Laser cooling of an indium atomic beam enabled by magnetic fields

We demonstrate magnetic field enabled optical forces on a neutral indium atomic beam in a light field consisting of five frequencies. The role of dark magnetic ground state sublevels is studied and enables us to cool the atomic beam transversely to near the Doppler limit with laser frequencies tuned above the atomic resonance. The effect of laser cooling can be explained with transient effects in the light potential created by the standing wave light field where the atoms are optically pumped into the dark states and recycled by Larmor precession.

Figure 1

Energy level scheme of ${}^{115}\text{I}\text{n}$. The lasers in the experiment address the $P_{1/2}$ $F=4,5$ to $S_{1/2}$ $F^{\prime }=5$ and $P_{3/2}$ $F^{″}=4,5,6$ to $S_{1/2}$ $F^{\prime }=5$ transitions.

Figure 2

Schematic of the experimental setup. The two interaction zones are 60 cm apart.

Figure 3

Fluorescence of the atomic beam interacting with a laser beam which connects the $S_{1/2}$ $F^{\prime }=5$ level with all possible ground states. A homogeneous magnetic field with varying field amplitude has been applied perpendicular to the atomic and the laser beam. The linear polarization was 45° with respect to the magnetic field.

Figure 4

Spatial distribution of the atomic beam in the probe region without (1) and with (2) applied laser beams. The lower axis gives the spatial coordinate perpendicular to the propagation axis of the atomic beam. The difference in the signal-to-noise ratio is due to optical pumping into the $P_{3/2}$ level.

Figure 5

Measured velocity change of the atomic beam in terms of $v_r$ caused by laser traveling waves with all five laser frequencies tuned on resonance. The power has been changed for all beams proportionally. The polarizations were the same as used in Fig. 3 and the magnetic field strength was 4 G. The solid lines show the theoretical predictions for $\Delta =0\Gamma$ (upper) and $\Delta =0.35\Gamma$ (lower).

Figure 6

Part (a) shows the spatial distribution of the atomic beam 60 cm beyond (above) a laser standing wave for both wavelengths ($\lambda =410\text{nm}$: on resonance; $\lambda =451\text{nm}$: $\Delta =+2.4\Gamma$, 0, and $-2.4\Gamma$ from top to bottom). All profiles have the same base line. The shaded area gives the width of the atomic beam in the standing wave zone. (b) shows the difference of the two upper curves of (a). The vertical axis is enlarged for clarity, but the horizontal scale matches that of (a).

Figure 7

Spatial distribution of the atomic beam 60 cm beyond (above) a single (top) and two (bottom) laser standing waves for both wavelengths ($\lambda =410\text{nm}$: on resonance; $\lambda =451\text{nm}$: $\Delta =-2.4\Gamma$).

Figure 8

The inset shows the difference between the unperturbed beam and the heated beam. A difference of two Gaussians has been fitted to the signal and the width of the inner one is plotted for different laser powers. The power has been changed for all beams proportionally. The line is proportional to the number of scattered photons.

Figure 9

Fraction of the laser heated atoms of Fig. 8 plotted for different laser powers. The power has been changed for all beams proportionally. The line is proportional to the number of addressable atoms within a capture range given by Fig. 8.