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3 Scanning Near-Field Optical Microscopy (SNOM)

3 Scanning Near-Field Optical Microscopy (SNOM)

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Microscopy – Nanoscopy

Fig. 2.11 High-resolution topographical imaging of biomolecular assemblies by atomic force

microscopy (AFM). (a) 3 kbp (base pairs) plasmid DNA (pDNA) on mica; scale bar 150 nm

[2.45]. (b) Dense packing of human rhinovirus (HRV) particles with regular patterns of small

protrusions ∼ 0.5 nm high and ∼ 3 nm in diameter; width of the figure, ca. 70 nm [2.46]. (c)

Topographical image of the purple membrane to which a single antibody is bound and (d) a 3D

representation of two Fabs (fragment antigen binding regions of an antibody) bound to the bacteriorhodopsin (BR) molecules of mutant purple membranes from Halobacterium salinarum [2.44,

2.47]. (Reprinted with permission from [2.45] (a), [2.46] (b) and [2.47] (c) (d). © 2007 Elsevier

(a), © 2005 Elsevier (b), © 2004 Nature Publishing Group (c) (d))

fact that electromagnetic waves interacting with an object are always diffracted into

two components:

1. Propagating waves with low spatial frequencies (< s/λ), and

2. evanescent waves with high spatial frequencies (> s/λ)

where s is the tip-to-specimen spacing in near-field microscopy.

Whereas classical optics are concerned with the far-field regime where only the

propagating fields survive, the evanescent waves are confined to sub-wavelength

distances from the object corresponding to the near-field regime. Information about

the high spatial frequency components of the diffracted waves is lost in the far-field


Scanning Near-Field Optical Microscopy (SNOM)


regime and therefore sub-wavelength features of the object to be imaged cannot be

retrieved. On the other hand, by operating a microscope in the near-field regime the

Abbé resolution limit can be surpassed [2.49]. The invention of the STM triggered

increased effort in developing scanning near-field optical microscopy (SNOM).

As a characteristic of all scanning probe near-field microscopy methods the tipto-specimen spacing s is small and typically far below the optical wavelength λ of

the particular interaction to be studied. This means that the scanning probe near-field

microscopes are operated in the near-field (NF) regime (s ≤ λ) where the spatial

resolution is determined by the tip-to-sample spacing and the effective radius of

curvature of the probe tip, rather than the wavelength λ. In the far-field (FF) regime

(s > λ), however, the optical resolution is diffraction limited.

2.3.1 Scanning Near-Field Optical Microscopy (SNOM)

In SNOM experiments a tiny aperture illuminated by a laser beam from the rear

side is scanned across the sample surface (see Fig. 2.12). To achieve a high lateral

resolution, which was of the order of 25 nm (λ/20) in the first experiments [2.50],

the aperture had to be maintained at a distance of less than 10 nm from the object.

This requirement arises because with increasing distance s from the aperture the

evanescent waves are damped out rapidly with the field intensity I ∝ s−4 . This

fourth power dependence on distance in the NF regime is in contrast to the behavior

in the FF regime where the field intensity decreases quadratically with distance.

Fig. 2.12 (a) Set-up of a scanning near-field optical microscope (SNOM) combined with a conventional optical microscope. (b) SNOM signal of a Cr pattern on a glass substrate, imaged in

transmission. (Reprinted with permission from [2.52]. © 1996 Elsevier)



Microscopy – Nanoscopy

Fig. 2.13 Schematics of the photon scanning tunneling microscope (PSTM). The tip probes the

sample-modulated evanescent field produced by an internally reflected light beam. (Reprinted with

permission from [2.53]. © 1989 American Physical Society)

The optical probes (see Fig. 2.12) are formed by a sharpened glass or fiber tip

coated with a thin metallic layer. The capability of fabrication of a nanometersized aperture is essential but is gained only at the expense of signal intensity.

The tip-surface distance can be controlled by exploiting the force interaction [2.51].

SNOM can be performed in reflection or in transmission. In addition, SNOM can be

combined with all techniques known in classical optical microscopy including the

investigation of luminescence, polarization, or phase contrast (see [2.20]).

In close analogy to STM, a sharpened optical fiber tip can be used to probe the

evanescent field above a dielectric in which total internal reflection (TIR) is made to

occur. The specimen may form the TIR surface and spatially modulates the evanescent field (see Fig. 2.13). The tunneling of photons to the tip end of the optical fiber

is detected by a photomultiplier tube connected to the other end of the fiber, while

the object surface is scanned relatively to the tip by means of a piezostage. The lateral resolution achieved with this photon STM (PSTM) or evanescent field optical

microscope (EFOM) is typically 50–100 nm.

2.3.2 Near-Field Scanning Interferometric Apertureless

Microscopy (SIAM)

Unlike in regular near-field optical microscopy (SNOM), where the contrast results

from a weak source (or aperture) dipole interacting with the polarizability of the

sample, SIAM relies on sensing the dipole–dipole coupling of two externally driven

dipoles (the tip and the sample dipoles) as their spacing is modulated. One can

measure the scattered electric-field variation caused by a vibrating and scanning

probe tip in close proximity to a sample surface (Fig. 2.14a) by encoding it as a

modulation in the phase of one arm of an interferometer. In the transmission mode

a laser beam is focused on the back surface of a transparent substrate holding the


Scanning Near-Field Optical Microscopy (SNOM)


Fig. 2.14 (a) Principle of the near-field scanning interferometric apertureless microscope (SIAM)

imaging method. (b) Optical SIAM image showing oil droplets as bright scattering regions on

mica; smallest resolvable feature ≈ 1 nm. (Reprinted with permission from [2.54]. © 1995 AAAS)

sample (Fig. 2.14a). A tip vibrating with a frequency fz = 250 kHz and an amplitude

of 6–10 nm is brought close to the focused spot over the sample surface with an

attractive mode atomic force microscope. The return beam Er + Es (reflection from

the substrate plus tip-sample scattering) is detected with an interferometer by combining it with a reference beam Er . The output signal of the interferometer measures

either the amplitude of (Er + Es ) or its phase difference with Er , which represents

the contrast mechanism [2.54]. Optical images and AFM can be recorded simultaneously. The optical image (Fig. 2.14b) shows oil droplets as enhanced scattering

centers on mica. The smallest feature resolved optically is about 1 nm across and a

phase change of 10−4 radian can be measured.

2.3.3 Mapping Vector Fields in Nanoscale Near-Field Imaging

The vectorial nature of electric fields can be mapped optically down to the nanoscale

[2.55, 2.56] in addition to the detection of the intensity of light by conventional optical probes. Making use of an apertureless scanning near-field optical microscope, a

polarizer is placed immediately before the CCD camera that is used to collect the

light scattered by the nanoparticle tip in the near-field position (Fig. 2.15). For each

tip position during the scan process the polarizer is rotated between 0◦ and 360◦

and a complete map of the vector state of the electric field at the tip position is



Microscopy – Nanoscopy

Fig. 2.15 The vector-field imaging device [2.55] combines an apertureless scanning near-field

optical microscope with a polarizer, which allows for mapping both the size and the direction of

electric-field vectors. (Reprinted with permission from [2.56]. © 2007 Nature Publishing Group)

derived. The knowledge of the electric vector field on the nanoscale could help in the

design of miniaturized optical components and may be of importance in biosensing

(see [2.56]).

2.3.4 Terahertz Near-Field Nanoscopy of Mobile Carriers

in Semiconductor Nanodevices

Ultraresolving terahertz (THz) near-field microscopy based on THz scattering at

atomic force microscope tips can specifically image charge carrier concentrations

in semiconductor devices [2.6]. This is due to light-matter interactions at the

low-energy THz frequencies exciting molecular vibrations and phonons, as well

as plasmons and electrons of non-metallic conductors.


Far-Field Optical Microscopy Beyond the Diffraction Limit


Fig. 2.16 Transmission electron micrograph (TEM) of a transistor. The highly doped regions

below the source and drain NiSi contacts are marked by dashed yellow lines. (b) Infrared

(λ ≈ 11 μm) image of a transistor. (c) High-resolution THz (λ = 118 μm) image of the same

transistor as in (b) showing all essential parts of the transistor: source, drain, and gate. The THz

profile extracted along the dashed white line allows the estimation of a spatial resolution of about

40 nm. (Reprinted with permission from [2.6]. © 2008 American Chemical Society)

Nanoscale resolution is achieved by THz field confinement to a tip apex to

within 30 nm, yielding a 40 nm (λ/3000) spatial resolution at 2.54 THz (wavelength λ = 118 μm) for imaging of semiconductor transistors (see Fig. 2.16).

Near-field THz microscopy provides a detection technique for mobile carriers in a

concentration range centrally important for semiconductor science and technology

(n = 1016 − 1019 carriers/cm3 ) where visible and infrared methods lack sensitivity. This is demonstrated in Fig. 2.16b, c where a transistor is illuminated with an

infrared (IR) CO2 laser at 28 THz (λ = 11μm) or a continuous-wave 2.54 THz

CH3 OH gas laser, respectively. In contrast to the IR image (Fig. 2.16b), the THz

image (Fig. 2.16c) also reveals the highly doped poly-Si gate and the highly doped

Si regions (both n ≈ 1019 cm−3 ) just below the metallic NiSi source and drain contacts. In addition, THz near-field microscopy allows for probing the mobile carriers

in the 65 nm wide region between source and drain which may open the possibility of future measurements of the carrier mobility in this most important part of

nanoscale conductor devices.

2.4 Far-Field Optical Microscopy Beyond the Diffraction Limit

Light microscopy plays an enormous role in life sciences. It allows the detection of

specific cellular constituents such as proteins, nucleic acids, and lipids under physiological conditions through fluorescence tagging but is conventionally restricted to

the Abbé limit of spatial resolution of about 200 nm. Near-field optical techniques

are qualified to enable high-resolution optical microscopy but only on surfaces and

not in the interior of, e.g., living cells, where electron microscopy with atomic

resolution power is inadequate because of intolerable radiation damage. Recently



Microscopy – Nanoscopy

developed far-field optical microscopy techniques such as stimulated emission

depletion (STED) microscopy [2.7], stochastic optical reconstruction microscopy

(STORM) [2.8], or photoactivated localization microscopy (PALM) [2.57] can substantially surpass the Abbé diffraction limit down to spatial resolutions of ∼ 10 nm

and potentially to molecular sizes [2.8, 2.9]. In addition, they can be used for 3D

imaging [2.58, 2.59] and for video-rate imaging of fast nanoscale processes in living cells [2.9]. In these techniques, photoswitching of fluorophore molecules [2.60]

plays an important role.

2.4.1 Stimulated Emission Depletion (STED) Optical Microscopy

This far-field fluorescence optical microscopy [2.61, 2.62] yields a focal plane

resolution of 15–20 nm in biological samples which is a 20–40-fold increase in

resolution beyond the diffraction limit. STED microscopy typically uses a scanning

excitation illumination spot that is overlapped with a doughnut-shaped counterpart

(see Fig. 2.17) for deexcitation of fluorophores by light, a phenomenon referred

to as stimulated emission depletion. Oversaturation in the deexcitation suppresses

emission in the doughnut and squeezes the fluorescence spot to sub-diffraction

dimensions so that superresolved optical images emerge by scanning this spot

through the object.

Making use of the 20 nm resolution provided by STED, optical studies of the

nucleus of a fixed but otherwise intact mammalian cell can be performed. This

appears to be suitable to bring further critical insight into how nuclear organization

ensures regulated gene expression.

Figure 2.18 displays the protein-heavy subunit of neurofilaments in the

human neuroblastoma cell line SH-SY5Y (retinoic acid–BDNF–differentiated),

which establishes cross-links to organize and stabilize neurofilaments in axons

(long process of a nerve fiber that conducts impulses to the nerve cell).

Neurofilaments play an essential role in many neurodegenerative diseases, such as

Fig. 2.17 Stimulated emission depletion (STED) microscopy. ExsPSF: measured focal spot for

excitation (wave length 470 nm; blue; FWHM 190 nm); STED PSF:STED excitation, doughnut


shape; (603 nm; orange); Eff PSF: the final spot yields a 22 nm effective size; Iˆ STED : crest intensity

of the STED beam. (Reprinted with permission from [2.61]. © 2006 National Academy of Sciences


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