Traditional optical experiments, such as splitting rays of light into various colours with a prism, have had the attraction of being visible to the naked eye. Modern methods of confining light within microscopic structures, and tailoring its interaction with matter on atomic scales, are taking optics into the quantum realm. Making the results visible is not straightforward. But a beautiful example comes from C. Chicanne et al. (Physical Review Letters 88, 097402; 2002), who have designed a way of taking snapshots of intricate light interference patterns in tiny photonic structures. One of their snapshots is shown here.

The images are reminiscent of the 'quantum corrals' for electrons, first created by Donald Eigler and colleagues in 1993, and produced with the scanning tunnelling microscope (STM). This instrument probes surfaces with a sharp needle, which can also be used to move loose atoms around on a metallic surface and position them into a closed loop. The electrons confined within these corrals interfere with one another and produce beautiful patterns, which the STM images make manifest. These experiments have been highly instructive for illustrating the quantum-mechanical principle that electrons can behave as waves.

Chicanne et al. now present an optical analogue of the quantum corral. Light has wave properties, of course, but interpreting the interference effects is not straightforward within a radius around a light source that is comparable to the wavelength of the light itself. To study this zone, Chicanne et al. made use of a relative of the STM, the scanning near-field optical microscope. Here, the surface probe is an optical fibre tapered to a sharp end that also illuminates the sample.

Much as in the experiments by Eigler and colleagues, Chicanne et al. first created corrals by carefully positioning particles of gold in a loop, in this example in the shape of a stadium. The optical corral is then imaged by scanning the fibre over the surface while collecting light transmitted though the transparent sample. The result is images of light interference patterns in a microscopic structure. The principles involved will help direct future observations, and tailored distribution, of light at atomic dimensions.