Spectroscopy with Single Atoms in Atomic Beams

Abstract

In the preceding chapter, the optical spectra of a signal field and methods of their measurement were defined and discussed. The signal fields are not free fields, but are generated by sources such as atoms or molecules. We now turn more specifically to the subject of spectroscopy with signal fields produced by single multilevel atoms radiating due to the interaction with the ordinary vacuum and irradiated by quasi-resonant fields. The study of spectral properties of the radiation field scattered by atoms or molecules is fundamental to a number of research disciplines in optics and laser physics and provides a convenient ground for rigorous examination of the characteristics of atoms and basic aspects of their interaction with the radiation field. We shall not, however, follow the usual treatment of atomic spectra, which utilizes optical spectra as the basis for the information about splitting of the atomic energy levels, transition frequencies between the levels, and their populations. Here, we shall focus on a different aspect of atomic spectroscopy, which utilizes optical spectra to tackle the problem of quantum fluctuations, their control and effect on the radiative properties of atoms. We consider fluorescence spectra of atoms in atomic beams of low atomic-number densities and will show that it is possible to exercise extraordinary control of the linewidths of the spectral features and their intensities by imposing strong driving fields on the atoms. The choice of a low density atomic beam takes advantage of the fact that the spacing between the atoms in the low density atomic beam is very much greater than the size of the atoms, so that interaction between the atoms can be neglected. This means that the effect of the driving fields on any atom is substantially the same as if it were the only a single atom present.