Nanotechnology/SNOM

=Scanning Near-field optical microscopy (SNOM)= The Abbe diffraction limit of optical microscopy can be circumvented if an evanescent wave is used instead of a travelling wave. The SNOM can be compared to a stethoscope : A doctor can locate your heart with a precision of a few cm, despite the fact that the sound of a beating heart he is listening to has a wavelength of the order 100 m. Apparently he has a resolving power of λ/1000 which is far better than what's dictated by the Abbe diffraction limit. A similar setup can be made with light waves: If the light is forced through a sub-wavelength sized aperture, the emitted field will be evanescent and decay exponentially on a length scale usually shorter than the wavelength. With this technique a resolution of 12 nm has been shown back in 1991. SNOM has the ability to make high resolution in all directions (x,y, and z) and can be adapted to fit onto the same laser systems and spectrometers that other microscopes also use.

Images by SNOM are made by scanning the probe over the sample like an LSCM, AFM or STM. The SNOM is a very versatile tool used by both physicists and biologists for many purposes, but the probe only interacts with the sample in close vicinity of the aperture, and hence the sample-probe distance becomes a concern for the fragile samples and probes.

One widespread distance control method is the shear force technique, invented in the 1992, where the SNOM probe is set in vibrations with an amplitude up to a few nm and the motion is detected and used in a feedback loop that senses the minute shear forces that occur when the probe tip is a few nm above the sample surface. Numerous shear force setups have been described in the literature. Both optical and non-optical methods are used to detect the vibrations. Groups using non-optical methods claim the optical methods are sources of stray light that will seriously affect the measurements, while e.g. find optical setups to be advantageous.

=References=

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