Nearfield Scanning Optical Microscopy

OutlineIntroduction

Historical developmentFar-field versus near-field optical microscopy

InstrumentationResolution and artifacts

SpectroscopyConclusion

Near-field scanning optical microscopy (SNOM)

Bojana Visic

Adviser: dr. Maja RemskarInstitut Jozef Stefan

January 2010

Bojana Visic Near-field scanning optical microscopy (SNOM)

OutlineIntroduction




1 Introduction

2 Historical development

3 Far-field versus near-field optical microscopy

4 Instrumentation

5 Resolution and artifacts

6 SpectroscopyFluorescenceRaman and surface enhanced Raman

7 Conclusion


OutlineIntroduction




Conventional optical microscopy-limited resolution

Two broad classes of techniques with better resolution:

radiation with shorter wavelength (electron, x-ray)scanning probe microscopy (scanning tunneling, atomic force)

Problems: destructive, high cost, low speed, unreliable,sample preparation, high vacuum

Answer: combination of the interaction mechanisms of opticalmicroscopy, and high resolution of scanning probe microscopy

Resulting technique - Near-field scanning optical microscopy(SNOM)


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(1928) Original idea: E. H. Synge 1 - subwavelength-diameter aperture inan optically opaque screen

Figure: Synges original proposal

1E.H. Synge , Phil. Mag. 6, 356, (1928)Bojana Visic Near-field scanning optical microscopy (SNOM)

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(1932) Synges alternative idea: instead of the aperture -point light source

(1972) Ash and Nichols: 3 cm wavelength radiation 99K 060resolution

(1984) Pohl at IBM Research Laboratory and Lewis at Cornelluniversity - developed microscope similar to Synges scheme


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Geometrical optics 99K paraxial approximation 99K paraxial ray tracing

Figure: A ray tracing diagram for a converging lens.


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1

f=

1

s+

1

s m =

s

s=

h

h(1)

NA = sin =CA

2s(2)

N =s

CA NA = 1

2N(3)

NA = sin =CA

2s (4)

CA = 2s sin CA = 2s sin (5)

m =NA

NA(6)


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Fraunhofer diffraction (far-field) - fundamental limits ofperformance for lensesAiry disc 99K radius d = 1.22NRayleigh criterion 99K M x = 1.22N = 0.61NAmodern objectives: NA (1.3 1.4)M x 2

Figure: Far-field optical microscopy


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Fresnel diffraction (near-field)Resolution - functionally dependent only on aperture size andprobe-to-sample separation 2

Figure: Aperture near-field microscopy

2G. A. Massey, Applied Optics 23, 5 (1984)Bojana Visic Near-field scanning optical microscopy (SNOM)

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The angular representation of the field in a plane z = z0 near an arbitrary object:

E(x , y , z0) =

A(kx , ky )e

i(kx x+ky y+z0

k20k2xk2y )dkxdky (7)

A(kx , ky ) 99K the complex amplitude of the field, k0 = /c 99K the vacuumwave vector

Sum of plane waves and evanescent waves propagating in different spatialdirections

kx and ky smaller than k0 99K homogenous plane waves that propagate in freespace, low spatial frequencies.

kx and ky larger than k0 99K the field components become evanescent,propagates at the x, y plane but it is exponentially attenuated in the z-direction.These fields, associated with high spatial frequencies (fine detail of an object),are not detected by the objective of a classical microscope.


OutlineIntroduction




conventional microscope 99K entire image at once, aperture scanned inthe back image plane, resolution SNOM 99K point-by-point, sample plane, resolution smaller than

Figure: Far-field and near-field imaging


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Figure: Standard SNOM setup consisting of (a) illumination unit, (b)collection and redistribution unit and (c) a detection module.


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Figure: Schematics for various SNOM probes: (a) heat-pulling method usingcommercial pipette puller coupled with a CO2 laser, (b) SEM micrographs of (i)pencil-shaped and (ii) triply tapered SNOM probe produced by selective etching, (c)meniscus etching using an organic protection layer over a HF etching solution, (d)tube etching: the whole etching process takes place inside a micro-cavity formed bythe polymer coating, (e) micro-processed hollow cantilever SNOM probe and (f)SNOM probe with nanometer scale photon detector or emitter at the apex (Suh andZenobi, 2000).


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spatial resolution - complicated because of auxiliary gap width controlmechanism 99K z-motion artifact

Figure: Resolution in SNOM as a function of the probe-to-sampleseparation. The separation in each case is (a) near contact, (b) 5 nm, (c)10 nm, (d) 25 nm, (e) 100 nm and (f) 400 nm (E. Betzig and R.Chichester, 1993).


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Is subsurface imaging possible? 99K resolution degrades withincreasing distance from the probe.

Must the sample be thin? 99K No.Must the sample be flat? 99K Optically active region at theexact apex of the sharp tip, distance regulation mechanism.


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FluorescenceRaman and surface enhanced Raman

Possible to take advantage of the the various contrasttechniques available to the optical microscopy but with muchhigher resolution.

Change in the polarization light or the intensity of the light asa function of the incident wavelength 99K possible to makeuse of contrast enhancing techniques such as staining,fluorescence, phase contrast and differential interferencecontrast.

Also possible to provide contrast using the change inrefractive index, reflectivity, local stress and magneticproperties among others.


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The most widespread and easily implemented mode for chemical imagingby near-field optical methods.

A fluorescent tag must be attached to the molecules of interest, unlesstheir natural fluorescence can be exploited. Due to high signal intensity, itis even possible to image single fluorescent molecules (E. Betzig and R.Chichester, 1993).

Optically excite the molecules, raster scan the sample plane, collect thefluorescence of individual molecules by the large NA objective.

Orientation of the absorbtion dipole moment of each molecule determinedby recording the spatial variation of the fluorescence as the aperturemoved over the molecules. A molecule is excited 99K component of theoptical electric field is polarized along its transition dipole moment.

Laterally and longitudinally polarized electric fields near the aperture 99Krandomly oriented molecules can be excited.

Emission patterns of single molecules 99K suitable for the determinationof molecular orientations.


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Figure: Series of three successive SNOM fluorescence images of the same area(1.2 by 1.2 m) of a sample of DiIC18 (carbocyanine dye) molecules embeddedin a 10 nm thin film of PMMA. The excitation polarization was rotated fromone linear polarization direction (a) to another (b) and then changed to circularpolarization (c). The fluorescence rate images of the molecules changeaccordingly. The molecule inside the dotted circle as a dipole axis perpendicularto the sample plane. Scale bar: 300 nm (J. A. Veerman et al, 1998)


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Raman spectroscopy 99K identifying and analyzing themolecular composition of complex materials.

Vibrational spectra 99K directly reflect the chemicalcomposition and molecular structure of a sample.

Raster scanning the sample and pointwise detection of theRaman spectra 99K chemical maps with nanoscale resolution.Drawback 99K low scattering cross-section, typically 14 ordersof magnitude smaller than those of fluorescence.


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Surface Enhanced Raman Scattering (SERS) 99Kenhancement of the electric field in the proximity ofnanometer sized metal structures.

Enormous enhancement factors of up to 14 orders ofmagnitude have been reported, allowing even for singlemolecule Raman measurements.

The strongest contribution 99K electromagnetic enhancementcaused by locally enhanced electric fields.

Sharp metal tip99K enhancement effect confined to a verysmall volume at the end of the tip 99K localized enhancementTip-enhanced Raman spectroscopy has been used on a varietyof different systems (dyes, fullerene films, SWCNT)


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Figure: High spatial resolution near-field Raman image (a) and simultaneouslydetected topographic image (b) of SWCNT on glass. Scan area 11m2. TheRaman image is acquired by detecting the intensity of the G band upon laserexcitation at 633 nm. (c) Cross section along the dashed line in the Ramanimage. (d) Cross section along thedashed line in the topographic image. Theheight of individual tubes is 1.4 nm. Vertical units are photon counts persecond for (c) and nanometers for (d) (B. Pettinger et al, 2002)


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Advantages

lateral resolution - 20 nm; vertical resolution - 2-5 nmvarious environmental conditionsno fundamental restrictions on sample - transparent, opaque;flat, corrugated; organic; semiconductorcontrast mechanisms

Disadvantages

long scan times for high resolution images or large specimenareas.only features at the surface of specimens can be studied.fiber optic probes 99K problematic for imaging soft materialsdue to their high spring constants, especially in shear-forcemode.topological artifacts


OutlineIntroductionHistorical developmentFar-field versus near-field optical microscopyInstrumentationResolution and artifactsSpectroscopyFluorescenceRaman and surface enhanced Raman

Conclusion

Nearfield Scanning Optical Microscopy

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