingwithSNOMissimilar to scanning confocal optical microscopy except thatthe illumination spot is smaller. Of course, one has to keep inmindthat SNOMisasurfaceselectivetechnique, whereasscanning confocal optical microscopy can advantageously beappliedtothicksamplesthatrequireopticalsectioning.12,13Nevertheless, all thecontrast mechanismssuchasabsorp-tion, phase, anduorescencecontrast, well-knownincon-ventional optical microscopy, can be transferred more or lessdirectly to SNOM applications. An aperture represents a veryconnedlight source without anybackground. This is incontrast to the apertureless techniques where a rather large,FIG. 1. Different types of scanning near-eld optical microscopes:a ap-ertureSNOMwithangularresolveddetection, baperturelesscongura-tion, andc scanning tunneling optical microscope.7762 J. Chem. Phys., Vol. 112, No. 18, 8 May 2000 Hecht et al.Downloaded 24 May 2011 to 200.19.190.5. Redistribution subject to AIP license or copyright; see http://jcp.aip.org/about/rights_and_permissionsintensive laser spot is focused onto a tiny tip apex. For uo-rescencestudies,whichareamongthemostrelevantappli-cationsofSNOM,alocalizedilluminationisimportantbe-cause excessive photobleaching of the sample has to beavoided.It is straightforward and tempting to extend existing con-ventional optical microscopes by a SNOM zooming stage. Atypical SNOMsetup, asit isrealizedinmanylaboratoriesandcommercial instruments, is sketchedinFig. 2. Laserlight of a suitable wavelength is coupled into an optical berthat has an aperture probe at its far end. Control of the po-larization and spectral ltering of the light is necessary be-fore coupling into the ber. The tip is mounted onto a me-chanical support that includes adjustment screws to facilitatea coarse approach of the tip to the sample into the range ofthe z-piezo displacement. Furthermore, it is often necessaryto align the tip laterally onto the optical axis of the conven-tional microscopes objective or move it to specic areas ofinterest on the sample.Inordertomeasureandcontrol thegapwidthbetweentip and sample, a strongly gapwidth-dependent signal isneeded. Early SNOMs relied on electron tunnelingfeedback.17,18Today, the majority of SNOMs utilizes amethod similar to noncontact atomic force microscopyAFMwhichiscalledshearforcefeedback.41,42Theberprobe is vibrated at one of its mechanical resonances parallelto the sample surface, ideally at amplitudes below 15 nm.Amplitude and phase of this minute ber oscillation aremonitored by a suitable displacement sensor.4146During thenal approach (020 nm due to the action of shearforces, the resonance frequency is detuned with respect to thedriving oscillator leading to a decrease in amplitude and to aphase shift. The origin of the shear force effect is not entirelyclear. Theremight bearangeofmechanisminvolved, in-cluding viscous damping in thin water lms,47,48intermittentcontact,49and electrostatic image forces.50The phase signalreacts faster to changes in the state of oscillation, because itis independent of the dissipation of kinetic energy stored inthe resonance. Therefore, the use of pure phase feedback51ora combination of amplitude and phase feedback44,46,52is ad-vantageous in fast shear force feedback systems. Lightsource, ber, optical probe, shear force sensor, and mechani-cal support often make up an independent unit in an SNOMdevice as sketched in Fig. 2a.Light that is emittedbytheaperturelocallyinteractswith the sample. It can be absorbed, phase shifted, or locallyexciteuorescence, dependingonthesampleandthecon-trast mechanism of interest. In any case, light emerging fromthe interaction zone has to be collected with the highest pos-sibleefciency.Forthispurpose,highNAoilimmersionmicroscopeobjectivesormirrorsystemsareoftenused. Adistinct advantage of an oil immersion objective when usinga plane parallel substrate is that it can collect light that wouldotherwiseundergototal internal reectionandthusfail toreach the detector. This is the so-called forbiddenlight.25,44,52,53Forbidden light detection can lead to en-hanced contrast and resolution in phase and amplitude con-trastimages.54Inthecaseofmirror-baseddetection,hemi-spherical substrates providing a solid immersion aresometimes used to capture the forbidden light.25,44,52,53Thecollectedlight is directedviaa dichroic mirroreithertoavisualinspectionportofthemicroscopeortoasuitabledetector seeFig. 2c. Filterscanbeinsertedtoremove unwanted spectral components. Light collection,redistribution, and ltering is advantageously done by meansof a standard inverted optical microscope depicted inFig. 2b.B. Fabrication of near-eld optical probesGreat efforts have been devoted to the microfabricationof NFO probes.55,56However, few of the present designs arecommerciallyavailableorhaveactuallybeenusedinNFOexperiments. Therefore, the NFOcommunity still reliesheavilyonapertureprobesbasedontaperedoptical bers.Since ber probes of sufcient quality are still expensive orevenimpossibletoobtaincommercially, tipfabricationre-mainsofmajorinteresttoscientistsworkingintheareaofnear-eld optics.The fabricationprocess for ber-basedoptical probescan be divided in two main steps: a the creation of a trans-parenttaperwithasharpapex, andbsubsequentcoatingwith aluminum to obtain an entirely opaque lm on the conewalls and to form a transmissive aperture at the apex. Thereare two methods for the preparation of tapered optical berswith a sharp tip and reasonable cone angle:i the heatingand pulling method, andii chemical etching.The heating and pulling method is based on local heatingusing a CO2 laser or a lament and subsequently pulling theberapart. Theresultingtipshapesdependheavilyonthetemperatureandthetimingof theheatingandpulling, aswell as on the dimensions of the heated area.44,57,58The pull-ingmethodhastheadvantagethattheglasssurfaceonthetaper is very smooth, which positively inuences the qualityof the evaporated metal layer. Furthermore, pulled tips oftenFIG. 2. StandardSNOMsetupconsistingof aanilluminationunit, bcollection and redistribution unit, andc a detection module.7763 J. Chem. Phys., Vol. 112, No. 18, 8 May 2000 Scanning near-eld optical microscopyDownloaded 24 May 2011 to 200.19.190.5. Redistribution subject to AIP license or copyright; see http://jcp.aip.org/about/rights_and_permissionsexhibitatfacetsattheapexduetoanalfractureduringpulling which facilitates the formation of an aperture duringevaporation. It is, however, difcult, though not impossible,toobtainalargeconeangle;correspondinglythetransmis-sioncoefcient for agivenaperturesizeislow, astobediscussed in Sec. III A1.Chemical etchingofoptical bersinHFsolutionsbe-came popular since Turners etching technique59wasintroduced.60It comprises dipping the ber into an HF solu-tioncoveredwithanoverlayer of anorganicsolvent. Tipformation takes place at the meniscus that forms at the berat the interface between HF and solvent. The mechanism isbased on the fact that the meniscus height is a function of theremaining ber diameter. Chemical etching allows more re-producible productionof larger quantities of probes inasingle step, thus opening the road towards a laboratory-scalemass production of optical probes. A specic advantageofTurnersmethodisthatthetaperanglecanbetunedbyvaryingtheorganicsolvent usedastheoverlayer. Accord-ingly, optical probes with correspondingly large transmissioncoefcient canbeproduced.61,62Amajor disadvantageofTurners technique turned out to be the microscopic rough-ness of the glass surface on the taper walls. It leads to pin-holesandimperfectionsinthemetal coatingthat interferewith the true NFO signals.This problem was solved only recently by introducing amethodcalledtubeetching.63,64IntubeetchingtheberisnotstrippedbeforedippingitintoHF.Tipformationtakesplace inside a small volume dened by the polymer coatingduetoconvectiveowof HF. Thedelicatemeniscus be-tween HF and the organic overlayer no longer plays a directroleinthetipformationprocess. Themethodisthuslessprone to produce roughness on the taper while still providinglarge cone angles.Inbothtechniques, theapertureis formedduringtheevaporation of aluminum. Since the evaporation takes placeunderanangleslightlyfrombehind, thedepositionrateofmetal at theapexismuchsmallerthanonthesides. Thisgeometrical shadowing effect leads to the self-aligned forma-tion of an aperture at the apex, as is illustrated in Fig. 3.InFig.4wecomparetypicalberprobesproducedbyheating and pullingleft side and tube etchingright side,respectively. A comparison of the different global shapes oftheprobesinFigs. 4aand4cbeforealuminumcoatingevidences the improved cone angle and overall shape of thetube etched tips Fig. 4c. Figures 4b and 4d show scan-ning electron microscopySEM images of the apex regionof pulled and tube etched tips after aluminum coating. It isobvious that both tips exhibit a comparably high quality ofthe aluminum layer. The aperture can be observed for bothtips.III. SQUEEZING LIGHT THROUGH ASUBWAVELENGTH APERTURETo achieve the ultimate performance possible in apertureSNOM, the optical probe should combine two main proper-ties: i thespot sizedeterminedbytheaperturediametershould be as small as possible. At the same time, ii the lightintensity at the aperture should be as high as possible. Thiscan be achieved either by optimizing the overall lightthroughput or byimprovingthe damage thresholdof themetal coating thus increasing the maximum input power. Fi-nally, forpracticalreasons, deliveringthelightintothevi-cinityoftheapertureshouldbeasconvenient aspossible.Thisrequirement iswell met byapertureprobesproducedfrom standard optical bers. Optical probes with small spotsizes and large transmission coefcients could open up fas-cinatingnewareasofresearchsuchasnanoscalenonlinearoptics andoptical surface modicationonthe nanometerscale. It isthereforeofprincipal interest tounderstandthelimitations and possibilities of aperture probes.FIG. 3.Evaporation geometry of the aluminum coating process: evaporationtakes place under an angle slightly from behind while the tip is rotating. Thedeposition rate of metal at the apex is much smaller than on the side walls.FIG. 4. Aluminum-coatedapertureprobespreparedbypullinga,bandetchingc,d: a,cmacroscopicshape,SEMandopticalimage. b,dSEM close-up of the aperture region, scale bar corresponds to 300 nm.7764 J. Chem. Phys., Vol. 112, No. 18, 8 May 2000 Hecht et al.Downloaded 24 May 2011 to 200.19.190.5. Redistribution subject to AIP license or copyright; see http://jcp.aip.org/about/rights_and_permissionsA. Transmission coefcient of aperture probes1. The taper regionThetransmissioncoefcient of anapertureprobeat agiven wavelength is dened as the power of the light coupledinto the taper region divided by the light power emitted bythe aperture. In the case of an optical ber, the powercoupled into the taper region is easily determined by cleav-ingtheberbehindthecouplerandmeasuringtheoutput.Themeasuredvaluecorrespondstothepowerintheberminus4%reection loss.The power emitted by the aperture is often measured inthe far eld. In principle, this is correct since the nonpropa-gating eld components around the aperture do not contrib-ute to the time-averaged Poynting vector.65However, in al-most all experimental situations theinstantaneous electriceldsdominatelight-matter interaction. Thus, thefar-eldtransmissioncoefcient doesnot reect theeldenhance-ment effectsintheneareld. However, incomparingtipswith the same aperture size, the far-eld transmission coef-cient can be valuable information.The transmission coefcient of an aperture at the apex ofaconicaltipisdeterminedbythecharacteristicsofitstwomain structural components: i the part that guides light intothe vicinity of ii the actual subwavelength aperture. In stan-dardber probes, as depictedinFig. 5, theguidingpartcorresponds to a tapered, metal-coated dielectric waveguide.The efciency of guiding light to the aperture is determinedby the distribution of propagating modes in the taperedwaveguide. In the special case of a single-mode ber with ataper produced by heating and pulling, the initial effect of thetaperisaspreadingoftheguidedmodesincetheguidingcore is getting thinner and thinner. When the mode starts tofeel the vicinity of the metal coating, the waveguidechangesitscharacterfromadielectrictoametallichollowone lled with a dielectric. In the case of a taper produced bychemical etching, themodespreadingeffect isabsent be-cause the core diameter is not affected by the chemical etch-ing. As a result the inuence of the metal coating starts to beimportant at smaller waveguide diameters closer to the aper-ture. In both cases, the mode structure in the metallic wave-guide is completely different from the one in an unperturbedberleadingtoareorganizationofpropagatingmodesac-companiedbyarather strongback-reectionof light asarst source of attenuation.Themodestructureinametallicwaveguideat opticalfrequencieswascalculatedbyNovotnyandHafner66asafunctionofthedielectriccorediameter. Theresultscanbeinterpreted such that in a tapered waveguide, where the corediameter decreases gradually, one mode after the other runsinto cutoff until only the nal HE11 mode is still propagating.An aluminum-coated (alu34.5i8.5) dielectric (core2.16)waveguide supports the pure HE11 mode at a wave-length488nmforinnerdiametersbetween250and160 nm as sketched in Fig. 5. The transmission coefcient oflight up to this point is mainly determined by the fraction ofpower contained in cutoff modes as compared to the powerinthestill-propagatingHE11mode. Themagnitudeofthisfactor is likely to depend on the geometry of the taper. Thisconnection has been poorly understood up to now. Power notcomprised in the propagating modes is either reected backinto the waveguide or absorbed in the metal coating, leadingto a considerable heating of the metal.6770Below an inner diameter of 160 nm, even the HE11 moderuns into cutoff. Cutoff means that the wave vector becomesimaginary and thus the mode eld decays exponentially. Thepowerthatisactuallydeliveredtotheaperturedependsonthe distance between the HE11 cutoff diameter and the aper-tureplanewhichis determinedbytheconeangleof thetaper. Thisfact wasrst noticedbyNovotnyet al.,71whopredicted that larger cone angles would drastically improvetheoveralltransmissioncoefcientofataperedwaveguidestructure.2. The apertureThe transmission coefcient of a subwavelength hole inan innitely thin perfectly conducting screen was rst calcu-lated rigorously by Bethe.72Errors in his expression for theneareldwerecorrectedinapaperbyBouwkamp.73Theso-called Bethe/Bouwkamp model of a subwavelength aper-ture is important because it provides closed analytic expres-sions for the resulting electric and magnetic eld. Althoughthe model fails to accurately describe reality, the expressionsstill include the most characteristic features of the near- andfar-eld distribution of a realistic subwavelength aperture.WithintheBethe/Bouwkampmodel,itisexpectedthatthe transmission coefcient of a subwavelength hole shouldscale as a4, where a denotes the aperture diameter.7274Anincrease of the aperture diameter from 20 to 100 nm henceresults in a transmission coefcient increased by a factor of625. In order to illustrate this, we use transmission data cal-culated by Novotny et al.71for a realistic tip structure withdifferent full taper angles and aperture diameters of 10 and20nm. Thecalculations employedthemultiplemultipolemethod75to solve Maxwells equations.Scaling the numerical data according to the Bethe/Bouwkampmodel providesasimpleapproximationof thetransmission of a 100 nm aperture as a function of the taperanglesee Fig. 6. For values up to30, where the twocurves run parallel, scaling the 10 nm curve by 104and the20nmcurveby54leadstotransmissioncoefcientsthatFIG. 5. Mode propagation in a tapered metal-coated optical ber at a wave-length of 488 nm. Cutoff diameters taken from Ref. 66.7765 J. Chem. Phys., Vol. 112, No. 18, 8 May 2000 Scanning near-eld optical microscopyDownloaded 24 May 2011 to 200.19.190.5. Redistribution subject to AIP license or copyright; see http://jcp.aip.org/about/rights_and_permissionsnearly coincide. At larger taper angles the interpolated datapoints start to diverge. This is likely due to the breakdown oftheBethe/Bouwkampmodel becausethenitepenetrationdepth of light into the aluminum (7 nm starts to wash outthe difference in diameter between the two apertures and thetransmission through the metal screen becomes dominant.From Fig. 6 it can be deduced that for a 100 nm apertureand a taper angle of about 30, a transmission between 106and105maybeexpected. For evenlarger taper anglesaround 42, as they occur in etched ber probes and in an-isotropically etched silicon structures, the transmission coef-cient should reach values around 103. These numbers areinreasonableagreement withtransmissioncoefcients re-ported in the literature and measured in ourlaboratories.61,63,76It should even be possible from such con-siderations to estimate the effective aperture size from trans-mission measurements if the taper angle at the apex is knownfrom an independente.g., SEM measurement.Workingwithreallysmall apertures(4050nm isoften impossible since the transmission coefcient decreasesdramaticallywithdecreasingaperturesize. Thiscannot beovercome by an increased input power because of the gen-erally lowdamage threshold of the metal coating (10mW.TherelevantNFOsignalsbecomessosmallthatthesignal to noise ratio is no longer sufcient to provide reliableresults. This is the reason why most NFO studies today arecarried out with apertures of 80100 nm.B. Field distributionSince we focus this article on aperture SNOM, it is im-portant to investigate the character of the electric eld distri-bution behind a small aperture. Let us consider a geometrythat is even simpler than the Bethe/Bouwkamp conguration,namely a slit in a thin, good-conducting metallic screen Fig.7. The screen is illuminated at normal incidence by a planewave with a wave number k02/, where is the wave-length Fig. 7a. Right behind this aperture, the transmittedeld will be more or less conned to its width.77The character of this eld is best understood by consid-ering its angular Fourier spectrum.10The electric eld dis-tribution behind the aperture can be obtained by convolutingtheangularspectrumoftheincident eldwiththeFouriertransform of the aperture.10The Fourier transformation mustbeperformedintheplaneof thescreenandprovides anangular spectrum of plane and evanescent waves with eldamplitudes(k , kz). Here, kandkzarerespectively, thetransverse and longitudinal components of the k vector.FIG. 6. Transmission coefcient of an aperture probe as a function of thefull taper cone angle . Squares and rhombs: calculated values from Ref. 71fora20and10nmdiameteraperture, respectively. Circlesandtriangles:interpolatedvaluesdeterminedbyscalingthenumericaldataaccordingtothe Bethe/Bouwkamp a4law by a factor of 104and 54, respectively. Thisprovidesasimpleapproximationforthetransmissioncoefcientofa100nm aperture.FIG. 7. Character of the electric eldsbehindanaperture: aAsmallaper-ture of size a in an innitely thin andperfectlyconductingscreenisillumi-nated at normal incidence with a planewave propagating in the z directionwithavectorkzk02/. bdThespectrumofthetransmittedeldis given for three different aperturesizes. In each case the vertical axisrepresents the eld amplitude of thek vector with components (k , kz),wherethetransversecomponentkisreal and the longitudinal component kzcanbeeither real or imaginary. Thelatter corresponds to an evanescenteldstronglylocalizedatthevicinityof the aperture. The ratio of propagat-ing and evanescent elds strongly de-pends on the aperture size a relative tothe wavelength.7766 J. Chem. Phys., Vol. 112, No. 18, 8 May 2000 Hecht et al.Downloaded 24 May 2011 to 200.19.190.5. Redistribution subject to AIP license or copyright; see http://jcp.aip.org/about/rights_and_permissionsSince our aim here is not to give an exact treatment of thescattering by the slit but rather to illustrate the physical phe-nomena relevant to aperture SNOM, we will assume that thetransversecomponent kisreal. However, thelongitudinalcomponent kz can be either real or imaginary. It is related tokviak02k2kz2. 1Together, kandkzdescribetheradiationpropertiesoftheplane and evanescent waves that constitute the eld behindthe aperture. The angular spectra for different aperture sizesaredepictedinFigs. 7b7d. Fieldamplitudes(k , kz)concentrated at small kcorrespond to large apertures,whereas a distribution of(k , kz)over a broad range of kvalues corresponds to small apertures.From Eq. 1 we see that, depending on the magnitude ofk , two different types of solutions are obtained for kzkzk02k2kk0ik2k02kk0.2As an example, let us consider an aperture of width a. Theangular spectrumof this aperture will containsignicanteldamplitudes (k , kz) at k2/a. For thisparticulark , the expression for the longitudinal wave vector compo-nent readskz2 22a 2. 3It is obvious that for a, kzbecomes imaginary.ThetwosetsofsolutionsdescribedbyEq. 2arede-pictedasadashedlineinFigs. 7b7d:for kk0theextremityofthekvectordescribesacircleinthe(k , kz)plane,thekzcomponentbeingreal.Ontheotherhand,forkk0thesolutionsfollowasquare-root curve, withanimaginary kzcomponent. A eld component with an imagi-nary wave vector in the propagation direction corresponds toan evanescent eld. It hence remains strongly bound to theaperture and does not propagate into the far eld. The stronglocalization is responsible for the resolution that can beachieved in scanning near-eld optical microscopy.78For a givenwavelength, the ratioof propagating(kzrealandevanescent(kzimaginaryeldamplitudesintheangular spectrum strongly depends on the size a of the aper-ture as compared to the wavelength see Figs. 7b7d.When a, the angular spectrum of the transmitted eld isverysimilar tothat of theincident eld, i.e., most of thetransmittedeldpropagatesintheforwarddirectionwithapropagation vector kzk0, k0Fig. 7b. Few eld com-ponents with a small k can appear, leading to a slight diver-gence of the beam transmitted through the aperture.When the aperture size becomes comparable to thewavelength, the lateral connement increases and the spec-trumbroadens. As a result the transmittedelddivergesstrongly Fig. 7c. The angular spectrum of the transmittedeldnowalsoincludesasmall rangeof niteamplitudeswith kk0.Finally, when the aperture is much smaller than thewavelength, thespectrumof thetransmittedeldinthezdirection becomes dominated by the evanescent components;i.e., for the vast majority of the transverse components, k islargerthank0Fig. 7d. Thismeansthat thetransmittedeld is strongly localized at the vicinity of the aperture anddecreasesrapidlyawayfromtheaperture. Asaresult, thefar-eld power emitted by the aperture decreases, whilestrongly conned and enhanced elds appear at the vicinityoftheaperture. Experimentalevidencefortheexistenceofnonradiativeeldsclosetoanaperturearetheforbiddenlight emission,24,44,54surface plasmon excitation,79as wellas an increased uorescence from single molecules close toan aperture.80IV. TIPSAMPLE INTERACTION AND IMAGEFORMATIONWe previously emphasized the importance of a localizedlight sourcefor SNOM. However, inmost SNOMexperi-ments the eld is detected at a large distance of the tip, in thefar eld. The mechanisms by which some components of theevanescent illuminating eld can be transformed into propa-gating eld components that carry information about thesample are at the core of image formation in SNOM.Accurate and versatile computational techniques aremandatory to understand these intricate contrast mecha-nisms. Oneoftheintrinsicdifcultiesinthesimulationofnear-eld optical images is related to the fact that a practicalexperiment contains elements that have very different sizes:Thesampleisusuallyquitesmall, but it isdepositedonasubstrate with a quasi-innite extension. The tip is very largetoo, but its interaction with the sample quite subtle. Finally,as already mentioned, while the relevant lightmatter inter-action takes place in the near eld close to the sample, theimageisformedbydetectingintensitychangesinthefareld, at very large distances from the sample.A great effort has been devoted, over the last 10 years, tothetheoretical understandingof imageformationinnear-eldoptical microscopy, andtwoexcellent reviewarticleshave been published by Girard and Dereux81and Greffet andCarminati.82However, it is only recently that self-consistentcalculations taking into account these different length scales,andinparticular the interactionbetweenthe tipandthesample in a fully three-dimensional manner, have beenpresented.83,84These results were obtained with the Greenstensor technique.85The decisive advantage of this approachtosimulateSNOMexperimentsliesinthefact that it canaccommodate complex systems that include both innitesubstrates and strongly localized samples. Furthermore, it al-lows the computation of the measured far eld while takinginto account the near-eld interaction in a fully self-consistentmanner.Unfortunately,thisapproachisnotveryaccuratewhenhighpermittivitymaterialsareinplay. Thiscan, however, besupersededbyusingthelteredGreenstensor technique.86Figure8presents results obtainedwiththis approach.Thegeometryunder studyisdepictedinFig. 8a, whichshows a relatively large tip that is used to illuminate a smallglass cube on a glass surface. In Fig. 8b we show an imagecalculated when the tip is raster-scanned over the cube at aconstant height above the surface. The eld transmitted7767 J. Chem. Phys., Vol. 112, No. 18, 8 May 2000 Scanning near-eld optical microscopyDownloaded 24 May 2011 to 200.19.190.5. Redistribution subject to AIP license or copyright; see http://jcp.aip.org/about/rights_and_permissionsthrough the system is computed, as in a realistic experimentsee Fig. 2. We observe in Fig. 8b a strong asymmetry inthe measured signal. In order to understand this effect, onemust recall that whenasystemliketheglasscubeonthesurface is illuminated with some external eld E0, it gener-ates a depolarizationeldEdsuchthat the total eldEtE0Edfullls the boundary conditions imposed by Max-wellsequations.87Inourcalculation,theilluminationeldE0waschosentobepolarizedinthexdirection. Suchanincident eld is therefore parallel to the top and bottom sidesof the sample Fig. 8b. Since an electric eld parallel to aninterfacemustbecontinuousacrossthisinterface,theinci-dent eld already satises the electromagnetic boundary con-ditions andnodepolarizationeldis createdalongthesesidesFig. 8c.Ontheotherhand, thisx-polarizedincident elddoesnot comply with the boundary conditions along both verticalsides, since it is normal to the sample interfaces. As a matterof fact, the total electric eld Etshould now have a discon-tinuityproportionaltothesamplepermittivityatthisinter-face when the electric eld is normal to an interface, it is theproductoftheeldwiththepermittivitythatmustbecon-tinuous across the interface, Fig. 8c. Therefore, the samplegenerates a strong depolarization eld so that the total eldfullls the boundary conditions. It is this strong depolariza-tionsignal that is observedinthe near-eldimage Fig.8b.If insteadof scanningat aconstant height abovethesurface, a constant gapwidth is kept between the tip and thesample, one measures a very different signalFig. 8d. Inthis scanning mode, the interaction between the incident eldandthesamplesidestakesplaceduringtheentireupwardmotionof the tipnecessarytofollowthe contour of thesample. This leads to the prominent depolarization peaks thatare observed in Fig. 8d when the incident eld is normal tothe sample side. These peaks are located far away from thesample because they include the tip radius in addition to thesample side. The strong signal is hence merely related to thetip motion and constitutes a good illustration of a topo-graphic artifact.88Such an artifact is very difcult to distin-guishfromthereal near-eldsignal. Scanningat constantheight thereforeis thepreferredmodeof operationwhenworking with slightly corrugated samples. On the other hand,our results also show that a careful control of the illumina-tion polarization can provide another handle in order to keeptheseartifactsundercontrol. Inparticular, aseriesofmea-surement of the same system with different illumination po-larizationsshouldallowdiscriminationbetweentopographyartifact and real near-eld signal.FIG. 8. Simulationofimageformationinnear-eldmicroscopy. aAnaccuratemodelforanSNOMexperimentshouldincludeanaperturetipwithareasonably large extension, a sample, and an innite substrate supporting the sample. b Total transmitted electric eld intensity computed for the geometrydepicted in a. The sample is made of glass (2)and deposited on a glass substrate. The tip has a 50 nm opening with a 70 nm thick aluminum coating.Thesamplesizeis404040 nm3. Theincident eldispolarizedinthexdirection. Toreproduceanexperimental image, eachpixel onthisgurecorresponds to a different calculation with a particular tipsample position, the tip apex being kept at a constant height 2 nm above the top sample surface.c The strong signals observed in b are related to depolarization elds created on the sample sides that are normal to the incident eld. d Comparison ofdifferent scanning modes: constant height dashed line and constant gap solid line measured along the dashed line in b. The corresponding tip motion isdepicted at the bottom of the gure. When the sample is scanned in constant gap mode, i.e., when the tip motion follows the sample outline, the depolarizationsignal is much stronger as it is in constant height mode.7768 J. Chem. Phys., Vol. 112, No. 18, 8 May 2000 Hecht et al.Downloaded 24 May 2011 to 200.19.190.5. Redistribution subject to AIP license or copyright; see http://jcp.aip.org/about/rights_and_permissionsV. SNOM APPLICATIONSA. Amplitude and phase contrastIn order to obtain a phase or amplitude contrast image ofasampleas discussedinSec. IV, light emittedfromtheprobesample interaction zone is collected and recorded di-rectly by means of a suitable photodetector.1. Metal island lmTodemonstrateamplitudecontrast, wechosea15nmthick aluminum island lm produced by means of the latexsphere shadow mask technique.89,90The structure is sketchedin Fig. 9a. The objects of interest are the metal patches atthe interstices between the spheres. They are formed byevaporationandsubsequent dissolutionof thespheres. Tocharacterize their size, we propose to choose the altitude ofthe inscribable equilateral triangle as a standard. The lengthof this altitudeis h(33/2)d0.23d, wheredis thesphere radius. For the d220 nm latex spheres used for thepresent test sample, the altitude hence is 50 nm. The nearest-neighbor distance between metal patches is d/3127 nm.The calculated distance between the patches is in agree-mentwiththeshearforceimageFig. 9bofthesample.Theshearforceimagerepresentsaconvolutionofaratherblunt protrusion on the aperture rim cf. Figs. 4c and 4dwiththereal sampletopography. Therefore, thetriangularshapeofthemetal patches, whichwasconrmedindepen-dently by SEM, is not visible.Constant-height-mode optical images Figs. 9c and9d were obtained with the same tip as used for Fig. 9bandatthesamelocationonthealuminumislandlm,col-lectingtheforbiddenlight emittedfromtheprobesampleinteraction zone, only. The light emitted by the optical probewas linearly polarized arrows in Figs. 9c and 9d in twoorthogonal directions. The extinction ratio measured wasabout 15. Figure 9e presents a cut through two neighboringdots at the position indicated by the white bar in Fig. 9c. InFig. 9f ahigh-pass-lteredrepresentationof Fig. 9c isdisplayed achieving a better visibility of the small structuresby removing long-rangedfar-eld modulations.Theoptical scanimages Figs. 9c and9d showadistinct ne structure. The hexagonal pattern, correspondingtothemetal patches, iswell visible. Thestructuresintheoptical images show systematic distortions. In particular, thepatchesareelongatedinthedirectionofpolarization. Thismight be attributed to the polar concentration of the electricenergy on opposite sides of the rim of a small aperture.71,73,91A few nanometers behind the aperture plane this rather sharpdouble-peakstructureis washedout, forminganapproxi-matelyellipticspot.Thedirectionofstrongestconnementhencecorrespondstothedirectionperpendiculartothepo-larization in agreement with the observed elongation. It hastobepointedout that themetal patchesstronglyinuencethe eld distribution at the aperture,92therefore, our discus-sionof spot shapesmust remainqualitativeif norealisticsimulations are available.The optical resolution in optical images Figs. 9c, 9d,and 9f, respectively can be estimated from the line cut Fig.9e to be at least 50 nm.2. Dielectric gratingPhase contrast is obtained when imaging samples inducedifferences in the optical path. A typical example for such asample is a dielectric grating. An AFM topograph of such agrating is shown in Fig. 10a, accompanied by a height pro-lein10b. Thegratingstructurehasaperiodof383nmand a step height of 8 nm which was etched into a thin glasssubstrate.Figure 10c shows a very high resolution constantheight mode optical image recorded at a gapwidth below 1nm. Figures 10d and 10e show constant height mode op-tical imagesrecordedgapofafewnmand100nm, re-spectively. This series of images clearly evidences the strongdependence of the resolution on the gapwidth.B. Fluorescence imagingHighresolutionoptical imagingof biological sampleswith uorescent labels is among the most promising elds ofapplication for SNOM. SNOM has the potential to image theFIG. 9. Metal island lm: a Sketch of metal patches arranged in hexagonsin the interstices of densely packed 220 nm latex spheres. b Shear forceimage. c, dConstant height forbiddenlight imagestakenat thesameposition as b. The light emitted by the optical probe was linearly polarizedin c and d in two orthogonal directions symbolized by the white arrows.e Intensity prole along the white line in c. f High-pass-ltered versionofc suppressing long-ranged intensity changes. The wavelength used forillumination was 633 nm.7769 J. Chem. Phys., Vol. 112, No. 18, 8 May 2000 Scanning near-eld optical microscopyDownloaded 24 May 2011 to 200.19.190.5. Redistribution subject to AIP license or copyright; see http://jcp.aip.org/about/rights_and_permissionsdistribution of such labels down to the level of single uo-rophoressee, forexample, Refs. 4,5,93, and94andrefer-ences therein.Controllabledepositionofwell-denedspeciesofbio-logically active molecules on surfaces is of interest in biol-ogyandbiochemistry, sinceit opensupthepossibilityfornanometer-scale patterning and to carry out chemical assayson a nm scale with minute quantities of reagents.9597We investigated microcontact-printed patches of proteinmolecules, specically chicken immunoglobulin labeled withtetramethyl-rhodamine-isothiocyanate TRITC,95usingSNOM. The patches consist of a monolayer of close-packedproteinmolecules, 41014nm3insize. Eachproteinmoleculeislabeledbyabout 4dyemolecules. Theuoro-phores were excited at 514 nm and detection was performedabove 570 nm, as described in Sec. II A using an oil immer-sion objective and a single photon counting avalanche pho-todiode. Figure 11a shows a near-eld uorescence imageof two rectangular patches of proteins, nominally 5003000nm2insize. Someareasofthesampleshowsmallirregularities. This feature makes the sample suitable for anSNOMresolution test. Figure 11b shows the simulta-neously recorded shear force topographs of the structure. Toestimate the optical resolution, intensity proles were takenin the optical image Fig. 11a at two positions marked byarrows 1 and 2. The proles are depicted in Figs. 11c and11dasgraylines. Thewidthofthesmall featureinFig.11d suggests a resolution of about 80 nm. To exclude thepossibility of a z-motion artifact,88line cuts were taken at thesame positions inthe topographic image. The results areshown in Figs. 11c and 11d as black lines and prove thatthefeatureintheoptical imageisnot inducedbyatopo-graphic crosstalk. A comparison of the line proles taken inthe optical and topographic image in 11c suggests a shift ofFIG.10. Dielectricgrating: aAFMtopography. bLinecutthroughb. cVeryhigh-resolutionconstantheightmodeimageclosetocontact. d,eConstant height mode allowed images recorded at a gapwidth of a few, and100 nm, respectively.FIG. 11. a High-resolution (512512pixelscanningnear-elduo-rescence image of a contact-printedpattern of TRITC-labeled chicken im-munoglobulinmoleculesimmobilizedona glass surface acquiredsimulta-neouslytotheshearforceimageb.Thewhitearrows1and2markthepositionswheretheprolesshowninc and d were taken. c Fluores-cence and topography prole along 1,andd Fluorescenceandtopographyprole along 2.7770 J. Chem. Phys., Vol. 112, No. 18, 8 May 2000 Hecht et al.Downloaded 24 May 2011 to 200.19.190.5. Redistribution subject to AIP license or copyright; see http://jcp.aip.org/about/rights_and_permissionsabout 100nmbetweenthe intensityandthe topographicmaximum. This is an effect that often occurs in SNOM im-ages. It canbeexplainedbyassumingthat aprotrusionislocatedontherimofthemetalcoatingseeFigs. 4cand4d that actually is responsible for the shear force interac-tion. Thus, the topographic features are expected to be dis-placed from the center of the aperture.C. Near-eld Raman spectroscopyIn combination with vibrational spectroscopies, near-eldopticscanyieldmolecular contrast onalateral scaledeterminedbytheaperturesize. Themaindifcultiesthatare encountered applying this combination of techniques arethe low efciency of Raman scattering and the lack of suit-ablebermaterial forinfraredspectroscopy. Nevertheless,there are examples of both near-eld Raman,98101and near-eld infrared spectroscopy.33,102104A method to increase thesmall Raman scattering cross section to make it signals de-tectableinSNOMexperiments is theuseof roughnoblemetal substrates. Surfaceenhancement effectsobservedinthesignalsfromsamplesdepositedonsuchsubstratescanreach several orders of magnitude depending strongly on themorphology and shape of the metal features. The reason forthe strong enhancement is still subject to somecontroversy.105,106Two different mechanisms seem to be in-volved. Theso-calledpureelectromagneticeffectisduetoplasmonresonancesof theroughmetal lmsor particles.Theelectromagneticneareldofametal particlewithinthis model can exceed the applied eld by orders of magni-tude. Anadditional enhancement canbecausedbychargetransfer or bond formation between the sample and the me-tallic substrate, which can strongly enhance the polarizabilityof the molecule.Figure12showstopographyaandnear-eldRamanimagesb, c, dandspectrafromselectedsampleposi-tions, of stained DNA strands on a specially prepared silversphere substrate. Since the substrate has a rough topography,itisimportanttocarefullydistinguishbetweentopographi-cally induced contrast and intensity changes due to absorp-tionof thesubstrate, andnallytheRamansignal of thesampleitself. FortunatelyreectivityandRamanintensitycan be separated, at least to a rst approximation.The two images, Figs. 12b and 12c, which were re-corded sitting on different Raman bands, show quite differ-ent features. Figure12b wasrecordedat aglassRamanband of the near-eld probe, whereas Fig. 12c correspondto a Raman signal of the labeled DNA. The signals of 12brepresent the reectivity absorption of the silver spheresubstrate. Ofcourse, ahigherreectivityofthesamplere-sults in an increase in the detected Raman intensity as well;therefore, weusethereectivitysignal tocorrect for thesampletopography107inall our spectra. Theresult isdis-played in Fig. 12d. It demonstrates that one of the promi-nent spots of Fig. 12c is actually due to a locally increasedFIG. 12. SNOMSERS imaging of brilliant cresyl blue labeled DNA fragments on an SERS substrate. Lateral dependence ofa topography measured inshear force mode, b Raman intensity at 800 cm1corresponding to Raman scattering of the tip material glass, c Raman intensity at 1641 cm1BCBbefore normalization, and d near-eld surface enhanced Raman intensity at 1641 cm1after normalization with the glass signal. In b, c, and d, the pixelsize was only 100100 nm, and for each graph the intensity of one line of the complete Raman spectrum was plotted. In c andd, the contrast betweenthe lowest and the highest Raman intensity was a factor of 3. The arrows in the spectral images correspond to the lateral positions of the Raman spectra shownine. The square marks a BCB band and the circle denotes a glass band. Note that the BCB signal never vanishes completely.FIG. 13. Far-eld a, b and near-eld Raman spectra c, d of p-xylene.a,c Parallel polarization;b,d Vertical polarization geometry.7771 J. Chem. Phys., Vol. 112, No. 18, 8 May 2000 Scanning near-eld optical microscopyDownloaded 24 May 2011 to 200.19.190.5. Redistribution subject to AIP license or copyright; see http://jcp.aip.org/about/rights_and_permissionsreectivity. The relation between reectivity and Raman sig-nal can be seen more clearly by comparing the selected un-correctedRamanspectrainFig. 12e. Thearrowsintheoptical images correspond to the lateral positions of the cho-senspectra. Obviously, asubstantialchangeintheratioofthe BCBDNA band square and the glass scatteringcircle occurs onlyfrom1to23, whereas theabsoluteintensitiesof BCBDNAandglassintensityvaryineachspectrum.Inthecaseof surfaceenhancedRamanscatteringthesignalintensitiescanbeveryhigh.Infact,single-moleculesensitivity in experiments using conventional optical micros-copy has already been claimed by several groups.108,109ForNFO experiments the detection of a few hundred moleculeswas shown.110The method thus seems to have the potentialfor molecular recognition on a nm scale.Near-eldRamanspectroscopyis not limitedonlytosolid surfaces, but can be used as well in liquids.111Figure13comparesthenear-eldandfar-eldRamanspectraofxylene. Thedatademonstratethat thedistinct polarizationproperties in the near eld of an optical probe are useful toinvestigate, for instance, the conformation and the degree ofcrystallinity of a sample.D. Pulsed laser ablation through SNOM tipsOptical probes produced by tube etching are stableenoughtowithstandseveral tenths of mJ of pulsedlaserradiation coupled into them.112The intensity at the aperturecanbeinexcessof108W/cm2, whichisenoughtocauseablation of a sample close to the tip. Several mechanisms arepossiblefor laser-inducedablationthroughSNOMtips.113The photochemical model assumes that the energy is depos-itedinthesampleviaoptical absorption, andablationfol-lows as a consequence of chemical bond breaking. Similarly,in a photothermal scenario, the absorbed optical energywould be converted into heat, causing ablation from the lo-callyheatedspot. Third, aballisticmechanismisconceiv-able, where material fromthe tip is sputtered onto thesample, causing ablation of material from its surface. A bal-listic mechanism was the most likely explanation for an ex-periment carried out on an anthracene sample using a com-pletely metallized SNOMtip.114In the absence of anyphotonsreachingthesample, sputteringbyeithermetalat-oms from the tip, or by material adsorbed on its surface, wasbelievedtobethereasonfortheablationprocess. Fourth,one also has to take into account transient thermalexpansion69,115,70as a possible mechanism for the creation ofsurface indentations, but it is likely to be negligible.114We now discuss an experiment that gave very clear re-sults about the ablation mechanism from a rhodamine B lm.This sample was irradiated either on resonance532 nm oroff resonance 650 nm with respect to its optical absorptionmaximumFig. 14a. SNOM tips with apertures of100nm diameter were used. No signicant drop in optical outputof the SNOM tip was measured for similar input energies at400 and 650 nm input wavelength, although some decreaseis expected due to the increasing difference between wave-length and aperture size. Figures 14b and 14c show shear-force topographic images recorded after several laser pulseshad been red onto the sample. No ablation was observed forthe off-resonance wavelength Fig. 14b, 650nm, 2.1 Jpulseenergy. Incontrast, well dened, 70nmdiameterFWHM, 5nmdeepholeswerecreatedbyawavelengththat is strongly absorbed by the sampleFig. 14c,532nm, 1.4Jpulseenergy. Thisclearlypointstoanopticalablationmechanism, eitherphotochemicalorphotothermal.Energy transfer by absorption of photons appears to be moreefcient than by a ballistic process. The photon energy usedhere is not sufcient for inducing chemical bond dissociationin the rhodamine lm, in contrast to ablation processes withultraviolet laser radiation, whereaphotochemical ablationmechanism is often used to rationalize the observedphenomena.116We therefore suggest that a photothermalmechanism is responsible for the ablation of the rhodaminelm: webelievethat theabsorbedphotonenergyis con-verted to vibrational excitation, causing a rapid temperatureincrease that leads to thermal desorption of the molecules.Another interesting observation in Fig. 14c is that theablated material is redeposited on the sample surface close totheablationcrater. Thedistributionofredepositedmaterialwas not symmetric in this experiment, but was always to theleft side of the crater, independent of the scan direction. Inother SNOM ablation experiments, the distribution was moresymmetric. The origin of the directionality appears to be thetip itself: frequently, the apex of an SNOM tip exhibits someuncontrolledasymmetry,causingtheablatedmaterialtobeblown off in a specic direction. This has interesting appli-cations for the analytical nanosampling of material surfaces.FIG. 14. Near-eldoptical ablation: aAbsorptionspectrumofrhodamineBat aconcentrationof105Minmethanol solution. Arrowsindicatethewavelengths used for the ablation experiments before recording the images on the right. b,c Topography of a rhodamine B lm after coupling 2.1 J intothe SNOM tip at 650 nm b, and after coupling 1.4 J into the SNOM tip at 532 nm c, a wavelength near the maximum absorption of rhodamineB. The total topographic contrast in the z direction is 10 nmb, and 26 nmc, respectively.7772 J. Chem. Phys., Vol. 112, No. 18, 8 May 2000 Hecht et al.Downloaded 24 May 2011 to 200.19.190.5. Redistribution subject to AIP license or copyright; see http://jcp.aip.org/about/rights_and_permissionsVI. PERSPECTIVESWiththisreview,byfocusingonfundamentalsandse-lected applications of aperture SNOM, we cover only a smallpart of the research currently pursued in the area of NFO. Fora more complete picture the reader is referred to other NFOreview articles in this issue and to recent reviews.25,9Therapiddevelopment ofNFOtechniquesisfarfromcoming to an end. Many problems remain to be solved andmany careful experiments are still to be done.With respect to the technical problem of tip fabrication,there is still plenty of room for improvements, in particularregardingtipstability, damagethreshold, andtransmissioncoefcients. Microfabricated near-eld optical probes55,56will help to solve these problems and improve reproducibil-ity by providing better dened experimental conditions.Also, new tip concepts which deviated from the simple ap-erture scheme may be of advantage.Another possibility to improve the performance of aper-tureprobesistheuseofultraviolet UVlight.117122Thiswill shift the cutoff region in the taper towards the apex andtherefore strongly increase the transmission coefcient. Thiscan be exploited by using smaller apertures. In addition, di-rect excitationof certainbiomoleculesmight becomepos-sibleinsteadofusinguorescent markers. Furthermore, inRaman spectroscopy it is advantageous to use UV excitationbecause of the 4dependence of the scattering efciency andbecauseof thepotential of resonant eldenhancement atsmall silver particles in this spectral region for a review, seeRef. 123.Apertureless techniques arebecomingmoreandmoreimportant.2634,124The reported resolutions are exciting, buttheir utilization for relevant applications such as uorescenceimagingseemstobelessstraightforwardthanforapertureSNOM. This is mainly related to the difculties of discrimi-nating between effects generated in the enhanced eld at thetip apex and the background related to the diffraction limitedspot illuminating the tip. For uorescence imaging, uores-cence resonant energytransfer FRET microscopy,125128another apertureless contraption, is one of the most promis-ing candidates for a signicant improvement of resolution.We also did not mention all of the numerous new con-trast mechanismsthat areconstantlyevolving. Wedidnotmention, e.g., time-resolvedSNOM, whichcombines na-nometer scale optical resolution with femtosecond time reso-lution. Anumber of groups are already working in thisarea.129132Pulsedlasertechniquesinprincipleprovidethepossibility to utilize nonlinear optical techniques like secondharmonic generation, hyper-Raman, etc. Such techniques canhelptoavoidstraylight problemsinSNOM34,124andalsocan improve the signal-to-noise ratio.As another example, the combination of local desorptionusingSNOMwithmassspectrometricdetectionprovidesapromising new analytic method.133Fromour point of view, SNOMis aeldwheretheinteractionbetweenmodelingandexperiment canbe ex-tremely fruitful. This interaction, however, has not yet beenas successful as one might wish. This is probably related tothe intrinsic limitations of numerical simulations to accountfor a realistic experimental conguration including tip,sample, and detection scheme and conversely to experimen-tal difculties to precisely control all the relevant parameters.As a matter of fact, our simulations illustrated, for example,in Fig. 8, the importance of the illumination polarization forimage interpretation; still, the exact polarization state of theeld emerging from a real SNOM tip is difcult to control.Onemightwish, forthefuture, thatexperimentaliststrytodesign their experiments such that they can be compared totheoretical models. On the other hand, theoreticians have toextendtheirnumericalcapabilities.Bothsideswouldprotfrom such collaborations.ACKNOWLEDGMENTSThe authors wish to thank A. Bernard for the kind gift ofthecontactprintedsampleandL. Novotnyforhelpfuldis-cussions and for providing the tip transmission data. 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