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Conference on Lasers and Electro-Optics

Coherent backscattering of light by cold atoms

Christian Miniatura, G. Labeyrie, Cord Axel Müller, J.-C. Bernard, Robin Kaiser

Published Aug 29, 2005• 2 citations
Publisher

Abstract

Summary form only given. A wave propagating in a strongly scattering medium experiences many scattering events which rapidly erase the memory of its initial direction on a typical scale length known as the transport mean free path. It is now known that interference can profoundly alter the wave transport, a spectacular manifestation being weak and strong localisation of waves. We shall briefly review the basics of coherent backscattering (CBS) and the results obtained with classical scatterers. We shall then describe our experiment using cold atoms and discuss the experimental data. The non trivial role of the atomic quantum internal structure in the enhancement reduction will be singled out in a simple situation and results of analytical calculations will be presented.

DOI
10.1109/iqec.2000.907953

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Summary form only given. A wave propagating in a strongly scattering medium experiences many scattering events which rapidly erase the memory of its initial direction on a typical scale length known as the transport mean free path. It is now known that interference can profoundly alter the wave transport, a spectacular manifestation being weak and strong localisation of waves. We shall briefly review the basics of coherent backscattering (CBS) and the results obtained with classical scatterers. We shall then describe our experiment using cold atoms and discuss the experimental data. The non trivial role of the atomic quantum internal structure in the enhancement reduction will be singled out in a simple situation and results of analytical calculations will be presented.
Since the work of Anderson on localization, interference effects for the propagation of a wave in the presence of disorder have been extensively studied, as exemplified in coherent backscattering (CBS) of light. In the multiple scattering of light by a disordered sample of thermal atoms, interference effects are usually washed out by the fast atomic motion. This is no longer true for cold atoms where CBS has recently been observed. However, the internal structure of the atoms strongly influences the interference properties. In this paper, we consider light scattering by an atomic dipole transition with arbitrary degeneracy and study its impact on coherent backscattering. We show that the interference contrast is strongly reduced. Assuming a uniform statistical distribution over internal degrees of freedom, we compute analytically the single- and double-scattering contributions to the intensity in the weak-localization regime. The so-called ladder and crossed diagrams are generalized to the case of atoms and permit to calculate enhancement factors and backscattering intensity profiles for polarized light and any closed atomic dipole transition.
Propagation in a random medium is often described by a random walk and a diffusion coefficient. However in the mesoscopic regime, where wave properties cannot be neglected, interferences alter the diffusion process leading in extreme situations to strong (Anderson) localisation. The recent observation of coherent backscattering of light by laser cooled atoms has opened the way to new studies in previously not accessible regimes. Here we present some of the questions addressed by these experiments, allowing for a more complete understanding of the physical processes involved in this quantum mesoscopic regime
Weak localization of light or coherent backscattering (CBS) of light is a phenomena in which partial waves traversing momentum-reversed (time-reversed) scattering paths interfere constructively in the backscattering direction leading to an enhancement of scattered light intensity. The enhancement of intensity is proportional to a reduction of light transported in the scattering medium. Thus the CBS signal is a measure of localization of light in the scattering medium. We are building an apparatus to observe weak localization of photons in a high density sample of ultra-cold atoms. First we slow the atoms from room temperature (300 m/s) to less than 10 cm/s in a magneto-optical trap (MOT) where the atom number density is 10 11 atoms/cm 3 . In order to increase the atomic density the atoms in the MOT are transferred to a far detuned microscopic dipole trap created by a laser focused to a waist of approximately 5m. We have successfully loaded the MOT and are now in the process of loading the high density microscopic dipole trap. Once this is achieved we will look at the CBS signal as a function of the atom number density and the scattering cross section. The goal of the project is to reach the Joffe-Regel region where mean free path length is about the wavelength of the scattering radiation.