Enhancement of Phase Space Density by Increasing Trap Anisotropy in a Magneto-Optical Trap with a Large Number of Atoms

The phase space density of dense, cylindrical clouds of atoms in a 2D magneto-optic trap is investigated. For a large number of trapped atoms ($>{10}^8$), the density of a spherical cloud is limited by photon reabsorption. However, as the atom cloud is deformed to reduce the radial optical density, the temperature of the atoms decreases due to the suppression of multiple scattering leading to an increase in the phase space density. A density of $2\times {10}^{-4}$ has been achieved in a magneto-optic trap containing $2\times {10}^8$ atoms.

Figure 1

A schematic of the experimental apparatus. The ferromagnetic foils shown below the mirror generate the magnetic field gradient for the MOT.

Figure 2

The temperature of the atoms for various radial optical densities. The data in (a) and (b) are taken at light shift parameters of 0.4 and 0.8, respectively. The radial optical densities indicated are measured after 17 ms of free expansion. These values are measured for the largest number of atoms in each data set.

Figure 3

The variation of the temperature with laser detuning. The number of atoms in the MOT is around $2.5\times {10}^8$ for each data set. The radial OD’s measured after 17 ms of free expansion are 0.75 (□), 0.45 (◇), and 0.3 (▲). The dashed line shows the predicted temperature of ${\sigma }^{+}-{\sigma }^{-}$ molasses for the same laser intensity. Inset: The minimum temperature vs the radial optical density.

Figure 4

The peak density of the atom cloud for different radial optical densities. The change in behavior from the density limited regime (■) to the temperature limited regime is clearly seen. The radial field gradients for each data set are $10\mathrm{G}/\mathrm{cm}$ (■), $7\mathrm{G}/\mathrm{cm}$ (□), $30\mathrm{G}/\mathrm{cm}$ (○), and $60\mathrm{G}/\mathrm{cm}$ (▽). The indicated ODs follow the same convention as in Fig. 2.

Figure 5

The phase space density of the atoms for various radial optical densities. The Rabi frequency per laser beam, detuning, and field gradient for each data set are: ▼ ($0.75\Gamma$, $-2\Gamma$, $10\mathrm{G}/\mathrm{cm}$); □ ($1.2\Gamma$, $-5.5\Gamma$, $7\mathrm{G}/\mathrm{cm}$); △ ($1.2\Gamma$, $-5.5\Gamma$, $10\mathrm{G}/\mathrm{cm}$); ○ ($1.2\Gamma$, $-8\Gamma$, $70\mathrm{G}/\mathrm{cm}$); ■ ($1.0\Gamma$, $-8\Gamma$, $70\mathrm{G}/\mathrm{cm}$). The indicated ODs follow the same convention as in Fig. 2.