Recoil-Limited Laser Cooling of ${}^{87}\mathrm{S}\mathrm{r}$ Atoms near the Fermi Temperature

A dynamic magneto-optical trap, which relies on the rapid randomization of population in Zeeman substates, has been demonstrated for fermionic strontium atoms on the ${}^{1}S_0-{}^{3}P_1$ intercombination transition. The obtained sample, $1\times {10}^6$ atoms at a temperature of $2\mu \mathrm{K}$ in the trap, was further Doppler cooled and polarized in a far-off resonant optical lattice to achieve 2 times the Fermi temperature.

Figure 1

(a) Configuration of the standard 1D magneto-optical trap. A pair of counterpropagating ${\sigma }^{+}$ and ${\sigma }^{-}$ polarized lasers is applied in the presence of an inhomogeneous magnetic field, $B(x)=\beta x$. (b) Position-dependent transition frequencies for $m_g\to m_e=m_g\pm 1$ for the case of ${\mu }_e/{\mu }_g\simeq 1$. The sign of the frequency shift depends on the atom position and light polarization, independently of the ground Zeeman substates of $m_g$. Hence, a restoring force mechanism is discussed, as in the $F_g=0\to F_e=1$ transition. (c) Relative transition probabilities for the larger $m_g$ states in the $F_g=9/2\to F_e=11/2$ transition. (d) For ${\mu }_e/{\mu }_g\gg 1$, the transition frequencies are solely determined by the Zeeman shifts of the magnetic substates $m_e$ of the excited state. Therefore, the position-dependent resonant condition alone does not lead to a restoring force (see text).

Figure 2

(a) Hyperfine structure of the ${}^{87}\mathrm{S}\mathrm{r}$ atoms in the ${}^{1}S_0-{}^{3}P_1$ transition. The $F_g=9/2\to F_e=11/2$ transition is used as a “trapping” cycle, while the $F_g=9/2\to F_e=9/2$ transition with a smaller $g$ factor is used as a “stirring” cycle. (b) Decay of atoms in the MOT. By applying the trapping and stirring lasers simultaneously (curve A), a stable operation of the DMOT is realized. The curve B shows the atom decay in the absence of a stirring laser.

Figure 3

Contour plot of the trapped atom number as a function of detunings of the trapping and the stirring lasers. The atom number is normalized by its maximum value. (a) Experimental results and (b) the Monte Carlo simulations.

Figure 4

Time-of-flight measurement of atoms in the lattice FORT. After polarizing and cooling the atoms, a bimodal velocity distribution appeared, indicating the atom population among several Zeeman substates. The dashed curve is a Gaussian fit to a background distribution corresponding to $2.2\mu \mathrm{K}$. The experimental configuration is shown in the inset.