ECDL
2.2 Laser cooling setup
2.2.2 The 2D-MOT and 3D-MOT
Several interesting cold atom sources arose out of the desire to provide a bright atomic beam for laser cooling, collision, and precision measurement applications that improved on the somewhat cumbersome Zeeman slower. The first so-called low- velocity intense source (LVIS) featured a standard vapor-cell MOT with a hole in one of the beams through which cold atoms would leak out unidirectionally [117]; versions developed at JPL and elsewhere featured a large pyramidal mirror combined with a retro-optic with a hole drilled in it to provide the ‘leak’ [116, 118–121]. The notion of two-dimensional cooling and trapping as an incubator for a cold atomic beam has existed in the form of ‘funnel’ setups as far back as 1990, leading into the development
5This ion pump was only necessary for bakeout; typically it shut down after several weeks post–
bakeout running, due to being saturated by rubidium. After this point the faraway stronger ion pump (and the occasionally pulsed sublimation pump) took care of anything coming through from the vapor cell.
6In the summer 2005 vacuum reconstruction and bakeout we replaced the old 600 differential pumping tube with a new 400 version. No significant effect on MOT loading or UHV pressure was noticed.
2D-MOT cuvette
Rb sample Ion pump Ion & Ti:sub pumps
Central chamber, w/ CO2 lenses
CO2 entrance CO2 exit
Differential pumping tube
Figure 2.8: Schematic of vacuum chamber.
of the 2D-MOT as a viable and well-studied source of cold atoms [122–128].
Our 2D-MOT was formed by the intersection of two pairs of counterpropagating beams through the cross-sectionally square glass cuvette used as a vapor cell. Im- mediately adjacent to the cell were two coils forming an effective two-dimensional quadrupole field. The coils were simple loops of particularly high aspect ratio; more complicated ‘baseball’-style winding was deemed unnecessary if the coils were made long enough.
Both 2D- and 3D-MOTs rely on the laser-cooling transition: 780 nm light detuned to the red from the |F = 2i −→ |F0 = 3i resonance. The 2D-MOT beams were expanded using telescopes to 200 diameters; ideally (as suggested by [126, 128]) one wants the illuminated region of the cuvette to be as long as possible. Repump light for the 2D-MOT was combined with trapping light using polarizing beamsplitters. While it is true that the repump division was a finite-sum situation, with any increases in
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Figure 2.9: The 2D-MOT in action
repump intensity at the 2D-MOT resulting in corresponding losses for the 3D-MOT, it was observed that both MOTs were operating with sufficient repump light via small changes in light division as well as the negligible effect of the insertion of a OD=0.1 neutral density filter. An image of the 2D-MOT in operation is shown in Fig. 2.9.
The characteristic cigar-shaped central fluorescence is immediately noticeable. The consequence of the two-dimensional cooling and trapping is a pair of cold atomic beams, one of which is directed downstream through the copper pipe into the UHV region. Transverse cooling in the beam is of course provided by the MOT action itself;
beam velocity is kept low through a selection effect whereby only longitudinally slow atoms experience the cooling region long enough to be significantly affected.
The 3D-MOT is formed by six intersecting beams of specific polarization, as de-
scribed elsewhere [37, 129]. The beams originate from a nearby single-mode fiber, the coupling into which has typically been optimized such that the output power is 50–80 mW. Collimation is achieved such that clipping on 100 optics is not visibly wasteful.
Repump light remains free-space, directed along two axes of the 3D-MOT at approx- imately 250 µW/cm2. Fluorescence from the trap was measured by a photodiode mounted outside one of the windows.
Our MOT coils were constructed of 200 turns of copper wire (each) and were arranged on the spherical cube in an approximation to anti-Helmholtz geometry.
Typically 3–4 A was used for MOT loading, as well as for providing the necessary gradients for Stern-Gerlach spectroscopy of the condensate mF distribution. We calculated that 4 A corresponded to a maximum gradient of 7 G/cm along the axis of the coils.
The laser detuning used was common to both 2D- and 3D-MOTS, which surely resulted in some inefficiency of trap loading. Optimal loading for a given alignment (as measured by frequency-corrected fluorescence from the trap) was typically found to be δ = −10 MHz, or less than 2Γ; most likely this provided optimal beam flux, but at some cost to the quality of the 3D-MOT, particularly at the high intensities used. The high intensities were necessary for good dipole trap loading, as detailed in
§3.1.
Despite considerable time spent in alignment of the 2D-MOT, no general proce- dure was ever found that led convincingly to high-flux beams. The figure of merit, always, was maximum size of the 3D-MOT as measured relatively by the nearby photodiode and CCD camera. Most likely the largest timesink in the latter stages of this experiment was maintaining quality 3D-MOT loading; possible improvements will be suggested in§4.2.2. It was noticed, though, that an awareness of the need for care in the alignment of the front edge of the cooling region (so as to prevent strong deflection of the emerging cold atomic beam) was usually rewarded.
Periodic heating of the rubidium sample on the vapor-cell side of the chamber resulted in significant improvement of trap loading, at the temporary cost of higher
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UHV pressure from straight-through atoms, observable by measurement of dark load- ing, whereby a small 3D-MOT formed even when no resonant light or field gradient were applied to the vapor cell region. Typically strong heating of the sample was only needed every few months; weak heating of the sample was more frequent but had inconsistent results and likely was more motivated by neurosis.
Approximate number calibration of the MOT was of the number resulting from fluorescence recorded by the CCD camera, which conveniently serves as a MOT mon- itor as well as the vehicle by which absorption images are recorded. This convenience stems from the fact that both procedures require lens placement such that the region at MOT center is imaged. The number of atoms is estimated via the ‘counts’ in any given image of the MOT, and is given by
N = 8π Γ
1 + 4(δ/Γ)2+ 6I/Is 6I/Is
Ncounts
texpηdΩ (2.2.1)
wheretexp is the exposure time, η is the efficiency of the CCD in counts/photon, and dΩ is the solid angle subtended by the light-gathering optics. A convenient check on the MOT number calibration was made through the use of a photodiode placed near a vacuum window; the reading from this photodiode provided realtime (if relative) feedback on MOT number, whereas the CCD images were most often used to monitor MOT shape and stability.
Loading of the MOT typically followed the standard loading formN(t)∝1−e−t/τ, and the decay of the MOT was shown even at high intensities to have a 1/e lifetime of 20–30 seconds. The lifetime of a large MOT in a high-intensity laser field is a more difficult case to ascribe causes to—such collisional issues are beyond the purview of this thesis but can be explored further in [130].
The MOTs that were used for high-quality BEC runs were in the range 108 <
N < 109, with the photodiode and CCD calculations agreeing within 50%. Larger MOT number unfortunately did not guarantee good dipole trap loading, and low MOT number did not prevent the same, but it appeared that having a large MOT
presented one with a larger “capture range” in parameter space when adjusting the various trap-loading parameters; see §3.1 for more detail.