Near-Field Optical Microscopy Mahdiar Noorbala Sharif University of Technology, Department of Physics

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.

Advantages of optical microscopy: Imaging a broad range of samples in a variety of environments. Noninvasive nature. Being convenient and user-friendly. Cheapness.

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.

Light emerging from an aperture: NF and FF

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:

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.

Near- vs. Far-Field Microscopy

NSOM modes: a) collection, b) illumination, c) collection/illumination, d) oblique collection and e) oblique illumination

Generic Design: Tip production methods. Tip position control. NSOM combines surface topography and optical imaging.

Tip Production Methods: Heat-pulling Chemical etching (Figure)

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.

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.