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

ed-detuned light) potential is created. In the experiments described in the following
chapters, an attractive dipole potential for lithium atoms is formed by focusing a CO2
laser beam (λ = 10.6 µm), and repulsive potentials for rubidium atoms are created by
multiple laser beams at a wavelength of λ = 532 nm.
2.7 Zeeman Slower
A magneto-optical trap, see chapter 2.9, is able of capturing atoms at a speed of
a few tens of m/s. However, to get a large enough number of lithium atoms into the gas
phase to create an effusive atomic beam, it is necessary to heat the lithium reservoir to a
temperature of about 350 ◦ C. At this temperature atoms within the effusive beam move
at a mean velocity of 1100 m/s (the most probable velocity is about 860 m/s) and are
thus too fast to be captured directly in the MOT. The standard way of slowing atoms
to lower speeds is to use a Zeeman slower [5–7, 22, 35]. Due to the higher vapor pressure
of rubidium, a Zeeman slower is not needed in the rubidium setup. Instead a double
MOT setup is used, see chapter 4.
A Zeeman slower has two main components: a magnetic field which strength
varies along the propagation axis of the atomic beam, and a counter-propagating laser
beam. This basic setup is schematically shown in figure 2.13. The Zeeman slower decel-
erates atoms in the effusive atomic beam by transferring momentum from the scattered
photons from the resonant laser beam to the atoms. With each scattering event an atom
in the atomic beam loses on average a momentum of p = h/λ. In order to transfer mo-
mentum efficiently from the photons to the atoms, the frequency of the laser beam has
to be close to the resonant frequency. As the speed of the atoms decreases, the transition
frequency changes as a consequence of the Doppler-effect. A spatially varying external
magnetic field (usually created by electromagnetic coils) induces a Zeeman-shift of the
energy levels of the atoms. By matching the Doppler- and Zeeman-shifts it is possible
to maintain a resonant scattering condition and thus a large scattering rate. The ideal
magnetic field is thus given by
ω0 + µBB(z)
19
= ωlaser +kv. (2.29)