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 ^[1]: 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 diffration 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 12nm has been shown back in 1991 ^[2]. SNOM has the anbility to make high resolution in all directions (x,y, and z) and can be adapted to fit onto the same laser systems/spectrometers that other microscopes also use ^[3].

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 viscinity of the aperture, and hence the sample-probe distance becomes a concern for the fragile samples and probes ^[4].

One widespread distance control method is the shear force technique, invented in the 1992^[5], 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 descibed 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 ^[6], while eg. ^[7] find optical setups to be advantageous.

References

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1. ↑ Optical Stethoscopy - Image Recording With Resolution Lambda/20, Pohl Dw, Denk W, Lanz M, Applied Physics Letters , Vol. 44 (7): 651-653 1984.
2. ↑ Breaking The Diffraction Barrier - Optical Microscopy On A Nanometric Scale, Betzig E, Trautman Jk, Harris Td, Weiner Js, Kostelak Rl, Science vol 251 (5000) p. 1468-1470 Mar 22 1991.
3. ↑ Manfaits webpage on Le Groupement De Recherche 1860 at the Centre National de la recherche scientifique,
4. ↑ A multipurpose scanning near-field optical microscope: Reflectivity and photocurrent on semiconductor and biological samples, Cricenti A, Generosi R, Barchesi C, Luce M, Rinaldi M, Review of Scientific Instruments, vol. 69 (9): 3240-3244 SEP 1998
5. ↑ Near-field scanning optical microscopy, Dunn RC, Chemical Reviews, vol. 99 (10): 2891 OCT 1999
6. ↑ Distance control in near-field optical microscopy with piezoelectrical shear-force detection suitable for imaging in liquids, Brunner R, Bietsch A, Hollricher O, Marti O, Review Of Scientific Instruments, vol. 68 (4) p. 1769-1772 APR 1997
7. ↑ A multipurpose scanning near-field optical microscope: Reflectivity and photocurrent on semiconductor and biological samples, Cricenti A, Generosi R, Barchesi C, Luce M, Rinaldi M, Review of Scientific Instruments, vol. 69 (9): 3240-3244 SEP 1998