ECDL
3.1 Loading the dipole trap
3.1.2 Our approach
Our approach used a single focused CO2 laser beam instead of a crossed pair. While we initially built the experiment to duplicate that geometry, it was thought that the combination of higher CO2 laser power and higher MOT number (109 vs. 107) would balance the loss in mean trap frequency. This assumption turned out to be incorrect, but not disastrously so; condensation was still achieved, albeit with the evaporation process slowed down somewhat.
Fig. 3.2 shows pictures taken during a typical loading sequence, which we largely adapted from the 2001 paper. The sequence begins with the CO2laser being turned on using the AOM. Immediately, the MOT is first rendered ‘dark’ by strongly reducing the repump intensity. After 20 ms the MOT beams are detuned another 50 MHz beyond the default detuning of δ =−10 MHz. After 40 ms of this configuration, all resonant light is shut off, as well as the quadrupole field used by the MOT. The repump field is shut off 2 ms before the cooling transition, in order to ensure optical pumping
1This process was investigated recently in all-optical sodium BEC experiment, confiming the early onset (and importance) of free evaporation [136].
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Figure 3.2: CCD images taken of the trap loading process. Images proceed in time left-to- right and top-to-bottom. The first two images are full-screen shots of the MOT as repump intensity is reduced for a 20ms ‘dark’ phase. The trap is then detuned 60 MHz. The next four images show the presence of the dipole trap, revealed by a narrow line of fluorescence centered near the MOT. After 40 ms of detuning, all light is extinguished and untrapped atoms are left to fall away. The final two images show the two exposures of the probe beam pulses: the first in the presence of a released dipole trap that has undergone 1.5 ms of ballistic expansion, and the second image a reference shot with no atoms. The final two images are divided to obtain absorption images such as in Fig. 3.4.
into the F = 1 ground state via off-resonant excitation of the F = 2 → F0 = 2 transition. The dipole trap is left to evolve in the dark for a period of time no less than 90 ms (in order to let untrapped atoms fall away) before being probed via absorptive imaging.
The dipole-trap loading process was the central mystery of this experiment. Ob- taining enough atoms to begin confident evaporation with was the goal of many experimental diversions and proposals. It is clear, however, that for a given trap geometry, it simply does not help to have extremely large MOTs as reservoirs; the expected increase in trap loading from having access to a large MOT as reservoir was quite na¨ıve in retrospect. The loading dynamics within the dipole trap appear to place stringent limits on trap number as a consequence of limited trap volume. The startling increase in density made available by the detuning/dark SPOT technique was deceptive in that it could not be increased arbitrarily. The obvious solution to this problem is to cool into a much larger trap. An ironic note is that our initial designs had much larger trap volume, yet our familiarity with the vicissitudes of trap loading at the time was low; we thus went as tight as possible and proceeded from there. Possible solutions to this issue will be discussed in the final section.
Given an apparent upper limit of ∼2.5×106 for our tightest trap, it was desir- able (at least) to achieve that amount on a regular basis. Three factors dominated the landscape of ‘tweaking’ that determined quality of trap loading given an average MOT of several 108 atoms. The most sensitive degree of freedom was dark-SPOT repump (DSRP) intensity at the trap location. Typical estimates of our optimal DSRP intensity were in the 10–25 µW/cm2 regime, although this was by no means a constant. Yet for a given sequence of runs, a 20% change in intensity in either direction would yield strong changes in trap loading, often as great as 50%. Period- ically performing runs at slightly higher or lower DSRP intensity kept this variance in check, however. Secondly, since the MOT was not colocated with the dipole trap due to offsets in alignment, coil geometry, and ZnSe lens placement, some effort was needed to make sure that the MOT compressed and darkened at the correct location.
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This was accomplished largely through the use of bucking fields originally in place to zero out the background field. Bucking fields of ∼0.5 G were effective at moving the MOT around and placing it such that fluorescence from atoms colocated with the dipole trap was observed during loading (see Fig. 3.2). Unfortunately this had the rather obvious side effect of presenting a significantly nonzero field during the loading phase, which significantly affected the dynamics of the MOT cloud as it was detuned by up to 150 MHz.
Given that detuning beyond the amount needed to cancel the CO2 laser light shift appeared to reap benefits, we expected that the efficacy would continue to improve all the way out to a detuning ofδ ∼ −140 MHz, at which point the laser light would become more accurately described as being to the blue of the |F = 2i → |F0 = 2i transition [137]. Indeed, details of the 2001 experiment showed that loading was completely ineffective until the light shift was cancelled, then rose sharply with further detuning, peaking beyond δ = 120 MHz, but not falling off significantly beyond that [133]. Our goal was thus to detune as far as this, necessitating the construction of an offset lock circuit capable of such significant jumps without the use of AOMs.
While the offset lock performed adequately, a significant problem with the loading process is that the detuning process was lossy, i.e., detuning the MOT for tens of milliseconds and then bringing it back did not come close to preserving atom number.
This problem was worse for larger detunings and longer times. We initially attributed this loss to transients in the offset lock jump, but it was quickly discovered that the number loss was linear in time spent detuned. The culprit, then, was deemed to be the strong background field, which surely interferes with the health of the detuned dark SPOT. Since the fields were necessary, we found an equitable solution of not detuning particularly far, typically settling at going out to −60 MHz, or −10Γ, for optimal loading.
Finally, MOT intensity during the loading phase was critically important to break through the final factor of two in initial atom number. While quality MOTs of order 109 atoms were loaded using a total MOT power (in six 100 beams) of 30 mW, it was
found that running at high power, up to 80 mW, yielded strong gains in loading.
Operating the MOT at these intensities (obviously far above saturation) did not change its performance significantly, but the necessity of sending enough power to the 2D-MOT placed a fundamental limit on how far we could push the MOT intensity.
In summary, loading the dipole trap to consistently high numbers was a formidable task with repeatability that (while significantly improved) remains questionable. It is the author’s hope that the next generation of the experiment will have a chance to address several of these issues.