Near-field optical microscopy

In 1981, Binnig and Rohrer invented the Scanning Tunneling electronic Microscope (STM), a discovery that sparked a revival in microscopy research and gave rise to a number of microscopes based on a completely new principle. In fact, their invention can be seen as the birth of a new kind of microscopy: local probe or near-field microscopy.

In a traditional microscope, the most important part is a lens, the objective. The object is illuminated by reflection or transmission, and the objective captures the field diffracted by the object to form an image. Since light is a wave, diffraction by the objective limits the microscope's resolving power: Rayleigh's criterion forbids separating two points on the object closer than half the wavelength. In visible light, the separating power is theoretically of the order of 0.25 µm and, in practice, is rarely less than one micrometer.

A local probe microscope has no lens; the most important part of these microscopes is a very fine probe, which is moved close to the object, in the near field, to illuminate it or pick up a signal. These microscopes are scanning microscopes; the image is obtained by moving the probe point by point and plotting the detected signal as a function of its position. They require the use of a computer not only to visualize the images, but also to control the position of the probe, which must move at nanometric distances from the object's surface.

These microscopes have a resolving power that is not limited by diffraction, and provide images with a resolution undreamt of until recently. Images with a resolution of 20 nm are being produced by many laboratories, and one team has achieved a resolution of 1 nm.

In this article, we briefly review the history of near-field optical microscopes. We then describe and compare the different configurations most commonly used today, explaining the operating principle and showing how the use of evanescent waves makes it possible to go beyond the Rayleigh criterion. We then present the various technical problems and their solutions, as well as current results and a few examples of applications. In conclusion, we look at the prospects and likely future development of this very recent technique, which has a great future ahead of it.

For further information, please refer to [1] and [2] .