Thus we now see that a spatially varying polarization in the light field
will produce a spatially varying shift in the energy levels of atoms.
When analyzing the dynamics of atoms in an OL, we consider only the motion
of the atoms in the ground state - the motion on the excited does not differ
significantly from the ground state and thus it provides no addition
information--so we ignore it.
Figure 4 illustrates the shift of the two ground states as a function
of position caused by a red detuned light field (the circles serve as visual
In the absence of a light field, the sublevels are degenerate.
When the atoms are in a position where the polarization is purely
sigma plus, the g-1/2
level is shifted down 3 times as
far as the g-1/2
, because the transition probability is 3 times
as great. Similar arguments apply when the polarization is linear
and sigma minus, and thus the sublevels will vary periodically with
When the OL is in steady state, the atoms will rest in the minima of these
In general, the precooled atoms are too energetic to be confined in
these potentials, so they must be cooled even further, by a process
known as Sisyphus cooling