Scanning Near Field Optical Microscopy (SNOM) was invented several years ago by one of the participants of the present consortium [D.W. Pohl, W. Denk, M. Lanz, Appl. Phys. Lett. vol 44 (1984) 651] and now it is a booming technique with many technological applications envisaged [E.Betzig, J.K. Trautman, Science 257 (1992) 189]. The most important feature of SNOM is that it allows optical imaging with subwavelength resolution. To circumvent the diffraction limit, in this technique the sample is imaged by scanning a subwavelength sized aperture across the surface at a distance of some 10 nm or less, i.e. in the optical near-field of the sample. The aperture can either operate as an emitter (i.e. as a nanoscopic light source) or as a receiver.
A number of spectacular advances have been achieved in the area of SNOM in the last few years. To list a few of them:
-demonstration of resolution of better than 20 nm in the transmission mode [Betzig et al, Science 251, 1468 (1991)] and resolution of better than 40 nm in the reflection mode on opaque samples [C. Durkan, I.V. Shvets, Ultramicroscopy vol 61 (1995) 227].
-optical images of biological samples such as chromosomes with resolution of the order of 50 nm [M.H.P.Moers, A.G.T.Ruiter, A.Jalocha, N.F. van Hulst, Ultramicroscopy vol. 61 (1995) 279].
-measurement of fluorescence spectra on single organic molecules [Betzig, Science ...].
- magnetooptic imaging in transmission mode through the Faraday effect [E. Betzig, J.K. Trautman, R. Wolfe, E.M. Georgy, P.L. Finn M.H. Ryder, C.H. Chang, Appl. Phys. Lett. 61 (1992) 142] and also in reflection mode through the polar Kerr effect [C. Durkan, I.V. Shvets, J.C. Lodder, Appl. Phys. Lett to be published in March 1997].
- studies of optoelectonic effects in semiconductors with resolution of better than 100 nm.
While the large potential of scanning near-field optical microscopy has by now been convincingly demonstrated, there are a number of shortcomings need to be overcome to allow routine use of the techniques for numerous applications.
The most important issues to be addressed in this respect are:
Clearly, most groups are strongly in favour of the particular experimental schemes they use. However, it is also clear that the different schemes are far from equal performance. There is no systematic effort so far to compare the imaging capabilities of these schemes on standard test samples. Such a comparison backed up by a theoretical analysis would be most helpful in this rapidly growing field.
This network is set up to address all these issues in a systematic way. All the members are among the best European researchers working on all these topics, such as:
Cross-fertilisation and exchange of researchers between the groups is required most of all at this stage to bring the field of SNOM into a new level of scientific and technological development.
(c) University of Ulm and the participants of the TMR-Network NanoSNOM.