Near-Field Optical Microscopy

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Near-Field Optical Microscopy

• Mahdiar Noorbala
• Sharif University of Technology,
• Department of Physics

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Outline

• Why another optical microscope?
• What are near-field and evanescent waves?
• Diffraction from sub-wavelength apertures.
• NSOM modes.
• Near-field vs. far-field microscopy.
• NSOM instrumentation.

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Advantages of optical microscopy

• Imaging a broad range of samples in a variety of
  environments.
• Noninvasive nature.
• Being convenient and user-friendly.
• Cheapness.

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The smaller the wavelength of light, the more
information it carries.

• Small wavelengths are not present in the
  far-field they decay within a small distance
  from the source. In near-field, however, they do
  exist! These are called evanescent waves.

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Light emerging from an aperture NF and FF
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Light emerging from an aperture NF and FF

• In the far-field region the intensity falls like
  1/r2, which is the characteristic of spherical
  waves

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Thus

• The aperture size is to be taken as small as
  possible.
• The detectors are to be placed as close to the
  aperture as possible.

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Near- vs. Far-Field Microscopy
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(No Transcript)
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NSOM modes
a) collection, b) illumination, c)
collection/illumination, d) oblique collection
and e) oblique illumination
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Generic Design

• Tip production methods.
• Tip position control.
• NSOM combines surface topography and optical
  imaging.

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Tip Production Methods

• Heat-pulling
• Chemical etching (Figure)

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Tip Position Control

• Prong oscillation at the resonance frequency of
  30-100 kHz.
• Increase in the signal valley further approach.
• Decrease in the signal hill retraction.

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Conclusion

• NSOM takes advantage of the information provided
  by evanescent waves with high spatial
  frequencies.
• This requires small aperture size and
  illumination of the sample at the proximity.
• To implement NSOM, two important elements are
  sought tip fabrication and tip position control
  .
• The overall result is a resolution of around 50
  nm.