Experiments to Control Atom Number and Phase-Space Density in ...

Figure 5.5: Atoms transferred into the optical trough as a function of the demon beam
detuning.
This density effect is hidden underneath a second effect specific to the rubidium
experimental setup. The walls of the glass cell are uncoated. This means that about 4%
of the light is reflected back into the chamber at each surface. Because the demon beam
is focused inside the glass cell, these reflections are divergent and are thus unfortunately
incident on atoms in the magnetic trap. This leads to a trap loss of atoms inside the
magnetic trap and decreases the number of atoms that can possibly be transferred into
the optical trough. A calculation shows how detuning the demon beam frequency can
alleviate this problem. Consider the saturation parameter sd = I/Isat of the demon
beam. The saturation parameter of the reflected light is then given by sr = αsd, α ≪ 1.
The ratio between the scattering events in the optical trough Rd to scattering events
inside the magnetic trap Rrefl as a function of the detuning is thus given by
Rd
Rrefl
= 1
α
1+αsd +(2∆/Γ) 2
. (5.1)
1+sd +(2∆/Γ) 2
Figure 5.6 shows a plot of equation 5.1 for values of sd = 6,α = 0.01 (blue) and
sd = 20,α = 0.01 (red). This shows that as the detuning of the demon beam is increased,
the effect of the partially reflected beam is minimized. It also shows that the negative
73