Near-field scanning optical microscopy (SNOM)

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1 Adviser: dr. Maja Remškar Institut Jožef Stefan January 2010

2 Fluorescence Raman and surface enhanced Raman 7

3 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)

4 (1928) Original idea: E. H. Synge 1 - subwavelength-diameter aperture in an optically opaque screen Figure: Synge s original proposal 1 E.H. Synge, Phil. Mag. 6, 356, (1928)

5 (1932) Synge s alternative idea: instead of the aperture - point light source (1972) Ash and Nichols: 3 cm wavelength radiation λ 0 60 resolution (1984) Pohl at IBM Research Laboratory and Lewis at Cornell university - developed microscope similar to Synge s scheme

6 Geometrical optics paraxial approximation paraxial ray tracing Figure: A ray tracing diagram for a converging lens.

7 1 f = 1 s + 1 m = s s s = h h N = NA = sin θ = CA 2s s CA NA = 1 2N (1) (2) (3) NA = sin θ = CA 2s (4) CA = 2s sin θ CA = 2s sin θ (5) m = NA NA (6)

8 Fraunhofer diffraction (far-field) - fundamental limits of performance for lenses Airy disc radius d = 1.22λN Rayleigh criterion x = 1.22λN = 0.61λ NA modern objectives: NA ( ) x λ 2 Figure: Far-field optical microscopy

9 Fresnel diffraction (near-field) Resolution - functionally dependent only on aperture size and probe-to-sample separation 2 Figure: Aperture near-field microscopy 2 G. A. Massey, Applied Optics 23, 5 (1984)

10 The angular representation of the field in a plane z = z 0 near an arbitrary object: E(x, y, z 0 ) = A(k x, k y )e i(kx x+ky y+z 0 k0 2 k2 x k2 y ) dk x dk y (7) A(k x, k y ) the complex amplitude of the field, k 0 = ω/c the vacuum wave vector Sum of plane waves and evanescent waves propagating in different spatial directions k x and k y smaller than k 0 homogenous plane waves that propagate in free space, low spatial frequencies. k x and k y larger than k 0 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.

11 conventional microscope entire image at once, aperture scanned in the back image plane, resolution λ SNOM point-by-point, sample plane, resolution smaller than λ Figure: Far-field and near-field imaging

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

13 Figure: Schematics for various SNOM probes: (a) heat-pulling method using commercial pipette puller coupled with a CO 2 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 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).

14 spatial resolution - complicated because of auxiliary gap width control mechanism 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).

15 Is subsurface imaging possible? resolution degrades with increasing distance from the probe. Must the sample be thin? No. Must the sample be flat? Optically active region at the exact apex of the sharp tip, distance regulation mechanism.

16 Fluorescence Raman and surface enhanced Raman 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 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.

17 Fluorescence Raman and surface enhanced Raman 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 component of the optical electric field is polarized along its transition dipole moment. Laterally and longitudinally polarized electric fields near the aperture randomly oriented molecules can be excited. Emission patterns of single molecules suitable for the determination of molecular orientations.

18 Fluorescence Raman and surface enhanced Raman Figure: Series of three successive SNOM fluorescence images of the same area (1.2 by 1.2 µm) of a sample of DiIC 18 (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)

19 Fluorescence Raman and surface enhanced Raman Raman spectroscopy identifying and analyzing the molecular composition of complex materials. Vibrational spectra directly reflect the chemical composition and molecular structure of a sample. Raster scanning the sample and pointwise detection of the Raman spectra chemical maps with nanoscale resolution. Drawback low scattering cross-section, typically 14 orders of magnitude smaller than those of fluorescence.

20 Fluorescence Raman and surface enhanced Raman Surface Enhanced Raman Scattering (SERS) 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 electromagnetic enhancement caused by locally enhanced electric fields. Sharp metal tip enhancement effect confined to a very small volume at the end of the tip localized enhancement Tip-enhanced Raman spectroscopy has been used on a variety of different systems (dyes, fullerene films, SWCNT)

21 Fluorescence Raman and surface enhanced Raman Figure: High spatial resolution near-field Raman image (a) and simultaneously detected topographic image (b) of SWCNT on glass. Scan area 11m 2. 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)

22 Advantages lateral resolution - 20 nm; vertical resolution 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 problematic for imaging soft materials due to their high spring constants, especially in shear-force mode. topological artifacts