Theory of radiation forces and momenta for mobile atoms in light fields

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Abstract
A theory for a bound system of charges interacting with electromagnetic fields is developed, with special emphasis on the radiation-pressure effects imposed by the fields on the gross motion of the charges. The theory is particularly simple and transparent for the case of two opposite charges of finite mass M in the electric-dipole approximation. This enables a rigorous description to be given for the quantum electrodynamics of the problem with the various interaction terms affecting the gross motion being easily identified. Here, too, it becomes clear that, even in the dipole approximation, the conventional interaction term should be supplemented by the so-called Röntgen term. Besides ensuring the consistency of the theory for the overall (charges plus fields) system regarding conservation laws, the Röntgen term has dynamical consequences. This is established by explicit calculations of the expectation values of the mechanical momentum 〈MṘ〉 and the radiation pressure force 〈MR¨〉 on a two-level atom in the dipole approximation. Results of calculations are displayed for the case of a single light beam and for counterpropagating beams. The feasibility of experimentally observing the transverse force is explored. Generalizations of the well-known friction force arising in one-dimensional optical molasses are given and discussed.