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Physical Review Letters

Observation of intensity-dependent fluorescence line-shape asymmetry for two-level atoms in a standing-wave field

Mara Goff Prentiss, Shaoul Ezekiel

Published Jan 6, 1986• 36 citations
Publisher

Abstract

Intensity-dependent fluorescence line-shape asymmetries have been observed for two-level atoms in a standing-wave excitation field. These asymmetries are attributed to the force experienced by the induced atomic dipoles in the field gradient of the standing wave. Preliminary calculations support the experimental observations.
DOI
10.1103/physrevlett.56.46

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Resonance fluorescence of two two-level atoms driven by a coherent laser field propagating parallel to the interatomic axis is examined, taking into account the dipole-dipole interaction. We show that the number of peaks in the fluorescence intensity depends on the direction of observation with respect to the interatomic axis and on whether the system is driven by a standing-wave or a running-wave laser field. When the system is driven by the standing-wave field the fluorescence intensity shows two peaks and a dip for an observation direction perpendicular to the interatomic axis and three peaks in the direction parallel to it. When the system is driven by the running-wave field only two peaks are observed for the perpendicular direction. We explain these features in terms of the collective states of the system and show that the directional effect can be attributed to the fact that the antisymmetric state of the system does not radiate in the direction perpendicular to the interatomic axis. Photon statistics also demonstrate the directional effect for the standing-wave field. Frequency regimes are found where sub-Poissonian photon statistics occur. Depending on the direction of the observation, a state close to a Fock state can occur for one or two frequencies of the driving field, corresponding to zero-photon-number fluctuations or maximum sub-Poissonian behavior. The fluorescence spectrum is found to depend on the direction of observation and the interatomic separation. For small interatomic separations and a standing-wave driving field the spectrum exhibits seven lines for the perpendicular direction and 13 lines in the direction parallel to the interatomic axis. For the running-wave field the spectrum shows 13 lines independent of the direction of observation. Moreover, we show that for particular interatomic separations dark regimes appear in the spectrum. We discuss the effect of the dipole-dipole interaction on the dark states and explain the spectral features in terms of the dressed-atom model. We also consider the absorption spectrum of a weak probe beam illuminating the two-atom system. We show that apart from a weak dispersion structure, the spectrum shows large peaks as one of the atoms is moved to successive nodes of the field.
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The author has studied the effect of an adiabatically and near-adiabatically increasing field intensity on the resonance fluorescence spectrum of the two-level atom. There is an asymmetry in the spectrum for off-resonance excitation and that this is due to the fact that the field turn-on repopulates the dressed state which has been depopulated by spontaneous emission. Also found are the conditions under which the temporal shape of the field turn-on may be found in a simple way. Corrections to this shape due to atomic recoil are discussed, and found to be small under typical experimental conditions.
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