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Journal of Experimental and Theoretical Physics

Loading of a krypton magneto-optical trap with two hollow laser beams in a Zeeman slower

Sunita Singh, Vibhuti Bhushan Tiwari, Satya Ram Mishra, H. S. Rawat

Published Sep 1, 2014• 2 citations
Publisher

Abstract

A significant enhancement in the number of cold atoms in an atomic-beam-loaded magneto-optical trap (MOT) for metastable krypton atoms is observed when hollow laser beams are used in a Zeeman slower instead of a Gaussian laser beam. In the Zeeman slower setup, a combination of two hollow laser beams, i.e., a variable-diameter hollow beam generated using a pair of axicon lenses superimposed on a fixed-diameter hollow beam, has been used to reduce the longitudinal velocity of the atoms in the atomic beam below the capture speed of the MOT. The observed enhancement in the number of atoms in the MOT is attributed to reduced destruction of the atom cloud in the MOT and increased cooling of the off-axis atoms in the atomic beam, resulting from the use of hollow beams in the Zeeman slower.

DOI
10.1134/s1063776114090209

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A significant enhancement in the number of cold atoms in an atomic-beam-loaded magneto-optical trap (MOT) for metastable krypton atoms is observed when hollow laser beams are used in a Zeeman slower instead of a Gaussian laser beam. In the Zeeman slower setup, a combination of two hollow laser beams, i.e., a variable-diameter hollow beam generated using a pair of axicon lenses superimposed on a fixed-diameter hollow beam, has been used to reduce the longitudinal velocity of the atoms in the atomic beam below the capture speed of the MOT. The observed enhancement in the number of atoms in the MOT is attributed to reduced destruction of the atom cloud in the MOT and increased cooling of the off-axis atoms in the atomic beam, resulting from the use of hollow beams in the Zeeman slower.
We report the studies on the effect of Zeeman slower beam power on the loading rate and collision loss rate in an atomic beam loaded krypton magneto-optical trap (MOT). The results show that an increase in Zeeman slower beam power initially increases the MOT loading rate and reduces the background collision loss rate to increase the number of cold atoms in the MOT to an optimum value. With further increase in the Zeeman slower beam power, the number of cold atoms in the MOT decreases due to increased background collision loss rate and decrease in the trap loading rate. However, the cold collision loss rate is observed to remain unaffected by the variation in the Zeeman slower beam power. Therefore, the study emphasizes the need to optimize the Zeeman slower beam power to trap maximum number of cold atoms in an atomic beam loaded MOT.
Magneto-optical traps (MOTs) based on the 626-nm, 136-kHz-wide intercombination line of Dy, which has an attractively low Doppler temperature of $3.3\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{K}$, have been implemented in a growing number of experiments over the last several years. A challenge in loading these MOTs comes from their low capture velocities. Slowed atomic beams can spread out significantly during free flight from the Zeeman slower to the MOT position, reducing the fraction of the beam captured by the MOT. Here we apply a scheme for enhancing the loading rate of the MOT wherein atoms are Zeeman slowed to a final velocity larger than the MOT's capture velocity and then undergo a final stage of slowing by a pair of near-detuned beams addressing the 421-nm transition directly in front of the MOT. By reducing the free-flight time of the Zeeman-slowed atomic beam, we greatly enhance the slowed flux delivered to the MOT, leading to more than an order-of-magnitude enhancement in the final MOT population.
We report here an atomic beam loaded magneto-optical trap (MOT) for metastable krypton atoms in which the fluorescence signal from the atomic beam is used to lock the cooling laser frequency. The fluorescence signal is generated by exciting the metastable krypton atomic beam using a probe laser beam (i.e. part of the cooling laser beam) intersecting the atomic beam at an angle. A spectral shift in the fluorescence signal can be achieved by varying the angle between the probe laser beam and the atomic beam to obtain the desired frequency detuning to lock the cooling laser frequency. This has been used to optimize the number of cold atoms in the MOT. The dependence of the peak height and slope of the atomic beam fluorescence (ABF) locking signal on the RF power in the discharge tube and pressure in the observation chamber of the setup has been studied to correlate its effect on the number of atoms in the MOT.
We report the observation of significant enhancement in the transfer of laser-cooled 87Rb^{87}\text{R}\text{b}87Rb atoms between two vertically separated magneto-optical traps (MOTs) when an appropriately tuned and aligned hollow laser beam was used. In the experimental setup, the lower MOT was loaded from a continuous flux of atoms ejected from the upper MOT by a resonant push beam. The observed enhancement appears to be mainly due to a slowing down of atoms by the hollow beam after being ejected from the upper MOT.
We describe an experimental setup for producing a large cold erbium (Er) sample in a narrow-line magneto-optical trap (MOT) in a simple and efficient way. We implement a pair of angled slowing beams with respect to the Zeeman slower axis, and further slow down atoms exiting from the Zeeman slower. The second-stage slowing beams enable the narrow-line MOT to trap atoms exiting from the Zeeman slower with higher velocity. This scheme is particularly useful when the Zeeman slower is at low optical power without the conventional transverse cooling between an oven and a Zeeman slower, in which case we significantly improve the loading efficiency into the MOT and are able to trap more than 108{10}^{8}108 atoms in the narrow-line MOT of 166Er^{166}\mathrm{Er}166Er. This work highlights our implementation, which greatly simplifies laser cooling and trapping of Er atoms and also should benefit other similar elements.
A magneto-optical trap of cesium atoms was generated by applying a circularly polarized cooling laser beam onto a reflective two-dimensional diffraction grating with an aperture and by retroreflecting the incident beam passing through the aperture while reversing the circular polarization. The cooling laser beams comprised the incident, retroreflected, and four diagonally diffracted beams at an angle of 50° with respect to the normal direction of the grating surface. The intensity of the retroreflected beam was carefully adjusted to balance the radiation forces acting on the atoms. A significant number of cold atoms (7.0 × 106) were captured, in spite of the difficulty in a magneto-optical trap of cesium atoms, using nonorthogonal cooling beams owing to the high nuclear spin. The significance of the retroreflected beam in the trapping process was highlighted when the intensity of the retroreflected beam was reduced, resulting in the absence of trapped atoms. Notably, the cold atom cloud was generated near the edge of the region, where all the cooling beams were overlapped. The phenomenon is explained by the numerical calculations of the radiation forces considering all the Zeeman sublevels in the cooling transition and the Gaussian intensity profile of the incident beam.