Efficient two-dimensional subrecoil Raman cooling of atoms in a tripod configuration

We present an efficient method for subrecoil cooling of neutral atoms by applying Raman cooling in two dimensions to a four-level tripod system. The atoms can be cooled simultaneously in two directions using only three laser beams. We describe the cooling process with a simple model showing that the momentum distribution can be rapidly narrowed to velocity spread down to $0.1v_{\mathrm{rec}}$, corresponding to effective temperature equal to $0.01T_{\mathrm{rec}}$. This method opens new possibilities for cooling of neutral atoms.

Figure 1

(a) An atomic ensemble interacts with three running optical waves. (b) The level diagram of a four-level tripod-type atom. The ${\sigma }_{+}$ and ${\sigma }_{-}$ circularly polarized contrapropagating waves couple transitions $|1\rangle \leftrightarrow |4\rangle$ and $|3\rangle \leftrightarrow |4\rangle$, respectively, and the $\pi$-polarized wave couples the $|2\rangle \leftrightarrow |4\rangle$ transition.

Figure 2

The momentum distribution of atoms originally prepared in state $|2\rangle$ after they interact with a pulse of Raman beams. (a), (c) The population transferred to states $|1\rangle$, $|3\rangle$ through Raman transitions $|2\rangle \leftrightarrow |1\rangle$ and $|2\rangle \leftrightarrow |3\rangle$, respectively; (b) cross appearing in state $|2\rangle$ due to these transfers. The duration of Raman pulse is $\tau =2$, the Rabi frequency of two-photon resonance is $\alpha =0.5$. The detunings from ground states are ${\delta }_m=0$ ($m=1,2,3$).

Figure 3

Momentum distribution of atoms after applying 420 elementary cycles of 2D Raman cooling. Atoms have been piled up in a narrow peak near zero momentum with effective temperature equal to $0.01T_{\mathrm{rec}}$. The height of the peak is about 80 times larger than that of the original distribution, the width (full width at half maximum, FWHM) is about 0.24 in units of $\hslash k$.