Home Up Uni Ulm Coordinator TMR Home Vacancies Nanofab Atomic/Molecular Manipulation

Justification
Justification Objectives

Home
Up

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:

 

development of a more robust and durable novel optical probe offering higher energy throughput. The problem with conventional metal coated pulled optical fibre probes is that they are often not reproducible in quality, size of the aperture, and also their life time is quite short (from less than an hour to a few days depending on the sample and probe height). The problem is also with a rather low efficiency of the probes: typically their throughput is only about 10 -6 .
understanding of the optical image formation mechanisms and development of the algorithms for the image interpretation. The problem with image interpretation is that there is still no adequate understanding how the optical image is formed and how this depends on the type of the probe and the properties of the sample such as its dielectric constant and conductivity. The complication is that image interpretation algorithms often require solving an inverse problem and it is substantially complicated by the strong dependence of the signal upon the probe height.
development of methods for uniform calibration and comparison of different SNOM imaging circuits.
There are almost a dozen of distinct schemes for the SNOM imaging such as : transmission, reflection imaging, so-called photon STM, imaging with the forbidden light, imaging with silicon or silicon nitride micromachined cantilevers, imaging with coated and uncoated fibre probes, imaging with the light collection into the probes and others. There also numerous methods employed for the controlling the distance between the probe and the surface.

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.

there was much effort put in spectacular image hunting and achieving the highest possible resolution. It would be helpful to put much more efforts in the fundamental aspects of SNOM such as: polarisation effects, spectroscopy, studies of the plasmons etc.
Further studies of these phenomena would considerably strengthen the progress in the 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:

SNOM instrumentation development.
Studies into novel optical probes.
Studies of the image interpretation and image formation mechanisms.
Studies of the fundamental effects such as plasmons, polarisation effects, optical spectroscopies with high spatial resolution.

 

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.

Home Up Uni Ulm Coordinator TMR Home Vacancies Nanofab Atomic/Molecular Manipulation
(c) University of Ulm and the participants of the TMR-Network NanoSNOM.                
If you have Questions or if you need help please contact the Webmaster.
Last Revision: 2000-07-13.