Field-dipole orientation mechanism and higher order modes in atom guides

In cavity quantum electrodynamics (CQED), cavity size, dipole position and dipole orientation are the main factors controlling cavity effects, for exa mple, suppression and enhancement of spontaneous emission. However, the dyn amical effects of dipole orientation in CQED have, to date, remained largel y unexplored, with most treatments simply concentrating on two (or three) o rthogonal directions to illustrate the variations of CQED effects with dipo le orientation. No mechanism which determines the evolution of the dipole o rientation has been put forward to describe typical situations where atoms move in the field of an excited cavity mode. We emphasize here that in the presence of a cavity mode, the average dipole orientation is automatically determined at every point to be parallel to the direction of the electric f ield vector of the cavity mode. Besides giving rise to a single value for t he spontaneous emission rate, this mechanism is shown to have important con sequences for the dynamics of atoms in atom guides. In particular, it deter mines the average trapping potential distributions and the average radiatio n forces which guide the atoms along a cylindrical cavity. The effects of t he field-dipole orientation mechanism are illustrated with reference to typ ical situations involving sodium atoms in sub-micron cylindrical guides. Th e role of a higher order cavity mode of the cylinder in the dynamics is hig hlighted in terms of its influence on the rotational and vibrational motion s in such guides.