Enhanced trapping of cold ${}^6\mathrm{Li}$ using multiple-sideband cooling in a two-dimensional magneto-optical trap

Trapping a large number of ${}^6\mathrm{Li}$ atoms in a simplified experimental setup is a long-term pursuit in the studies of degenerate Fermi gases. We experimentally and theoretically demonstrate the enhancement of ${}^6\mathrm{Li}$ trapping efficiency in a three-dimensional magneto-optical trap (3D MOT) by using the multiple-sideband cooling in a two-dimensional magneto-optical trap (2D MOT). In the 2D MOT, we increase the spectral width of the cooling light to 102 MHz by generating six frequency sidebands in order to couple fast atoms. The capture velocity is dramatically increased by employing the multiple-sideband cooling. The number of trapped atoms in the 3D MOT is $6.0\times {10}^8$, which is higher by a factor of 4 than in the case of single-frequency cooling. We have investigated the dependence of atom number on laser detuning, and our experimental result agrees well with the prediction of a simple two-level model. The efficiency of the multiple-sideband cooling for lithium (in contrast to many other alkali-metal atoms) is also confirmed by the analysis of the loss due to fine-structure changing collisions.

Figure 1

Schematic of the experimental setup. The system consists of two cooling and trapping regions, the 2D MOT and 3D MOT. The initial fast-moving atoms are emitted from a baked lithium oven which is located 170 mm below from the center of the 2D MOT. Atoms cooled in the 2D MOT are extracted toward the 3D MOT by using a pushing beam. The cooling light in the 2D MOT has multiple frequency sidebands, which has a spectral width of 102 MHz to increase the capture velocity. Trapped atoms are detected in the 3D MOT using the fluorescence imaging method. The solid red arrows show the cooling laser beams for 2D and 3D MOTs, and the black solid arrow indicates the pushing beam.

Figure 2

Generation of multiple sidebands. (a) Optical arrangement. PBS: polarization beam splitter; FR: Farady optical rotator; HWP: half-wave plate; QWP: quarter-wave plate; M1 and M2: mirrors; EOM: electro-optical modulator; RF: radio-frequency signal; PD: photodetector; SA: spectrum analyzer. (b) Power distribution of the multiple frequency components, which is recorded with a SA.

Figure 3

Capture velocity vs the cooling light intensity per beam in the 2D MOT ($d=4$ cm, ${\mathrm{\Delta }}_0=-8.2\mathrm{\Gamma }$, and ${\mathrm{\Delta }}_4=-11.9\mathrm{\Gamma }$). Both detunings are close to the optimal values for achieving the maximum atom number as shown in Fig. 4. The red curve indicates $v_c$ for the multiple-sideband cooling, and the black curve for the single-frequency cooling.

Figure 4

Number of trapped atoms in the 3D MOT vs the cooling light frequency detuning in the 2D MOT (${\mathrm{\Delta }}_0$ for the single-frequency cooling and ${\mathrm{\Delta }}_4$ for the multiple-sideband cooling). In the 2D MOT we have $d=4$ cm, $I=1.9$ mW/${\mathrm{cm}}^2$, and $\partial B/\partial x=\partial B/\partial y=50$ G/cm. In the 3D MOT the parameters are $d=3$ cm, $I=2.0$ mW/${\mathrm{cm}}^2$, and $\partial B/\partial x=\partial B/\partial z=7$ G/cm. Red squares and black circles show experimental results for the multiple-sideband and single-frequency cooling, respectively. Each experimental point comes from five measurements and the error bars indicate the standard deviation (SD). Solid red and black curves are theoretical predictions from Eqs. (3) and (2), respectively.

Figure 5

Fluorescence images of trapped atoms in the 3D MOT. The red and black solid curves are the 1D integrated fluorescence for the multiple-sideband and single-frequency cooling, respectively. The 2D fluorescence images are also inserted in the plot. In the multiple-sideband cooling $N=6.0\times {10}^8$ and in the single-frequency cooling $N=1.5\times {10}^8$. The color in the 2D images is scaled to the fluorescence amplitude on the CCD.