Dunn, R. C.
Near-field scanning optical microscopy. Chemical Reviews 99: 2891-2927 (1999).

The ability to view and study samples under high magnification is enormously important in many disciplines such as the biological sciences and materials research. Traditionally, optical techniques have been the most widely employed for these purposes given their long historical development, noninvasiveness, specificity, ease of use, and relatively low cost. However, the spatial resolution attainable with conventional optical techniques is limited to approximately half the wavelength of the light source used. For visible radiation, this results in a theoretical resolution limit of 200-300 nm which is restrictive for many applications. This limitation motivated the development of higher resolution techniques such as scanning electron microscopy (SEM) and transmission electron microscopy (TEM) along with the recent emergence of other scanning probe techniques such as atomic force microscopy (AFM) and scanning tunneling microscopy (STM). The introduction of these and related forms of microscopy have brought about fantastic gains in resolution to the point where it is now possible to image and study single atoms.