[School of Physics - Optics Group]

Near Field Scanning Optical Microscopy

Microscopy, in its many forms, is one of the most important techniques for the analysis of small objects. During the last few decades this technique has expanded in several directions. Currently these include the disciplines known as Scanning Tunnelling Microscopy, Electron Microscopy, Atomic Force Microscopy, Soft X-Ray Microscopy, Laser Scanning Confocal Microscopy and Optical Microscopy. Each of these techniques has certain advantages over it competitors, but only one, Optical Microscopy, has been in the game since the very beginning.

Optical Microscopy cannot compete with the previously mentioned techniques in resolution, but it does have several advantages that explain its ever present popularity. The technique is relatively inexpensive to set up and equally easy to use. Its reliability is unmatched and requires little or no sample preparation. In addition, performance is possible at room temperature and atmospheric pressure allowing a much wider range of possible applications. Of coarse all these advantages come with a price, and this is the lack of resolution. Due to the diffraction limit placed on waves of light in free space, the resolution obtainable is no less than about 300nm. It is precisely this reason that forced microscopy to be performed at shorter wavelengths. This shortfall of optical microscopy lead to the invention of the Electron Microscope

Scanning Near-field Optical Microscopy is a technique that allows us to achieve resolution an order of magnitude below normal Optical Microscopy by "cheating" the diffraction limit. Since it is essentially Optical Microscopy it retains many of the advantages while simultaneously eliminating the major disadvantage. The resolution of this system is governed primarily by the size of the aperture used to collect the light. Hence by fabricating apertures smaller than the wavelength of light used, we obtain sub-wavelength resolution.

Near field scanning optical microscopy is a relatively new form of microscopy capable of producing images with a resolution much less than the wavelength of light. We aim to make a fairly small modification to the standard near field microscope to enable it to produce not just images of the intensity of a light field, but also its phase. The change that is made is to add a reference arm to the probe, making an optical fibre interferometer (see below).

Why do we want to do this?

There are some interesting phase structures in light that can be studied if we have high enough resolution. Phase singularities can occur naturally in laser beams. One that is of particular interest is the so-called doughnut mode.

This has uses in atom optics with its predicted ability to focus an atomic beam. There is a phase singularity in the focal region of a lens that has only been observed with far field interference.

Optical Fibre Interferometry

Interferometry is the mixing of two coherent light waves, producing interference, to determine a phase difference between the two waves. The standard interferometer which most people are familiar with would have a light beam split by a semi-silvered beam splitter, and mixed again at a beam splitter to produce some sort of a pattern at the other end.

It is also possible to produce interference by mixing light from two optical fibres. This can be done with optical couplers which let the evanescent fields surrounding one optical fibre enter a second fibre which is held in close proximity. This means that light from one fibre is mixed between two. When both fibres have light coming into them, what you get out the end is the interference between the two. The optical fibre interferometer uses one coupler to split a light wave travelling down one fibre into two, and a second coupler to mix these two back together.

Why do these things interest us?

Optical fibre interferometers are used commonly in photonics, or as incredibly sensitive sensors. If the temperature on one piece of fibre changes by a small amount, the optical path length along that fibre changes. Therefore the difference in lengths between two fibres in an interferometer will change. Due to the extremely small wavelength of light, the interference between the two will be sensitive to length changes of the order of half a micrometer or less. This makes for extremely powerful temperature and motion sensors, but is a big nuisance if it gets in the way of what we're trying to measure.

Here is a measurement of the phase structure near the focus of a high numerical aperture lens. From left to right: cosine of phase, intensity, greyscale of phase around phase singularity vortex at bottom left, and shaded surface of same. Left two images are 12x12 microns.

Relevant publications

Walford JN, Nugent KA, Roberts A and Scholten RE, Quantitative imaging of phase dislocations, Optics Letters 27(5) 345-347 (2002).
Walford J, Nugent KA, Roberts A and Scholten RE, Three dimensional phase imaging with a scanning optical fibre interferometer, Applied Optics 38 3508-3515 (1999).
Huntington ST, Horsfall A, Rhodes SK, Walford JN, Barty A, Nugent KA, Roberts A and Scholten RE, Near-field scanning optical microscopy of electromagnetic field structures, Near field optics: principles and applications, X Zhu and M Ohtsu, Eds., p43-57, World Scientific, Singapore (2000).

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  Created: 10 March, 1995 Last modified: 

Authorised by: Assoc. Prof. R. Scholten, School of Physics

Maintained by: Assoc. Prof. R. Scholten, School of Physics.
Email: r.scholten at physics.unimelb.edu.au