Seeing Things Smaller Than Light
Dateline: 04/16/00

Engraving from Robert Hookeâs Micrographia (1665).
The primary sense that scientists use for observation is vision. Of course, vision depends on light (electromagnetic waves) bouncing off of the object and into the scientist's eye. Microscopes have been used for centuries to gather light so that scientists can see smaller and smaller objects. For some time, it seemed that improvement on this method (termed "optical microscopy") would allow us to see smaller and smaller objects without limit. For instance, this concept was expressed in 1733 by the poet Jonathan Swift when he wrote:
So, naturalists observe, a flea
Has smaller fleas that on him prey;
And these have smaller still to bite 'em;
And so proceed ad infinitum.

That concept may still be true; however, due to the delocalized nature of light, traditional optical microscopy encounters a far-field diffraction limit, which limits the resolution to half of the wavelength of light being used. Since the smallest frequencies of light that humans can see is around 400 nm, 200nm is the theoretical limit of simply focusing light into a human eye or detector (the practical limit is even larger). Thus, in order to observe and manipulate nanoscale objects, it is necessary to cross the diffraction barrier.

Scanning Probe Microscopy (SPM) is a recently developed family of techniques that can be used to produce images of nanoscale surfaces with resolution reaching down to the sub-angstrom level. Each of these methods has its own unique strengths and weaknesses.

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