title: Collective atomic scattering and motional effects in a dense coherent medium.
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 1 μK. We observe highly directional forward emission with a peak intensity that is enhanced, for the strong transition, by >10(3) 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.
authors.affiliation: JILA, NIST and Department of Physics, University of Colorado, 440 UCB, Boulder, Colorado 80309, USA.
authors.forename: S L
authors.initials: SL
authors.lastname: Bromley
authors.affiliation: JILA, NIST and Department of Physics, University of Colorado, 440 UCB, Boulder, Colorado 80309, USA.
authors.forename: B
authors.initials: B
authors.lastname: Zhu
authors.affiliation: JILA, NIST and Department of Physics, University of Colorado, 440 UCB, Boulder, Colorado 80309, USA.
authors.forename: M
authors.initials: M
authors.lastname: Bishof
authors.affiliation: JILA, NIST and Department of Physics, University of Colorado, 440 UCB, Boulder, Colorado 80309, USA.
authors.forename: X
authors.initials: X
authors.lastname: Zhang
authors.affiliation: JILA, NIST and Department of Physics, University of Colorado, 440 UCB, Boulder, Colorado 80309, USA.
authors.forename: T
authors.initials: T
authors.lastname: Bothwell
authors.affiliation: JILA, NIST and Department of Physics, University of Colorado, 440 UCB, Boulder, Colorado 80309, USA.
authors.forename: J
authors.initials: J
authors.lastname: Schachenmayer
authors.affiliation: JILA, NIST and Department of Physics, University of Colorado, 440 UCB, Boulder, Colorado 80309, USA.
authors.forename: T L
authors.initials: TL
authors.lastname: Nicholson
authors.affiliation: Université de Nice Sophia Antipolis, CNRS, Institut Non-Linéaire de Nice, UMR 7335, F-06560 Valbonne, France.
authors.forename: R
authors.initials: R
authors.lastname: Kaiser
authors.affiliation: Department of Physics, University of Connecticut, Storrs, Connecticut 06269, USA.|Department of Physics, Harvard University, Cambridge, Massachussetts 02138, USA.
authors.forename: S F
authors.initials: SF
authors.lastname: Yelin
authors.affiliation: Department of Physics, Harvard University, Cambridge, Massachussetts 02138, USA.
authors.forename: M D
authors.initials: MD
authors.lastname: Lukin
authors.affiliation: JILA, NIST and Department of Physics, University of Colorado, 440 UCB, Boulder, Colorado 80309, USA.
authors.forename: A M
authors.initials: AM
authors.lastname: Rey
authors.affiliation: JILA, NIST and Department of Physics, University of Colorado, 440 UCB, Boulder, Colorado 80309, USA.
authors.forename: J
authors.initials: J
authors.lastname: Ye