109 research outputs found
Cooperativity in light scattering by cold atoms
A cloud of cold N two-level atoms driven by a resonant laser beam shows
cooperative effects both in the scattered radiation field and in the radiation
pressure force acting on the cloud center-of-mass. The induced dipoles
synchronize and the scattered light presents superradiant and/or subradiant
features. We present a quantum description of the process in terms of a master
equation for the atomic density matrix in the scalar, Born-Markov
approximations, reduced to the single-excitation limit. From a perturbative
approach for weak incident field, we derive from the master equation the
effective Hamiltonian, valid in the linear regime. We discuss the validity of
the driven timed Dicke ansatz and of a partial wave expansion for different
optical thicknesses and we give analytical expressions for the scattered
intensity and the radiation pressure force on the center of mass. We also
derive an expression for collective suppression of the atomic excitation and
the scattered light by these correlated dipoles.Comment: 15 pages, 8 figure
Intensity fluctuations signature of 3D Anderson localization of light
Apart from the difficulty of producing highly scattering samples, a major
challenge in the observation of Anderson localization of 3D light is
identifying an unambiguous signature of the phase transition in experimentally
feasible situations. In this letter we establish a clear correspondence between
the collapse of the conductance, the increase in intensity fluctuations at the
localization transition and the scaling analysis results based on the Thouless
number, thus connecting the macroscopic and microscopic approaches of
localization. Furthermore, the transition thus inferred is fully compatible
both with the results based on the eigenvalue analysis of the microscopic
description and with the effective-medium Ioffe-Regel criterion
Spatial and temporal localization of light in two dimensions
Quasi-resonant scattering of light in two dimensions can be described either
as a scalar or as a vectorial electromagnetic wave. Performing a scaling
analysis we observe in both cases long lived modes, yet only the scalar case
exhibits Anderson localized modes together with extremely long mode lifetimes.
We show that the localization length of these modes is influenced only by their
position, and not their lifetime. Investigating the reasons for the absence of
localization, it appears that both the coupling of several polarizations and
the presence of near-field terms are able to prevent long lifetimes and
Anderson localization.Comment: 5 pages, 4 figures and Supplementary Informatio
Role of disorder in super- and subradiance of cold atomic clouds
The presence of superradiance and subradiance in microscopic and mean-field
approaches to light scattering in atomic media is investigated. We show that
these phenomena are present in both descriptions, with only minor quantitative
differences, so neither rely on disorder. In particular, they are most
prominent in media with high resonant optical depth yet far-detuned light,
i.e.. in the single--scattering regime
Coherence effects in scattering order expansion of light by atomic clouds
We interpret cooperative scattering by a collection of cold atoms as a
multiple scattering process. Starting from microscopic equations describing the
response of atoms to a probe light beam, we represent the total scattered
field as an infinite series of multiple scattering events. As an application of
the method, we obtain analytical expressions of the coherent intensity in the
double scattering approximation for Gaussian density profiles. In particular,
we quantify the contributions of coherent backward and forward scattering.Comment: 10 pages, 6 figure
Quantum effects in the cooperative scattering of light by atomic clouds
Scattering of classical light by atomic clouds induces photon-mediated
effective long-range interactions between the atoms and leads to cooperative
effects even at low atomic densities. We introduce a novel simulation technique
that allows us to investigate the quantum regime of the dynamics of large
clouds of atoms. We show that the fluorescence spectrum of the cloud can be
used to probe genuine quantum cooperative effects. Signatures of these effects
are the occurrence, and the scaling behavior, of additional sidebands at twice
the frequency of the classical Mollow sidebands, as well as an asymmetry of the
Mollow triplet
- …