Collective atomic scattering and motional effects in a dense coherent medium

Abstract

We investigate collective emission from coherently driven ultracold 88Sr atoms. We perform two sets of experiments using a strong and weak transition that are insensitive and sensitive, respectively, to atomic motion at 1 μK. We observe highly directional forward emission with a peak intensity that is enhanced, for the strong transition, by >103 compared with that in the transverse direction. This is accompanied by substantial broadening of spectral lines. For the weak transition, the forward enhancement is substantially reduced due to motion. Meanwhile, a density-dependent frequency shift of the weak transition (∼10% of the natural linewidth) is observed. In contrast, this shift is suppressed to <1% of the natural linewidth for the strong transition. Along the transverse direction, we observe strong polarization dependences of the fluorescence intensity and line broadening for both transitions. The measurements are reproduced with a theoretical model treating the atoms as coherent, interacting radiating dipoles.

Exact electrodynamics versus standard optics for a slab of cold dense gas

“…Our hypothesis is that there are two in principle independent dimensionless density parameters in the light-propagation problem, the on-resonance optical thickness 6πρh (6πρhk −2 in terms of full dimensional quantities) that is a MFT parameter, and the plain ρ (ρk −3 ) that governs the role of dipole-dipole interactions beyond MFT. In several recent experiments [3][4][5]10] the on-resonance optical thickness has proven to be the dimensionless parameter that governs the density dependence of the results. In contrast, our interpretation is that the nontrivial results in Fig.…”

“…Optical thickness is a characteristic dimensionless parameter of MFT, and while MFT remains valid, optical thickness may be expected to be the dimensionless parameter. Numerous theoretical analyses are phrased in terms of optical thickness, and a scaling with optical thickness has been demonstrated in recent experiments, e.g., [3][4][5]10]. It is not a surprise that one can observe superradiance even in standard optics, and that it scales with optical thickness; optical thickness makes optical resonances broader, whereupon the conventional wisdom about Fourier transformations automatically predicts shortening time scales.…”

“…The growing throughput of computers available to researchers is making such a plan practical. These methods, whether called classicalelectrodynamics simulations or coupled-dipole simulations, are now a routine theoretical tool [2,4,7,8,[14][15][16][17][18][19][20][21][22][23][24][25][26]. Closely related numerical techniques based on the analysis of the eigenstates of the coupled system of the light and the atoms [15,[27][28][29][30][31][32] or density matrices and quantum trajectories [33][34][35] are also widely used today.…”

“…This subject is obviously very old, but cold atomic samples afford an opportunity for experiments in unprecedented regimes and in unprecedented detail. Correspondingly, new experiments are emerging rapidly [1][2][3][4][5][6][7][8][9][10]. The measurements of the optical response have been dominated by trapped inhomogeneous atomic ensembles, but also those with atoms confined in a slab geometry are emerging in increasing numbers in both thermal vapor cells using a hot gas [11] and in the ultracold regime [12].…”

Collective effects in the radiation pressure force

“…Starting from an independent approach, cooperative scattering in the low-intensity (or 'single-photon') regime, related to Dicke states [16], has been investigated in the mid 2000s, first from a theoretical point of view [17], followed by experiments with presently ongoing efforts in many groups [18][19][20][21][22][23][24][25][26][27][28][29]. One result, which has been described with a cooperative scattering approach, has been the momentum transfer onto the center of mass of a cloud of cold atoms, measured via the radiation pres-scattering has been developed [33,34].…”