Atoms, dipole waves, and strongly focused light beams

We describe the resonant interaction of an atom with a strongly focused light beam by expanding the field in multipole waves. For a classical field, or when the field is described by a coherent state, we find that both intensity pattern and photon statistics of the scattered light are fully determined by a small set of parameters. One crucial parameter is the overlap of the field with the appropriate dipole wave corresponding to the relevant dipole transition in the atom. We calculate this overlap for a particular set of strongly focused light beams, whose spot size is only $0.1lambda^2$, as discussed in S.~Quabis {em et al.}, Appl. Phys. B {bf 72}, 109 (2001).