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arXiv:2101.08088 · Journal of the Optical Society of America B

Frequency-comb-induced radiation pressure force in dense atomic clouds

Mateo Kruljac, Danijel Buhin, Domagoj Kovačić, Vjekoslav Vulić, Damir Aumiler, Ticijana Ban

Published Mar 28, 2022• 1 citation
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Abstract

We investigate the frequency-comb-induced radiation pressure force acting on a cloud of cold 87 R b atoms. Reduction and spectral broadening of the frequency comb force are observed as the cloud’s optical thickness is increased. Since the radiation pressure force is uniquely determined by light scattered by an atomic cloud, we discuss different scattering mechanisms and point to the shadow effect as the dominant mechanism affecting the FC-induced force in resonantly excited dense atomic clouds. Our results improve the understanding of the interaction of frequency comb light with many-atom ensembles, which is essential for novel frequency comb applications in simultaneous multispecies cooling and self-ordering, multimode quantum memories, and quantum computing.

DOI
10.1364/josab.449798

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We investigate the frequency-comb-induced radiation pressure force acting on a cloud of cold 87 R b atoms. Reduction and spectral broadening of the frequency comb force are observed as the cloud’s optical thickness is increased. Since the radiation pressure force is uniquely determined by light scattered by an atomic cloud, we discuss different scattering mechanisms and point to the shadow effect as the dominant mechanism affecting the FC-induced force in resonantly excited dense atomic clouds. Our results improve the understanding of the interaction of frequency comb light with many-atom ensembles, which is essential for novel frequency comb applications in simultaneous multispecies cooling and self-ordering, multimode quantum memories, and quantum computing.
We investigate the modifications of the frequency-comb-induced radiation pressure force on cold 87^{87}87Rb atoms that are induced by the collective effects. Collective effects include both coherent and incoherent contributions and depend on the optical thickness of the atomic cloud. We observe reduction and broadening of the comb-induced force when the cloud's optical thickness increases, and compare the measured results with predictions of a coherent model based on the timed Dicke state approach and an incoherent scattering model based on the shadow effect explained by Beer-Lambert law. Both models describe the experimental results well, indicating that an incoherent scattering approach is sufficient to explain the observed modifications of the comb-induced force even for larger optical thicknesses. The results support the analogy between the frequency comb and continuous-wave laser-atom interaction and thus pave the way toward novel frequency comb applications in laser cooling, quantum communication, and light-atom interfaces based on structured and disordered atomic systems.
We study the emergence of a collective optical response of a cold and dense 87Rb^{87}\mathrm{Rb}87Rb atomic cloud to a near-resonant low-intensity light when the atom number is gradually increased. Experimental observations are compared with microscopic stochastic simulations of recurrent scattering processes between the atoms that incorporate the atomic multilevel structure and the optical measurement setup. We analyze the optical response of an inhomogeneously broadened gas and find that the experimental observations of the resonance line shifts and the total collected scattered light intensity in cold atom clouds substantially deviate from those of thermal atomic ensembles, indicating strong light-induced resonant dipole-dipole interactions between the atoms. At high densities, the simulations also predict a significantly slower decay of light-induced excitations in cold than in thermal atom clouds. The role of dipole-dipole interactions is discussed in terms of resonant coupling examples and the collective radiative excitation eigenmodes of the system.
Cooperative scattering of light by an extended object such as an atomic ensemble or a dielectric sphere is fundamentally different from scattering from many pointlike scatterers such as single atoms. Homogeneous distributions tend to scatter cooperatively, whereas fluctuations of the density distribution increase the disorder and suppress cooperativity. In an atomic cloud, the amount of disorder can be tuned via the optical thickness, and its role can be studied via the radiation force exerted by the light on the atomic cloud. Monitoring cold (87)Rb atoms released from a magneto-optical trap, we present the first experimental signatures of radiation force reduction due to cooperative scattering. The results are in agreement with an analytical expression interpolating between the disorder and the cooperativity-dominated regimes.
We measure the coherent scattering of light by a cloud of laser-cooled atoms with a size comparable to the wavelength of light. By interfering a laser beam tuned near an atomic resonance with the field scattered by the atoms, we observe a resonance with a redshift, a broadening, and a saturation of the extinction for increasing atom numbers. We attribute these features to enhanced light-induced dipole-dipole interactions in a cold, dense atomic ensemble that result in a failure of standard predictions such as the "cooperative Lamb shift". The description of the atomic cloud by a mean-field model based on the Lorentz-Lorenz formula that ignores scattering events where light is scattered recurrently by the same atom and by a microscopic discrete dipole model that incorporates these effects lead to progressively closer agreement with the observations, despite remaining differences.
The present state of research on the cooling of atomic ensembles by means of laser radiation pressure is discussed. First the basic theory of resonance radiation pressure is reviewed. The effect of recoil on the drift and fluctuations of the momentum of an atom interacting with resonance radiation is examined. There is an analysis of the forces acting on an atom in some simple light fields: a traveling plane wave, a Gaussian beam, and counterpropagating Gaussian beams. The review then focuses on the monochromatization of atoms by resonance radiation pressure and related methods for laser cooling of atoms. The roles played by the radiation pressure force and by momentum diffusion in shaping the narrow velocity distributions of the ensemble of atoms are analyzed. The lowest temperature obtainable in the radiation cooling of atoms is estimated. The basic experimental methods for laser cooling of atomic beams are described. Problems of the three-dimensional cooling of atomic ensembles and problems of localizing cold atoms in light fields, steady-state magnetic fields, and electrostatic fields are discussed. The review concludes with a discussion of some applications of cold atoms in high-precision spectroscopy, frequency standards, and atomic physics.
We experimentally investigate the radiative force and laser-induced fluorescence (LIF) in cold rubidium atoms induced by pulse-train (frequency-comb) excitation. Three configurations are studied: (i) single-pulse-train excitation, (ii) two in-phase counterpropagating pulse trains, and (iii) two out-of-phase counterpropagating pulse trains. In all configurations, measured LIF is in agreement with calculations based on the optical Bloch equations. The observed forces in the first two configurations are in qualitative agreement with the model(s) used for calculating mechanical action of a pulse train on atoms; however, this is not the case for the third configuration. Possible resolution of the discrepancy is discussed.