Scanning Near Field Optical Microscopy (SNOM): Applications in ...

S. Margherita di Pula 2003Summary:• Near-field microscopy (SNOM) in anutshell• Practical examples of results:– PtSi/Si– Spectroscopic SNOM: Diamond films– Biological growth medium– Pancreatic cells– LiF– Internal photoemission, Schottky Barrier– Boron nitride– Fluorescent markers in cells• The future: new sources -- 4GLS etc.

NashvillecountrymusicSwiss cheese& FrascatiwineThanks to all the partners of this work:• Frascati - ISM-CNR: A. Cricenti, G. Longo, V. Marocchi, V. Mussi, M. Girasole, R. Generosi, M.Luce, P. Perfetti• Monterotondo - ISM-CNR: F. Cattaruzza, A. Flamini, T. Prosperi• EPFL: D. Vobornik, G. Margaritondo, S. Catsicas, P. Steiner, H. Hirling• Vanderbilt: D. W. Piston, M. A. Rizzo, J. K. Miller, B. Ivanov, N. H. Tolk• US - NRL: D. Talley, P. Thielen, J. S. Sanghera, I. D. Aggarwal• Frascati - ENEA: G. Baldacchini, F. Bonfigli, F. Flora, T. Marolo, R. M. Montereali• MISDC -VNIIFTRI Mendeleevo, Moscow: A. Faenov, T. Pikuz• University of Roma: A. Congiu CastellanoS. Margherita di Pula 2003

The scanning near-field opticalmicroscope (SNOM): like the stethoscopeHeart:Frequency 30-100 HzWavelength λ 102 mAccuracy in localization 10 cm λ /1000SmallapertureCoatedsmall-tipopticsfiberSmalldistanceMicroscopiclightemittingobjectSNOM resolution: wellbelow the “diffractionlimit” of standardmicroscopy ( λ)S. Margherita di Pula 2003

The diffraction limit and how we can beat itFar-field microscope:∆yDiffraction by the lens aperture blurs theimages of the two objects: the minimumseparation ∆y that can be resolved is λ/2λNear-field scanningmicroscope:diffraction does notplay a roleTapered optic fiberS. Margherita di Pula 2003

oscillat ingd λ (far field) only the term 1/r survives andthe Poynting vector S and its flux have a well definite valueand the traveling wave can be detected far from the dipoleFor r≅λ (near field) The terms 1/r 2 and 1/r 3 do notcontribute to S, they represent the near field whoseintensity decay exponentially to zeroS. Margherita di Pula 2003

A simple approach to SNOMTask: resolving two small objects illuminated by a light-wavein this case two slits separated by a distance x 0wave, k = 2π/λxzThe uncertainty principle applied tothe wave requires: ∆x ∆ k x> 2πthe minimum ∆x corresponds tothe maximum ∆ k x ,butx 0k z =(k 2 -k x2 ) 1/2 =[(2π/λ) 2 -k x2 ] 1/2first casewe may have two casesk z is real, then -2π/λ< k x < 2π/λthe maximum value for ∆k x is 4π/λand the minimum value for ∆x is,from the uncertainty principleλ/2, far field limitS. Margherita di Pula 2003

second casek z =(k 2 -k x2 ) 1/2 =[(2π/λ) 2 -k x2 ] 1/2k z is imaginary (evanescent waves): then ∆k xcan be larger than 4π/λand one can have∆x< λ /2 (near field limit)How to get an imaginary k zwave, k = 2π/λaAfter a pinhole of width a, the wave willhave a strong Fourier component alongx-direction, with k x =π/2a, corresponding tok z =(k 2 -k x2 ) 1/2 =[(2π/λ) 2 -(π /2a) 2 ] 1/2if a< λ /4 then one has an evanescent wavewith imaginary k zS. Margherita di Pula 2003z .

SNOM transmission modesampleplane waveDETECTORsmall aperturea < < λS. Margherita di Pula 2003E. H. Synge Philos. Mag. 6, 356 (1928)

SNOM transmission modesampleplane waveDETECTORsmall aperturea < < λS. Margherita di Pula 2003

Multi-purpose SNOM module built at the ISM-CNR (Istitutodi Struttura della Materia), Frascati [Antonio Cricenti et al.]Fiber tipModes of operation:SampleTransmission Reflection CollectionS. Margherita di Pula 2003

IR SNOM -- the strong points:• It beats the diffraction limit thus reaching highlateral resolution• It can work in any environment and (almost)without any particular sample preparation• It provides real optical signal• It can perform spectroscopic measurements• It does not touch the sample: totally nondestructivetechnique[Image from theJasco Co. UKHomepage]S. Margherita di Pula 2003

Fiber Preparation:Chemical etchingfor IR fibers:Tampon solution(Tetra-methylpenta-decane)Chalcogenidefiber+ 30% H 2 S 2Piranha Solution70% H 2 SO 4MicroscopeS. Margherita di Pula 2003

Final structure(tapered chalcogenide fiber with Au coating):190 µmAuAs 38 Se 62As 40 Se 60120 µmS. Margherita di Pula 2003

Managing the distance fiber tip-sample:S. Margherita di Pula 2003

…our final component -- the Vanderbilt FEL!S. Margherita di Pula 2003

WHAT IS A SASE-FEL RADIATION SOURCE?SPONTANEOUS RADIATION:λ ph ≅ λ u /2γ 2 (1 + K 2 )Beam divergence ≅ 1/γ = m e c 2 /E e ≅ 1 00 µradI ph ≅ Ν eΝ e number of electrons (≅ 10 9 )S. Margherita di Pula 2003

THE INTERACTION OF THE ELECTRONS WITH THE SPONTANEOUSRADIATION CAUSES MICROBUNCHING AND THE SELF-AMPLIFICATION OFSPONTANEOUS EMISSION (SASE)IN THE SASE MODE THE INTENSITY:I ph ≅ Ν 2eSINCE Ν e number of electrons (≅ 10 9 ) THE AMPLIFICATIONGIVES AN EXTRAORDINARY HIGH PHOTON FLUXBeam divergence (few µrad.)S. Margherita di Pula 2003

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SNOM IN THE SPETTROSCOPIC MODETake the image at λ valueswhere the sample has a spectroscopicfingerprintS. Margherita di Pula 2003•reflectivity•absorption•fluorescence• ……

This is an example of how we can usea SNOM to investigate bulk opticalproperties with lateral resolutionwell below the diffraction limitQuickTime and aGIF decompressorare needed to see this picture.S. Margherita di Pula 2003

Absorption Spectrum of Artificial Diamond Filmsλ=3.5 µmS. Margherita di Pula 2003

Artificial CVD Diamond FilmNear FieldOptical imagesTopographicImagesS. Margherita di Pula 2003

SNOM/FEL Images of Biological Growth Medium•Surfaces immersed in water under physiological conditions arerapidly covered with biofilms, which consist of microrganismsembedded in a matrix of extracellular substance•Intensive research is in progress to elucidate the inflence of biofilmactivity on a variety of industrial, medical and environmentalprocesses•The first step of this research it to study the growth medium ofbiofilms•The growth medium is a minimal salt medium with severalconstituents like sulphur compounds and nitrogen oxideS. Margherita di Pula 2003

First IR spectroscopic SNOM results on biosystems:postgate growth medium on siliconNoise:

First IR spectroscopic SNOM results on biosystems:resolution tests20 µmLine scan:Resolution< 150 nm = λ/40S. Margherita di Pula 2003

Absorption of a Pancreatic Cellλ = 6.1 µmλ = 6.45 µmS. Margherita di Pula 2003

Shear-Force and SNOM Images (20x20 µm)of Pancreatic Cells with photons of 6.1 µmShear-ForceReflectivityS. Margherita di Pula 2003

IR spectroscopic SNOM results onbio-systems: fixed (with paraformaldehyde) cells inwater from a rat pancreatic β cell line (INS-1)λ = 6.95 µm(sulfide stretch)20 µmλ = 6.45 µm(amide II)λ = 6.1 µm (amide I,C=O stretch)TopographyReflectionS. Margherita di Pula 2003

IR spectroscopic SNOM study of metal (CdCl 2 )contamination of pancreatic cellstopographyλ = 0.488 µm 0.630 µm 0.785 µm 1.25 µm 4.05 µmspectroscopyS. Margherita di Pula 2003

First IR spectroscopic SNOM results onbio-systems: COS-7 cells (not fixed) in PBScellTopographyPBScrystalThe nucleusis best seenhere8.05 µm (phosphates, DNA)7.6 µm C-H bending6.95 µm (sulfide)only thePBS crystalreflects6.45 µm (amide II)6.1 µm (amide I)S. Margherita di Pula 200320 µm

How important is IR SNOM,in particular for IR bio-research?Very important: due to the long wavelengths, IRtechniques are severely affected by the diffraction limit!20 µmSpectroscopicSNOM image of celltaken at 6.1 µmSame image withdiffraction-limitblurringS. Margherita di Pula 2003

First example ofIR SpectroscopicSNOM:diamond filmsS. Margherita di Pula 2003

Lithium fluoride (LiF) films on siliconLiF - an interesting material for waveguides, microcavities,laser sources and generally optoelectronics devices:• Non hygroscopic• Irradiation by x-rays, e-beams and ion beams producesstable color centers (F-center) that can be used in devicemanufacturing. X-rays give high spatial resolution.+ -e -We need to clarify how the film growth parameters influencethe coloring process and to compare film coloring with bulkcoloringS. Margherita di Pula 2003

LiF crystal film on Si “colored” with X-rays through a12.7 µm grid.Topography, fluorescence SNOM andreflectivity SNOMFluorescenceTopographyReflectivityat 6.1 µmThe SNOM contrast at 9.2, 6.1,5.8 and 5.5 µm reveals acoloration-induced change inthe refractive indexS. Margherita di Pula 2003

LiF film pattern created with X-rays on a Si substrateShear-force (topography)S. Margherita di Pula 2003Reflectivity at 6.1 µmThe SNOM contrast at 9.2, 6.1, 5.8 and 5.5 µm reveals acoloration-induced change in the refractive index

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SNOM and Internal Photoemission (IPE)an improved local probeS. Margherita di Pula 2003

The first steps: “FELIPE” (FEL Internal photoemission)Conduction bandFELSchottky Barrier φ bE FhνValence bandSemiconductor MetalPhotocurrentpAThe Schottky Barrier is measured directlyhνS. Margherita di Pula 2003

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SNOM as a Useful Probe in Microelectronicse.g. SNOM Images of Boron Doped Silicon (100)Ion implantation is the key technique inintegrated Circuits TechnologyDepending on the Boron doses the post annealedsamples may present clusters formation or other defectsSNOM/IPE Techniques may be used as powerfuldiagnostic toolsS. Margherita di Pula 2003

IR Spectroscopic SNOM:Boron-doped (ion-implanted, annealed) silicon10 µmWavelength:1.33 µmS. Margherita di Pula 2003

IR Spectroscopic SNOM: BN thin films (+ oxides)SNOM topography imageWurtziteLine scan: featuresnarrower than λ/9SNOM reflectionspectroscopic imagesHexagonalS. Margherita di Pula 2003Cubic

A negative but important result:carboxylic acid terminated Si and silicon nitride surfaces(chemical processing for biological sensors)Surface—(CH 2 ) n (COOH)AFMtopographyReflectivityλ = 6.1~ µm6 µm20 µm20 µmIR spectroscopic images taken at this and otherwavelengths show no lateral fluctuationsS. Margherita di Pula 2003

Spectroscopic SNOM results on bio-systems:preliminary tests on internal cell structureslabeled by a fluorescent markerFluorescentmarkerCells at PetriDish bottomDetector(avalanchephotodiode)S. Margherita di Pula 2003

Intestinal adeno carcinoma epithelial cells labeledwith Fluorescein Isothiocynate25 x 25 µmS. Margherita di Pula 2003

Fibroblast cells (COS) with Oregon greenfluorophore (excitation at 496 nm, emission at522 nm) fixed to TransferrinNeuron cells withOregon greenfluorophore directlyattached to vesiclesinside the cellS. Margherita di Pula 2003

S. Margherita di Pula 2003Fluorescence: SNOM vs far-field

SNOM Fluorescence study of magnetic field (24 hrs50 Hz, 1 mT) effects on human keratinocyte cellsunexposedcellsTopographyTopographySNOM fluorescenceSNOM fluorescenceexposedcellsS. Margherita di Pula 2003

New types of synchrotron sources:• Ultrabright storage rings (SLS, newGrenoble project) approaching thediffraction limit• Self-amplified spontaneous emission(SASE) X-ray free electron lasers• VUV FEL’s (such as CLIO)• Energy-recovery machines• Inverse-Compton-scattering table-topsourcesS. Margherita di Pula 2003

THE “4 GLS” CONCEPT AT DARESBURYS. Margherita di Pula 2003

Conclusions:1. SNOM experiments with an FEL are possible and yieldresolution levels well beyond the diffraction limit2. Vibrational spectroscopy SNOM provides chemicalinformation on a microscopic scale3. Results on biological systems finally open up the possibiltyto perform infrared microscopy of cells and internal cellstructures4. IR Spectroscopic SNOM tests were successfully performedin different SNOM modes5. The experimental performances did not yet reach saturationand substantial improvements can be foreseen6. In the near future, even better IR sources will further bost thecapabilities of IR SNOMS. Margherita di Pula 2003

A powerful international coalition:NashvillecountrymusicSwiss cheese& FrascatiwineS. Margherita di Pula 2003

The first steps: “FELIPE” (FEL Internal photoemission)Conduction bandLightsourceConduction banddiscontinuityhνValence bandSemiconductor A Semiconductor BPhotocurrentpAProblem: the high value of the photon energy“mixes” the conduction band discontinuity and thelocal gapS. Margherita di Pula 2003hν

The first steps: “FELIPE” (FEL Internal photoemission)FELConduction bandhνValence bandSemiconductor A Semiconductor BS. Margherita di Pula 2003pAThe FEL removes the problem and the conductionband discontinuity is measured directly

Another problem, however, still remains: is thediscontinuity the same for the entire interface?Solution: FELIPE(essentially, a near-field SNOM system)First SNOM results with an FEL!S. Margherita di Pula 2003Lateral fluctuations!

SNOM: why does it work?yUncertainty principle:Photon beamx∆y ∆k y > 2 → ∆y > 2/∆k y∆k y < k y = (k 2 - k x2 ) 1/2k x real → ∆k y < k = 2/λ∆y > λ(Abbé’s diffraction limit)This conclusion is no longer valid ifk x is imaginary(absorbed, evanescent waves)S. Margherita di Pula 2003

IR SNOM: a multi-purpose module built at theISM (Istituto di Struttura della Materia, CNR),Frascati [Antonio Cricenti et al.]S. Margherita di Pula 2003Resolution< 0.15 µm

…our final component -- the Vanderbilt FEL!S. Margherita di Pula 2003

NashvillecountrymusicSwiss cheese& FrascatiwineThanks to all the partners of this work:• Frascati - ISM-CNR: A. Cricenti, G. Longo, V. Marocchi, V. Mussi, M. Girasole, R.Generosi, M. Luce, P. Perfetti• Monterotondo - ISM-CNR: F. Cattaruzza, A. Flamini, T. Prosperi• EPFL: D. Vobornik, G. Margaritondo, S. Catsicas, P. Steiner, H. Hirling• Vanderbilt: D. W. Piston, M. A. Rizzo, J. K. Miller, B. Ivanov, N. H. Tolk• US - NRL: D. Talley, P. Thielen, J. S. Sanghera, I. D. Aggarwal• Frascati - ENEA: G. Baldacchini, F. Bonfigli, F. Flora, T. Marolo, R. M. Montereali• MISDC -VNIIFTRI Mendeleevo, Moscow: A. Faenov, T. Pikuz• University of Roma: A. Congiu CastellanoS. Margherita di Pula 2003