Near Field Scattering Optical Microscopy (NSOM) or (SNOM) technique and analysis of its application based review paper.

1. Saransh Khandelwal (BM23RESCH04003)
Technical Superintendent,
Department of Biomedical Engineering
IIT Hyderabad, 502284
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NSOM

2. Content
Outline
Introduction
Fundamentals of NSOM
Concept of Techniques involved
Application-based Research Paper
Conclusion and Future Aspects
PAGE 2

3. NSOM
Near-field scanning optical microscopy
(NSOM) a.k.a. scanning near-field
optical microscopy (SNOM)
It offers higher resolution around 50
nm (or even < 30 nm), depending on
tip aperture size.
Scanning probe technique developed
to surpass the spatial resolution
constraints that traditionally limit
conventional optical microscopy.
It provides simultaneous
measurements of the topography and
optical properties.
Introduction
PAGE 3

4. What is Resolution?
 Digital Imaging:
Image resolution is typically described in
PPI, which refers to how many pixels are
displayed per inch of an image. Higher
resolutions mean that there more pixels
per inch (PPI), resulting in more pixel
information and creating a high-quality,
crisp image.
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NSOM
Paper Review Conclusions Questions
Fig. 1: Similar images with lower and higher resolution.

5. What is Resolution?
 Microscopy:
The ability of a microscope to distinguish
details of a specimen or sample. In other
words, the minimum distance between
two distinct points of a specimen where
they can still be seen by the observer or
microscope camera as separate entities.
𝑅𝑒𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛𝑑 =
𝜆
2 ∗ 𝑁𝐴
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Paper Review Conclusions Questions
Fig. 2: Microscopic image of the sample taken with same magnification
and varying Numerical Aperture. [1]

6. Numerical Aperture (NA)
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Paper Review Conclusions Questions
• NA is just a way of expressing the ability of an
objective lens to gather light information.
• ‘n’ represents refractive index
• ‘𝜃’ represent the maximum angle of acceptance
Fig. 3: Microscopic image of the sample taken with same magnification and
varying Numerical Aperture [2]

7. Abbe’s Limit
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In general, the light propagates through space in an
unconfined manner and is the "normal" light utilized in
conventional microscopy
𝑅𝑒𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛𝑑 =
.61𝝀
𝑁𝐴
In order to get better resolution, we need to trade-off
between Numerical Aperture (NA) or Wavelength (𝝀)….
How…?
Confining the wavelength i.e. by Aperture Manipulation

8. Near-Field Wavelength
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• Traditionally, the lens collecting the scattering light is
placed several wavelengths of the illumination light far
away from the sample surface. This causes the
commonly known diffraction limit, that is far-field
optical techniques are limited to resolve features
approximately on the order of half of the wavelength of
the illuminating light due to the diffraction of light.
• The near-field (or evanescent) light consists of a non-
propagating field that exists near the surface of an
object at distances less than a single wavelength of
light. Light in the near-field has its greatest amplitude
in the region within the first few tens of nanometers of
the specimen surface. Because the near-field light
decays exponentially [3,5] within a distance less than the
wavelength of the light, it usually goes undetected.
Fig. 4: Diagram illustrating near field optics [3]

9. Near-Field Wavelength
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The classical NSOM uses a tapered optical fibre probe and an aperture that is much smaller than the
wavelength of the light. After light passes through the cantilever tip with nano-aperture, a optical near-field (or
evanescent field) on the far side of this tip can be created that is not diffraction limited. Then the resolution in
near-field microscopy is directly affected by the size of the aperture and independent of the wavelength of the
light.
Fig. 5: Near-field technique [3]

10. NSOM Instrumentation
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The classical NSOM uses a tapered optical fibre
probe and an aperture that is much smaller
than the wavelength of the light. After light
passes through the cantilever tip with nano-
aperture, a optical near-field (or evanescent
field) on the far side of this tip can be created
that is not diffraction limited. Then the
resolution in near-field microscopy is directly
affected by the size of the aperture and
independent of the wavelength of the light.
Fig. 6: General principles of NSOM instrumentation [3]
Fig. 7: SEM image of a coated NSOM tip. Grains in the Aluminium and the aperture at the end are visible [4]

11. NSOM Instrumentation
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An illuminating probe aperture having a diameter
less than the wavelength of light is maintained in
the near field of the specimen surface. Because
close proximity or contact between the specimen
and probe (separation less than the wavelength)
is a general requirement for non-diffraction-
limited resolution.
An x-y-z scanner (usually piezoelectric) is utilized
to control the movement of the probe over the
specimen. The NSOM configuration positions the
objective in the far field, in the conventional
manner, for collection of the image-forming
optical signal.
Fig. 6: General principles of NSOM instrumentation [3]

12. NSOM Instrumentation
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• Depending upon the design, the x-y-z scanner can either be
attached to the specimen or to the local probe. If the scanner and
specimen are coupled, then the specimen moves under the fixed
probe tip in a raster pattern to generate an image from the signal
produced by the tip-specimen interaction.
• The size of the area imaged is dependent only on the maximum
displacement that the scanner can produce.
• A computer simultaneously evaluates the probe position,
incorporating data obtained from the feedback system, and controls
the scanning of the tip and the separation of the tip and specimen
surface.
• The information is collected and recorded by the computer point-by-
point during the raster movement.
• Two-dimensional data sets gathered are subsequently compiled and
displayed as a three-dimensional reconstruction on a computer
monitor.
Fig. 8: NSOM imaging schematic [5]

13. X-Y-Z Scanner Mechanism
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• The scanner must have low noise (small position fluctuations) and precision positioning capability.
• The required precision of the probe positioning usually necessitates that the entire instrument rest
on a vibration isolation table, or be suspended by some other means, to eliminate the transfer of
mechanical vibrations from the building to the instrument.
• it is necessary to maintain the probe in constant feedback above the specimen surface being
imaged.
• In case of unnoticed, damage to the probe tip or the specimen, is likely if the two come into
contact.
• The exponential variation of signal level with changing tip-to-specimen separation can produce
artifacts in the image that do not accurately represent optical information related to the specimen.
• Two most commonly employed mechanisms of tip positioning have been optical methods that
monitor the tip vibration amplitude (usually interferometric), and a non-optical tuning fork
technique.

14. Feedback Methods
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Oscillatory Feedback Methods
• NSOM tip is almost always oscillated at the resonance frequency
of the probe. This allows lock-in detection techniques to be
utilized, which eliminates positional detection problems
associated with low-frequency noise and drift.
Shear-Force Feedback
• The shear-force feedback method laterally dithers the probe tip
at a mechanical resonance frequency in proximity to the
specimen surface.
Piezo-Electric Feedback
• Piezoelectric quartz tuning forks were first introduced into
scanning probe microscopy for use in scanning near-field
acoustic microscopy. Fig. 10: Feedback mechanism in NSOM [6]

15. NSOM Image
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Fig. 11: NSOM image and topographic data of cleaned glass [6]

16. History of NSOM
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• Edward H. Synge, beginning in 1928, published a series of articles that first conceptualized the idea
of an ultra-high resolution optical microscope. Synge's proposal suggested a new type of optical
microscope that would bypass the diffraction limit, but required fabrication of a 10-nanometer
aperture (much smaller than the light wavelength) in an opaque screen.
• In addition, Synge accurately outlined a number of the technical difficulties that building a near-
field microscope would present. Included in these were the challenges of fabricating the minute
aperture, achieving a sufficiently intense light source, specimen positioning at the nanometer scale,
and maintaining the aperture in close proximity to the specimen.
• In 1972, E. A. Ash and G. Nicholls demonstrated the near-field resolution of a sub-wavelength
aperture scanning microscope operating in the microwave region of the electromagnetic spectrum.
• In 1984, a research group at IBM Corporation's Zurich laboratory reported optical measurements at
a sub-diffraction resolution level.
• The IBM researchers employed a metal-coated quartz crystal probe on which an aperture was
fabricated at the tip, and designated the technique scanning near-field optical microscopy (SNOM).
• The Cornell group used electron-beam lithography to create apertures, smaller than 50
nanometers, in silicon and metal. The IBM team was able to claim the highest optical resolution of
25 nm, or one-twentieth of the 488-nanometer radiation wavelength.

17. Advantages
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Paper Review Conclusions Questions
• Its ability to provide optical and spectroscopic data at high spatial resolution, in
combination with simultaneous topographic information.
• Typical resolutions for most NSOM instruments range around 50 nanometers,
which is only 5 or 6 times better than that achieved by scanning confocal
microscopy.
• For biological materials, specimen preparation is especially demanding, as
complete dehydration is generally required prior to carrying out sectioning or
coating.
• An additional limitation of other techniques such as AFM etc, is not able to take
advantage of the wide array of reporter dyes available to fluorescence
microscopy.

18. Applications
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Paper Review Conclusions Questions
• Near-field optical spectroscopy: NSOM can be used to study the optical properties
of materials, such as plasmons and excitons, with high spatial and spectral
resolution.
• Nanolithography: NSOM can be used for high-resolution patterning of surfaces and
for direct writing of nanostructures with sub-10 nm resolution.
• Imaging of biological samples: NSOM can be used to image biological samples,
such as cells and tissues, with sub-50 nm resolution, providing detailed information
about the structure and dynamics of these samples.
• Semiconductor device characterization: NSOM can be used for imaging and
analyzing the properties of semiconductor devices, such as transistors and LEDs, at
the nanoscale.
• Quantum information processing: NSOM can be used for quantum information
processing, such as single-photon generation and detection, which is critical for
developing quantum technologies.

19. Limitations
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Some of the limitations of near-field optical microscopy include:
• Practically zero working distance and an extremely small depth of field.
• Extremely long scan times for high resolution images or large specimen areas.
• Very low transmissivity of apertures smaller than the incident light wavelength.
• Only features at the surface of specimens can be studied.
• Fiber optic probes are cumbersome for imaging soft materials due to their high spring
constants, especially in shear-force mode.

20. PAGE 20
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Paper Review
Conclusions Questions
Paper Review
 Royal Society of Chemistry
 Indexed: PubMed, MEDLINE
and Science Citation Index
 Scattering-type Scanning Near-
field Optical Microscope
machine to an array of newly
available Quantum Cascade
laser” (QCL) diode lasers. It can
deliver SI datasets down to a
∼𝜆/1000 spatial resolution of
∼10 nm

21. PAGE 21
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Paper Review
Conclusions Questions
Results
 The resolution here is ∼140
times better than the
diffraction limit at this
wavelength and has
allowed to image chemical
contrast within the cell for
the first time.
Fig. 12: Left Image, AFM scan of the physical structure of a single red blood cell in a squamous epithelial
oesophageal biopsy. Right image, s-SNOM image [7]

22. PAGE 22
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Paper Review
Conclusions Questions
Results (Cont..)
 The s-SNOM phase
image, in this example,
approximately
corresponds to the
absorption in the
graphene layer, and
reveals regions of
differing thickness, and
the edge effects
responsible for the
device operation
Fig. 12: Graphene layer AFM topology, s-SNOM image and amplitude [7]
(a)
(b)

23. PAGE 23
NSOM Paper Review
Conclusions
Questions
Conclusion and Discussions
 For the s-SNOM it is the beginning, and the challenges are still mostly
technical. We believe it has a very bright future as a new scientific
research tool for a wide range of biomedical problems, but, already, we
are on the lookout for that elusive “unmet need”
 Although in its infancy, this instrument can already deliver ultra-detailed
chemical images whose spatial resolutions beat the normal diffraction
limit by a factor of ∼1000. This is easily enough to generate chemical
maps of the insides of single cells for the first time, and a range of new
possible scientific applications are explored

24. PAGE 24
NSOM Paper Review Conclusions
Questions
Questions…??

25. References
PAGE 25
1. https://microscopeclarity.com/what-is-microscope-resolution/
2. Live‐Cell Fluorescence Imaging, Jennifer C. Waters, Methods in Cell Biology, Elsevier, Volume 81, 2007, Pages 115-140
3. https://phys.libretexts.org/Courses/University_of_California_Davis/UCD%3A_Biophysics_241_Membrane_Biology/05%3A
_Experimental_Characterization_Spectroscopy_and_Microscopy/5.06%3A_Nearfield_Scanning_Optical_Microscopy_%28
NSOM%29
4. https://dunngroup.ku.edu/near-field-scanning-optical-microscopy
5. https://micro.magnet.fsu.edu/primer/techniques/nearfield/nearfieldintro.html
6. https://my.eng.utah.edu/~lzang/images/Lecture_16_NSOM.pdf
7. New IR imaging modalities for cancer detection and for intra-cell chemical mapping with a sub-diffraction mid-IR s-SNOM, H.
Amrania et. al, Royal Society of Chemistry, Faraday Discuss., 2016, 187, 539-553