Loading and cooling in an optical trap via hyperfine dark states

We present an optical cooling scheme that relies on hyperfine dark states to enhance loading and cooling atoms inside deep optical dipole traps. We demonstrate a sevenfold increase in the number of atoms loaded in the conservative potential with strongly shifted excited states. In addition, we use the energy selective dark state to efficiently cool the atoms trapped inside the conservative potential rapidly and without losses. Our findings open the door to optically assisted cooling of trapped atoms and molecules which lack the closed cycling transitions normally needed to achieve low temperatures and the high initial densities required for evaporative cooling.

Figure 1

(a) Relevant energy levels of ${}^{87}\mathrm{Rb}$. (b) $m_{F^{\prime }}$-dependent light shifts on the $5P_{3/2}$ levels caused by the FORT at 1560 nm—not to scale. The blue waves show the lasers involved in the Raman scheme; ${\mathrm{\Delta }}_{3^{\prime }}$ and ${\mathrm{\Delta }}_{2^{\prime }}$ indicate the cooler detunings at any given point in the trap.

Figure 2

(a) Atoms loaded into the FORT as a function of the cooling laser detuning from the $F=2\to F^{\prime }=3$ resonance during the sub-Doppler cooling phase, for an independent repumper (empty blue circles), and for a repumper in Raman condition with the cooler (filled red circles). (b) Population fraction of atoms in $F=2$ with the repumper in the two configurations as above; $N_{1,2}$ indicate the atomic populations of the $F=1,2$ manifolds. The violet points result from a numerical evaluation of the DSs forming at each detuning (see Appendix pp1).

Figure 3

HDS cooling near the $F=2\to F^{\prime }=2$ transition at different FORT depths indicated in colors. The sharp peak at $-43.5\mathrm{\Gamma }$ is given by atoms cooled just outside the FORT. The broader peak underneath corresponds to atoms residing within the FORT volume. Inset: The radially changing FORT power results in a position dependent excited-state light shift, represented for a FORT depth of $700\mu \mathrm{K}$. The effect can be exploited to selectively cool the optically trapped atoms at specific radial positions, or to progressively cool the atoms in the FORT from its borders and towards its center, by sweeping $\mathrm{\Delta }$ as indicated by the gray arrow.

Figure 4

Raman detuning optimizing the HDS cooling, as a function of the FORT power. The filled red points correspond to HDSs on the $F=2\to F^{\prime }=2$ transition, the empty blue ones to HDSs on the $F=2\to F^{\prime }=1$ transition (where no shift is required to optimize cooling). Inset: HDS cooling efficiency as a function of the Raman detuning frequency, without the FORT.

Figure 5

(a) Ballistic expansion of the atomic cloud released from the FORT: cloud size vs time of flight. The fits (solid lines) yield a temperature of $198\mu \mathrm{K}$ before the cooling sweep (black points) and $48\mu \mathrm{K}$ after it (red points). Absorption images of the atoms before (b) and after (c) the cooling sweep, taken with a time-of-flight of $700\mu \mathrm{s}$. The atom number in the cross configuration is $\simeq 6\times {10}^6$, with no significant difference between the two cases.