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Journal of Experimental and Theoretical Physics

Coherent backscattering under conditions of pulsed radiation trapping

V. M. Datsyuk, Igor M. Sokolov

Published May 1, 2006• 11 citations
Publisher

Abstract

Coherent backscattering of pulsed radiation emitted by optically dense atomic ensembles is considered. The diagrammatic technique is used for deriving analytic expressions for correlation functions of scattered light, which make it possible to take into account all main factors affecting the dynamics of the process, including the hyperfine and Zeeman structure of the ground and first excited states of atoms, polarization of probe radiation, the actual shape and size of an atomic cloud, its spatial inhomogeneity, motion of atoms, and angular-momentum polarization of atoms. On the basis of these relations, the time dependence of the total intensity and the dependence of enhancement factor of backscattered light on the pulse duration, type of polarization of the polarization system of observation, optical thickness of the scattering medium, and the carrier frequency of the pulse are investigated. The calculations are performed for an ensemble of rubidium-85 atoms cooled in magnetooptical traps.

DOI
10.1134/s1063776106050049

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Coherent backscattering of pulsed radiation emitted by optically dense atomic ensembles is considered. The diagrammatic technique is used for deriving analytic expressions for correlation functions of scattered light, which make it possible to take into account all main factors affecting the dynamics of the process, including the hyperfine and Zeeman structure of the ground and first excited states of atoms, polarization of probe radiation, the actual shape and size of an atomic cloud, its spatial inhomogeneity, motion of atoms, and angular-momentum polarization of atoms. On the basis of these relations, the time dependence of the total intensity and the dependence of enhancement factor of backscattered light on the pulse duration, type of polarization of the polarization system of observation, optical thickness of the scattering medium, and the carrier frequency of the pulse are investigated. The calculations are performed for an ensemble of rubidium-85 atoms cooled in magnetooptical traps.
Coherent backscattering of light by optically dense atomic ensembles is considered. The spectrum of scattered radiation is studied. The dynamics of the total intensity and enhancement factor is considered in the case of scattering of pulsed radiation. The spectral and temporal characteristics of coherent backscattering are analysed depending on the observation conditions. As an example, calculations are performed for an ensemble of 85 Rb atoms in a magnetooptical trap. It is shown that analysis of the correlation and dynamic properties of scattered radiation makes it possible to separate contributions from different orders of scattering and thereby to study the process of radiation trapping in dense media in more detail.
The coherent backscattering of light by an ensemble of cold atoms located in a magneto-optical trap is investigated theoretically. The dependence of the gain coefficient on the probe frequency is analyzed in a wide spectral range covering the entire hyperfine structure of the excited state. The calculation is performed for 85 Rb atoms. It is found that destructive interference can be observed at certain frequencies, which results in gain coefficients smaller than unity. The angular distribution of scattered light is investigated for corresponding frequencies and the dependence of the shape of the cone of destructive interference on the size of the atomic cloud and its optical thickness is analyzed.
We report the first observation of coherent backscattering of light from a sample of laser-cooled atoms. Rubidium 85 atoms are captured from a room-temperature vapor in a magneto-optical trap, and illuminated with a laser probe. The angular intensity distribution of the backscattered light is recorded on a cooled CCD detector. We observe a small enhancement factor in all polarization channels, including the helicity preserving channel where the enhancement is only 1.08. This surprising observation is attributed to the existence of contrast reduction processes involving the atom's internal structure.
Specific features of fluorescence of dense and cold nondegenerate atomic ensembles in an external constant magnetic field are analyzed theoretically. The angular distribution, polarization properties, as well as the spectral composition of fluorescence radiation are calculated. The time variation of these characteristics after the end of the excitation pulse is analyzed. The dependence of the properties of secondary radiation on the duration and carrier frequency of the pulse is investigated. It is shown that, for dense clouds in which the free path length of quasiresonance photons is commensurate with the interatomic distance, the magnetic field significantly modifies all the observable properties of the radiation. Under these conditions, the trapping time may increase by tens of times. Magnetic field enhances the effect of quantum beats observed on time scales commensurate with the lifetime of the excited states of atoms. For individual polarization channels, this field also intensifies the phenomenon of coherent backscattering (CBS). The phenomena found are explained by the effect of magnetic field on the character of resonance dipole–dipole interaction and, as a result, on the specific features of collective phenomena in dense atomic ensembles.
On the basis of a quantum microscopic approach we study the dynamics of the afterglow of a dilute Gaussian atomic ensemble excited by pulsed radiation. Taking into account the vector nature of the electromagnetic field we analyze in detail the angular and polarization distribution of single-photon superradiance of such an ensemble. The dependence of the angular distribution of superradiance on the length of the pulse and its carrier frequency as well as on the size and the shape of the atomic clouds is studied. We show that there is substantial dependence of the superradiant emission on the polarization and the direction of fluorescence. We observe essential peculiarities of superradiance in the region of the forward diffraction zone and in the area of the coherent backscattering cone. We demonstrate that there are directions for which the rate of fluorescence is several times more than the decay rate of the timed-Dicke state. We show also that single-photon superradiance can be excited by incoherent excitation when atomic polarization in the ensemble is absent. Besides a quantum microscopic approach, we analyze single-photon superradiance on the basis of the theory of incoherent multiple scattering in optically thick media (random walk theory). In the case of very short resonant and long nonresonant pulses we derive simple analytical expressions for the decay rate of single-photon superradiance for incoherent fluorescence in an arbitrary direction.