Scaling behavior of a very large magneto-optical trap

Abstract

We investigate the scaling behavior of a very large magneto-optical trap (VLMOT) containing up to 1.4 × 10 11 Rb 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.

Towards a measurement of the Debye length in very large magneto-optical traps

“…In particular, it satisfactorily explains the important observation that the atomic density in a MOT has an upper limit (preventing for instance the initially sought Bose-Einstein condensation). It also predicts a size scaling L ∼ N ∼1/3 , which is observed with reasonable precision in the experiments [12,[44][45][46]. However other mechanisms can lead to an upper bound on the density, such as light assisted collisions or other short range interactions [7,9,43].…”

“…A basic model to describe atoms in a large MOT has then emerged, where atoms, beyond the friction and external trapping force, are subjected to two kinds of effective interaction forces: an effective Coulomb repulsion of [2], which is dominant, and an effective attraction, sometimes called shadow effect, first described in [3]. Even though the shortcomings of this model are well known (such as a too large optical depth, space dependent trapping parameters [4], sub-doppler mechanisms [5,6], light assisted collisions [7] and radiative escape [8,9] or hyperfine changing collisions [10,11]), its predictions on the size and the shape of the atomic clouds are in reasonable agreement with experiments on very large MOTs [12].…”

“…The experimental apparatus used in this work as been described in detail elsewhere [12]. 87 Rb atoms are loaded in a magneto-optical trap from a dilute roomtemperature vapour.…”

“…This is achieved by imaging the trapping light scattered by the atoms, known as "fluorescence" light, with a CCD camera. However, the spatial distribution of fluorescence light usually does not reflect that of the atomic density, because of multiple scattering [12]. To minimize this effect, we acquire the fluorescence image at a large detuning of −8Γ.…”

Collective effects in the radiation pressure force

“…In the steady state regime, multiple scattering induces an effective longrange, repulsive force, which increases the size of the magneto-optical trap (MOT) and limits its spatial density [1][2][3][4]. In time-resolved experiments, it leads to 'radiation trapping', i.e., a long lifetime of the light inside the sample [5][6][7].…”