Coherent backscattering of light by cold atoms: Theory meets experiment

Abstract

Coherent backscattering (CBS) of quasi-resonant light by cold atoms presents some specific features due to the internal structure of the atomic scatterers. We present the first quantitative comparison between the experimentally observed CBS cones and Monte-Carlo calculations which take into account the shape of the atomic cloud as well as the internal atomic structure.PACS numbers: 42.25. Dd, 32.80.Pj, 05.60.Gg When light is elastically scattered off an optically thick disordered medium, the interference between all partial waves produces strong angular fluctuations of the intensity distribution (speckle pattern). Averaging over the positions of the scatterers washes out interferences and produces a smooth reflected diffuse intensity, except around backscattering where it is enhanced. This coherent backscattering effect (CBS), originates from a two-wave interference, namely the interference between waves traveling along the same scattering paths but in reverse order [1,2]. This interference is constructive at exact backscattering and only survives, after averaging, in a narrow angular range ∆θ ≃ 1/kℓ around it (k is the light wave-number and ℓ the scattering mean free path). The CBS enhancement factor measures the ratio of the maximum intensity (measured at backscattering) to the background intensity measured at angles θ ≫ 1/kℓ. Because it originates from a two-wave interference, the enhancement factor is bounded by 2.For vector waves like light, either linear or circular polarization can be considered. This leads to four different polarization channels in scattering experiments: lin lin where both the incoming and outgoing photons are linearly polarized along the same axes, lin ⊥ lin where they are linearly polarized along orthogonal axes, h h where they are both circularly polarized with the same helicity (because they propagate in opposite directions, they have opposite polarizations) and h ⊥ h where they are circularly polarized with opposite helicities, i.e. same polarization.For spherically-symmetric scatterers (and hence for point-dipole scatterers) and in the h h channel the CBS enhancement factor is equal to 2 [3] because the following conditions are met: (i) All scattering paths have a distinct reverse counterpart; (ii) The two paths in each pair contribute with the same amplitude and phase, so that a maximum interference contrast is guaranteed. Condition (i) is not true for single scattering paths which thus do not contribute to CBS. In general, this implies an enhancement factor slightly smaller than 2. However, for spherically-symmetric scatterers, there is no single scattering signal in the backward direction in the lin ⊥ lin and h h channels. Condition (ii) is met in the parallel channels h h and lin lin as a general consequence of reciprocity [4] (which is equivalent to time-reversal symmetry in the absence of absorption). Therefore, an enhancement factor much smaller than 2 has only been observed in the perpendicular channels or by breaking the reciprocity with the help of an external magneti...

Spectrum of coherent backscattering of light by two atoms

“…As regards the atomic degeneracy mediating Raman processes accompanied by photon polarization flips, its dephasing role is nowadays very well understood, with quantitative accordance between theory [7,8,9] and experiment [5,10], for ensembles of Rb atoms.…”

Spectrum of coherently backscattered light from two atoms

“…A much weaker sensitivity of the quality of the CBS signal was observed for Rb atoms [9]. In this latter case, inelastic processes occur on degenerate transitions [10,11,12,13,14], while Sr atoms offer dipole transitions with a nondegenerate ground state [15].…”

Spectral dependence of coherent backscattering of light in a narrow-resonance atomic system

“…A general theory of the coherent backscattering process in an ultracold atomic gas has been developed recently by several groups [15,16,17]. The theoretical development essentially maintains the earlier conceptions of weak localization in the atomic scattering problem [22], and takes into account the influence of the optical depth and sample size on the character of the coherent backscattering cone.…”