The Dipole Force

Abstract

Atoms in an inhomogeneous light field, such as a standing wave, experience a force that derives from the spatial gradient of their light shifts called the dipole force (see p. 10). The simple case of the dipole force on atoms in a standing wave can also be viewed as absorption from one beam followed by stimulated emission into the other of the two counterpropagating beams that constitute the standing wave. The ordering of these sequential events determines the direction of the force, and is itself determined by the relative phase of the counterpropagating beams at the position of the atom. This relative phase, of course, determines the slope of the envelope of the standing wave. These forces can be very much larger than the maximum value of the dissipative force F_max = ℏkγ/2 (see Eq. 3.14) because the dipole force is not limited by the requirement for spontaneous decay from the excited state. Since the slope of the potential associated with the light shift increases with light intensity without limit, the force can be arbitrarily large.