Copyright by Kirsten Viering 2006 - Raizen Lab - The University of ...

Figure 2.1: Schematic of the energy shift due to a periodic perturbation in a two-level
atom.
The potential created by the energy shift can be used to trap atoms. The force
due to the potential is a consequence of the dipole moment and is therefore called
dipole force,
Fg,e = −∇E = ∓
4δ ∇Ω2 ∝ 1
∇I. (2.20)
δ
The dipole force is proportional to the intensity of the light and inversely proportional
to the detuning of the light-frequency from the resonance frequency. Since the detuning
from resonance can be either positive or negative, i.e. blue or red detuned, one should
consider two separate cases. In case of blue detuned light, δ > 0, the energy shift
of the ground state is positve and the dipole force repulses atoms from the intensity
maximum. Therefore atoms will not be trapped in the beam. On the other hand, it is
doable to exert an attractive dipole force if δ < 0. Thus it is possible to trap atoms in a
red detuned laser beam, as it is done in an optical tweezer.
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