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Journal de Physique IV (Proceedings)

Magneto-Optical Trap in the limit of very large number of atoms : New scaling law for the MOT size?

G. L. Gattobigio, G. Labeyrie, Robin Kaiser

Published Oct 1, 2006• 0 citations
Publisher

Abstract

\nWe study the properties of a magneto-optical trap of Rubidium in the\nlimit of very large number of atoms (up to N$_{\\rm at} =10^{10}$). In\nthis regime we observe a new scaling law for the size of the MOT ($\n\\propto {\\rm N}_{\\rm at}$$^{1/2}$). Several physical ingredients could\npossibly contribute to this new behavior : a correction of the\nspring constant due to the shadow effect, multiple scattering beyond\ndouble scattering and non linear spatial dependence of the forces.\nThis study should allow to design new compression technique to\nachieve high spatial densities (ideally up to ${\\rm n}=10^{13}\\,{\\rm\ncm}^{-3}$) in order to study new localization effects of light in\ndense cloud of cold atoms.\n

DOI
10.1051/jp4:2006135103

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Multiple scattering of light has been the main limitation of the maximum atomic density achievable in magneto-optical traps (MOTs). We present a detailed experimental investigation of the size and density scaling laws for large MOTs with up to N=1010 atoms, larger than those usually studied in detail. Most of our observations can be explained with previous models and only a few regimes show unexplained deviations. We also propose a new repulsion mechanism, based on the rescattered repumper photons that might limit the atomic density of atoms when the optical thickness for repumper light becomes important, adding an additional ingredient in the complexity of large MOTs.
In this thesis we present a detailed experimental investigation of size- and densityscaling laws for large Magneto-Optical Traps (MOT) with up to 10^10 atoms. It is well known that laser-induced interactions, produced by multiple scattering, is the main limitation to spatial densities in a MOT. We therefore realized a ?Dark MOT? combined with a Dipole-Trap. Thus, the repulsions were considerably lower and an increase by two factor of magnitude (up to 10^12 atm/cm^3) has been observed. We also propose a qualitative model for the density (and thus for the size), which introduces the mechanical interactions induced by the repumper photons when its optical thickness becomes important. This work is part of the research project to observe Anderson localization of light in cold atoms, where densities of the order of (10^13 - 10^14 atm/cm^3) are required. For densities lower than this critical value gain mechanisms could be useful to amplify prelocalized modes. We thus implemented a pump-probe technique suitable for gain production in the atomic sample. In our case, we have been naturally lead to a Four-Wave Mixing (FWM) configuration. A detailed analysis of the properties and characteristics of FWM-signal has been presented, with, in particularly, a clear red-blue asymmetry of FWM-spectrum for high intensities of the pump beams. We explained this asymmetry by the spatial bunching of the atoms in the nodes or antinodes of the strong standing wave of the pump beams.
We investigate the scaling behavior of a very large magneto-optical trap (VLMOT) containing up to 1.4×10111.4 \times 10^{11}1.4×1011 Rb87^{87}87 atoms. By varying the diameter of the trapping beams, we are able to change the number of trapped atoms by more than 5 orders of magnitude. We then study the scaling laws of the loading and size of the VLMOT, and analyze the shape of the density profile in this regime where the Coulomb-like, light-mediated repulsive interaction between atoms is expected to play an important role.
We investigate the scaling behavior of a very large magneto-optical trap (VLMOT) containing up to $1.4\ifmmode\times\else\texttimes\fi{}{10}^{11}\phantom{\rule{4pt}{0ex}}{\mathrm{Rb}}^{87}$ atoms, loaded from a background vapor. By varying the diameter of the trapping beams, we are able to change the number of trapped atoms by more than 5 orders of magnitude. We then study the scaling laws of the loading and size of the VLMOT, and analyze the shape of the density profile in this regime where the Coulomb-like, light-mediated repulsive interaction between atoms is expected to play an important role.
We present in this paper a review of our work on the impact of long-range interactions on the static and dynamic behaviors of magneto-optical traps (MOT) for neutral atoms. In such systems, all forces (confinement and interaction) are mediated by light. The most obvious signature of longrange interactions is the dependence of the MOT size with number  of trapped atoms. We study in detail the scaling laws for the MOT size versus  , in a range of large N '3> 10^. We then show that the competition between confinement and repulsive interaction can lead, for  large, to a new dynamical instability characterized by spontaneous, self-sustained oscillations of the MOT. This instability is investigated experimentally and theoretically, using both a simple analytical model and a more refined numerical one.
We have observed self-sustained radial oscillations in a large magneto-optical trap, containing up to 10(10) Rb85 atoms. This instability is due to the competition between the confining force of the magneto-optical trap and the repulsive interaction associated with multiple scattering of light inside the cold atomic cloud. A simple analytical model allows us to formulate a criterion for the instability threshold, in fair agreement with our observations. This criterion shows that large numbers of trapped atoms N>10(9) are required to observe this unstable behavior.