In the experiment we show that Sisyphus cooling in a 1D gray optical lattice can be improved by using a bichromatic standing wave that increases the efficiency of
cooling as compared to typical lattices. The bichromatic standing wave is created by superimposing a repumper
lin. perp lin. standing wave on a standard lin. perp lin.$ standing wave used to form an optical lattice.
We refer to the relative displacement between the two
standing waves, at a given location along the wave, as the phase difference f.
Consider the case of having the lattice standing wave blue detuned with respect to the |F=2> g|F'=2> hyperfine transition on the 5S1/2 g 5P3/2 line of 87Rb, and the repumper resonant on the |F=1>g |F'=2> transition. The potentials in the |F=1> level will be flat because there is no light shift created by the on-resonant repumper. Fig. 1 illustrates Sisyphus cooling with this light field, for the extreme cases of 0 and p phase difference, which we will refer to as ``in phase'' and ``out of phase''.
As with the case of regular Sisyphus cooling, it is instructional to begin with an atom, at
z=0, on the lowest adiabatic potential
with mF=2, moving in the +z direction. As it climbs the potential hill, the atom loses kinetic energy.
If the atom scatters a photon near the top of the hill, there is some probability of it being optically
pumped to the mF= 1 state of the |F=1> level. Energy is removed
via the spontaneously emitted
photon and the atom moves freely on the flat |F=1> potential until it scatters a photon.
The case of f=0 is shown in Figure 1a. Since the probability of absorbing a s+photon is 6 times greater than that of a s - photon, due to the respective CGs, the atom will more likely scatter a photon at z=l /2 than at z=l /4. If the atom absorbs a photon at z=l /2, then it may be optically pumped to the |F=2, mF= 2> level, where it will be located at the minimum of a s+ well on the lowest adiabatic potential. From this point, the atom begins the Sisyphus cooling process again. Therefore, from this consideration it is evident that the efficiency of cooling is increased in the case of in phase standing waves.
If f =p, as displayed in Figure 1b, then at z=l/4, there will be a high transition probability from |F=1, mF= 1> to |F'=2,mF' =2>, from where the atom may relax down to |F=2, mF= 2>. However, at z=l /4, the |F=2, mF= 2> sublevel resides on one of the upper adiabatic potentials. This will lead to subsequent heating of the atoms for two reasons. First, the photon scattering rate on the upper potential is greater than on the lower potential. Second, a maximum of the upper potential occurs at z=l /4, which implies the atom will accelerate as it moves away from z=l /4. Therefore, from this consideration it is evident that the efficiency of cooling is diminished in the case of out of phase standing waves.
Using the TOF technique to determine the velocity distribution of all the atoms, we calculate the efficiency by measuring the fraction of all atoms that are cooled by the lattice. The compiled data is shown in the Figure 2 . The phase difference f is controlled by the means of a retro-mirror that is used to create the two standing waves. Translating the retro-mirror by 22mm corresponds to a full period of f. The results clearly demonstrate that efficiency depends on the relative displacement between the two standing waves formed by the two colors. The optimal configuration of the bichromatic light field yields a 40\% improvement in efficiency compared to the typical optical lattices, while maintaining the same atomic core temperature. This method for improving cooling efficiency can be easily implemented when needed, since no new laser beams are required.
For detailed description of the experiment see our paper: ''Enhancement of Sisyphus cooling using a bichromatic standing wave'', S. K. Dutta, N. V. Morrow, G. Raithel, Phys. Rev. A 62, 035401 (2000).