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2.1.4 Trapping of atoms in light fields
Chapter 2: Theoretical Background
A spatial varying intensity of light I(�r) leads to a trap if the potential has a
local minimum. For red detuned light this is reached by a local maximum of
the intensity, while for blue detuned light (∆ > 0) the atoms can be trapped in
a local minimum. Figure 2.2 schematically shows how such a trap works by a
spatially dependent shift of the energy levels of the atoms. In the experiment
described here (chapters 6, 7) we work with red detuned light only. Thus,
trapping occurs in local maxima of the intensity. For this, a single focused
beam is already sufficient, with the radial confinement given by the waist of
the beam and the axial confinement due to the Rayleigh range. This set-up
was used in the first experimental realisation of an optical dipole trap [Chu86].
Figure 2.2: A spatially dependent intensity of light can be used to trap atoms.
The intensity of a red detuned Gaussian beam is causing a trapping potential
by the ac-Stark effect. In high intensity areas the atomic ground state is
shifted to lower energies while in low intensity areas it remains unperturbed
and remains at a relatively higher energy.
Two crossed beams with perpendicular polarisation and same focal char-
acteristics have the advantage that the confinement is much more isotropic.
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