Optical microscopy has come a long way from Zacharias Jansen’s first microscope at the end of the 16th century to today’s highly developed microscopes. A number of different contrast mechanisms allow broad applicability as a routine tool in biology, medicine, materials research, and many more. It is hard to think of any other tool for routine and exploratory tasks that is as widespread as optical microscopy.
The Diffraction Limit of Spatial Resolution
In the late 19th century, the German Ernst Abbe and the Englishman Lord Raleigh introduced a concept that is known as the diffraction limit of spatial resolution. This fundamental law states that with light, as with any other wave phenomenon used for microscopy, it is not possible to spatially resolve details that are located closer together than approximately half the probing wavelength. For optical microscopy, typically operating at a wavelength of 500 nm (the visible spectrum ranges from 400 nm to 700 nm), the lateral resolution is thus limited to about 250 nm.
The Two Methods Used to Break the Diffraction Limit of Resolution
Recently, two different approaches have been demonstrated to break the diffraction limit of resolution. Both approaches differ from conventional optical microscopy in that they do not form images of the object by means of lenses, but rather collect the optical response of the sample point by point.
Laser Scanning Confocal Microscopy (LSM or LSCM)
In Laser Scanning Confocal Microscopy, both the sample illumination and the light collection are focused onto the same spot on the surface of the sample (or even inside the sample). The sample is scanned and at every location the light intensity is recorded by a computer. The increase of resolution is achieved by the overlap of two Gaussian focal profiles, effectively narrowing the profile width and thus improving the resolution.
Scanning Near-Field Optical Microscopy (SNOM)
The second approach, named Scanning Near-Field Optical Microscopy (SNOM), brings a small optical probe very close to the sample surface, in the region called "near-field". Here, at distances smaller than the wavelength away from the surface, those waves can be probed that do not propagate, but rather decay exponentially perpendicular to the surface. It can be shown fairly easily that it in this evanescent field, the k-vectors parallel to the surface can be fairly large, corresponding to small lateral (spatial) dimensions. As opposed to conventional as well as to laser scanning microscopy, which are far-field microscopies, SNOM requires the close proximity between probe and sample.
The Evolving History of Near-Field Microscopy
This concept of near-field microscopy had been proposed already in 1928 by Edward Hutchinson Synge; however, the technical difficulties could not be overcome. It is amazing to see how closely the proposed device resembles today’s instruments! A number of further proposals followed, probably without knowing of the earlier publications. Finally, after the demonstration of the Scanning Tunneling Microscope (STM), the principle was experimentally demonstrated in 1984. It is fair to say that SNOM is a rather young technique, and by far not yet established in the way conventional optical microscopy is.
Benefits of Using Scanning Near-Field Optical Microscopy (SNOM)
With SNOM we have a tool in hand that combines the advantages of optical microscopy (i.e. a whole variety of different contrast mechanisms, and the possibility of using spectroscopy for chemical identification), with the high resolution capability of scanned probe microscopies such as Scanning Tunneling Microsocopy (STM) and Scanning Force Microscopy (SFM). Today, SNOM has a proven spatial resolving power of around 50 nm. That is definitely less than what STM and SFM are capable of, but comes along with valuable information only accessible with optical contrast. One should look at it as a complementary tool with some room for improvement.
Figure 1. A scaled diagram showing the different operating ranges of optical microscopes.