Collective atomic scattering and motional effects in a dense coherent medium

TL;DR

This study explores how ultracold strontium atoms collectively emit light, revealing effects of atomic motion and density on emission directionality, spectral broadening, and frequency shifts, supported by a coherent dipole model.

Contribution

It provides experimental and theoretical insights into collective atomic emission, highlighting the influence of atomic motion and density in dense, coherently driven ultracold atomic media.

Findings

Strong forward emission with >1000x intensity enhancement for the strong transition.

Density-dependent frequency shift observed (~10% of natural linewidth).

Spectral line broadening and polarization effects depend on atomic motion and transition strength.

Abstract

We investigate collective emission from coherently driven ultracold $ ^{88} $ Sr atoms. We perform two sets of experiments, using a strong and weak transition that are insensitive and sensitive, respectively, to atomic motion at one microKelvin. We observe highly directional forward emission with a peak intensity that is enhanced, for the strong transition, by > $ 10 ^3 $ compared to 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.