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Optics Letters

Two-and four-beam magneto-optical trapping of neutral atoms

Carlos Chesman, E. G. Lima, F. A. M. de Oliveira, S. S. Vianna, J. W. R. Tabosa

Published Aug 15, 1994• 11 citations
Publisher

Abstract

We report the experimental realization of new stable three-dimensional schemes for trapping neutral cesium atoms in a vapor cell that use either two or four Gaussian laser beams. Probe-beam absorption is employed to characterize both types of trap, and densities of 2 x 10(8) and 1 x 10(10) cm(-3), respectively, are measured for the two-and four-beam traps. The two-beam trapping mechanism combines a longitudinal magneto-optical force with a transverse spontaneous radiation pressure force associated basically with the beam geometry. This theoretical analysis compares well with our experimental observations.

DOI
10.1364/ol.19.001237

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We report the experimental realization of new stable three-dimensional schemes for trapping neutral cesium atoms in a vapor cell that use either two or four Gaussian laser beams. Probe-beam absorption is employed to characterize both types of trap, and densities of 2 x 10(8) and 1 x 10(10) cm(-3), respectively, are measured for the two-and four-beam traps. The two-beam trapping mechanism combines a longitudinal magneto-optical force with a transverse spontaneous radiation pressure force associated basically with the beam geometry. This theoretical analysis compares well with our experimental observations.
We report on the observation of a new laser trap for neutral atoms. It uses three orthogonal pairs of counterpropagating laser beams having linear polarizations at 45° and a quadrupole magnetic field. 10 8 cesium atoms were thus confined in a 0.15 mm 3 volume at a temperature of 60 μK, a factor of 2 below the Doppler cooling limit. We interpret this trapping as being due to the new magneto-optical force recently observed by Grimm et al. and which is essentially a dipole force rectified by the magnetic field. This trap opens new possibilities for increasing the phase-space density of laser-cooled atoms.
A novel method has been demonstrated for increasing the number of 85Rb trapped atoms in a vapor-cell magneto-optical trap. By using a second set of trapping laser beams overlapped with the original beams and detuned further from resonance, we obtained a doubling of the trap-filling rate as higher initial velocities were slowed. Blocking out the centers of these new beams and reducing the intensity in the original beams also allowed us to obtain a reduction in the light-induced loss rate. Overall we have obtained a fivefold increase in the number of trapped atoms. With 20-mm-diameter beams, ~1.4 × 109 atoms could be trapped. A Gaussian density profile with a peak density of ~2.2 × 1011 cm−3 was observed.
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.
Laser cooled and trapped atoms have been the workhorse of atomic physics for the past four decades. The predominant method has been the highly versatile Magneto-Optical Trap. We describe an alternative laser trap involving a simple geometry of collimated laser beams that provides both a velocity and position dependent restoring force such that a dense cloud of cold atoms is formed. This technique produces similar atom number (>106>10^6>106) and density (101010^{10}1010\,atoms/cm3^{3}3) to the Magneto-Optical Trap, albeit with \emph{no magnetic field}. The beam geometry is compatible with conventional sub-Doppler cooling techniques, allowing the trapped cloud to be cooled to <10 μ< 10~μ<10 μK. We demonstrate the validity and robustness of the trap by capturing 87^{87}87Rb atoms directly from the background vapor and provide a theoretical discussion of the underlying principles. This trap has many unique properties that make it highly suitable for quantum sensing, timing, and computing applications as well as a new tool in fundamental science and metrology.
In this work, we have explored ab initio the capture process in a magneto-optical trap by theory, simulation and experiment. We measured the capture velocity vc of a cesium vapor cell magneto-optical trap (VCMOT) from its capture rate R and developed an exact model for the capture rate of a VCMOT in terms of its capture velocity, background density and trap laser beam diameter. We measured the capture velocity of a cesium VCMOT for various trap laser intensities and magnetic field gradients. We observed that the capture velocity is a damping force as well as a restoring force phenomenon. We supported our findings by performing simulations for single atom trajectories in a 1D cesium MOT. Finally, we concluded that two MOTs can have the same capture velocities but very different capture rates, thereby revealing that these are two fundamentally different characteristics of the MOT.