Near-field scanning optical microscopy (SNOM)

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Near-field scanning optical microscopy (SNOM)

Bojana Viˇsi´ c

Adviser: dr. Maja Remˇskar Institut Joˇzef Stefan

January 2010

Bojana Viˇsi´c Near-field scanning optical microscopy (SNOM)

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Conclusion

1

Introduction

2

Historical development

3

Far-field versus near-field optical microscopy

4

Instrumentation

5

Resolution and artifacts

6

Spectroscopy Fluorescence

Raman and surface enhanced Raman

7

Conclusion

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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 optical microscopy, and high resolution of scanning probe microscopy Resulting technique -

Near-field scanning optical microscopy (SNOM)

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Conclusion

(1928) Original idea: E. H. Synge¹ - subwavelength-diameter aperture in an optically opaque screen

Figure: Synge’s original proposal

1E.H. Synge , Phil. Mag. 6, 356, (1928)

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(1932) Synge’s alternative idea: instead of the aperture - point light source

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

^λ₆₀⁰

resolution

(1984) Pohl at IBM Research Laboratory and Lewis at Cornell university - developed microscope similar to Synge’s scheme

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Conclusion

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

⁰⁰

m = s

⁰⁰

s = h

⁰⁰

h (1)

NA = sin θ = CA

2s (2)

N = s

CA ⇒ NA = 1

2N (3)

NA

⁰⁰

= sin θ

⁰⁰

= CA

2s

⁰⁰

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CA = 2s sin θ CA = 2s

⁰⁰

sin θ

⁰⁰

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m = NA

NA

⁰⁰

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Conclusion

Fraunhofer diffraction (far-field) - fundamental limits of performance for lenses

Airy disc 99K radius d = 1.22λN

Rayleigh criterion 99K M x = 1.22λN =

^0.61λ_NA

modern objectives: NA ≈ (1.3 − 1.4) ⇒M x ≈

^λ₂

Figure: Far-field optical microscopy

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Fresnel diffraction (near-field)

Resolution - functionally dependent only on aperture size and probe-to-sample separation

²

Figure: Aperture near-field microscopy

2G. A. Massey, Applied Optics 23, 5 (1984)

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Conclusion

The angular representation of the field in a plane z = z0near an arbitrary object:

E (x , y , z0) = Z Z

A(kx, ky)e^{i (k}^x^{x +k}^y^{y +z}⁰

q

k₀²−k_x²−k²_y)

dkxdky (7)

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

Sum of plane waves and evanescent waves propagating in different spatial directions

kx and ky smaller than k099K homogenous plane waves that propagate in free space, low spatial frequencies.

kx and ky larger than k099K 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.

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conventional microscope 99K entire image at once, aperture scanned in the back image plane, resolution ≈ λ

SNOM 99K point-by-point, sample plane, resolution smaller than λ

Figure: Far-field and near-field imaging

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Conclusion

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 using commercial pipette puller coupled with a CO2laser, (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 by the polymer coating, (e) micro-processed hollow cantilever SNOM probe and (f) SNOM probe with nanometer scale photon detector or emitter at the apex (Suh and Zenobi, 2000).

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Conclusion

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

Figure: Resolution in SNOM as a function of the probe-to-sample separation. 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 with increasing distance from the probe.

Must the sample be thin? 99K No.

Must the sample be flat? 99K Optically active region at the exact apex of the sharp tip, distance regulation mechanism.

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Conclusion

Possible to take advantage of the the various contrast

techniques available to the optical microscopy but with much higher resolution.

Change in the polarization light or the intensity of the light as a function of the incident wavelength 99K possible to make use of contrast enhancing techniques such as staining, fluorescence, phase contrast and differential interference contrast.

Also possible to provide contrast using the change in

refractive index, reflectivity, local stress and magnetic

properties among others.

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

A fluorescent tag must be attached to the molecules of interest, unless their natural fluorescence can be exploited. Due to high signal intensity, it is even possible to image single fluorescent molecules (E. Betzig and R.

Chichester, 1993).

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

Orientation of the absorbtion dipole moment of each molecule determined by recording the spatial variation of the fluorescence as the aperture moved over the molecules. A molecule is excited 99K component of the optical electric field is polarized along its transition dipole moment.

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

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

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Conclusion

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

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

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

Raster scanning the sample and pointwise detection of the Raman spectra 99K chemical maps with nanoscale resolution.

Drawback 99K low scattering cross-section, typically 14 orders of magnitude smaller than those of fluorescence.

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Conclusion

Surface Enhanced Raman Scattering (SERS) 99K enhancement of the electric field in the proximity of nanometer sized metal structures.

Enormous enhancement factors of up to 14 orders of magnitude have been reported, allowing even for single molecule Raman measurements.

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

Sharp metal tip99K enhancement effect confined to a very

small volume at the end of the tip 99K localized enhancement

Tip-enhanced Raman spectroscopy has been used on a variety

of different systems (dyes, fullerene films, SWCNT)

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

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Conclusion

Advantages

lateral resolution - 20 nm; vertical resolution - 2-5 nm various environmental conditions

no fundamental restrictions on sample - transparent, opaque;

flat, corrugated; organic; semiconductor contrast mechanisms

Disadvantages

long scan times for high resolution images or large specimen areas.

only features at the surface of specimens can be studied.

fiber optic probes 99K problematic for imaging soft materials due to their high spring constants, especially in shear-force mode.

topological artifacts