Sub-wavelength resolution of optical fields probed by single atoms

The paper summarizes on the results of several experiments on single trapped ions which have been used to map optical fields. In these experiments, the ion was placed either in front of a (distant) mirror or in an optical cavity. Sub-wavelength resolution is achieved because the position of the ion relative to the mirror(s) is well-controlled and the ion´s spatial wavefunction is confined to a region much smaller than the optical wavelength (400 to 800 nm). This strong localization is due to the trapping potential of a Paul-type ion trap, and laser cooling. In the three experiments the ions were Doppler-cooled, and a resolution between about 10 and 50 nm was achieved. With this resolution, the spatial variation of an optical standing-wave field was detected by shifting the single ion through the field structure.