53 research outputs found
Deterministic Weak Localization in Periodic Structures
The weak localization is found for perfect periodic structures exhibiting
deterministic classical diffusion. In particular, the velocity autocorrelation
function develops a universal quantum power law decay at 4 times Ehrenfest
time, following the classical stretched-exponential type decay. Such
deterministic weak localization is robust against weak enough randomness (e.g.,
quantum impurities). In the 1D and 2D cases, we argue that at the quantum limit
states localized in the Bravis cell are turned into Bloch states by quantum
tunnelling.Comment: 5 pages, 2 figure
Light transport in cold atoms and thermal decoherence
By using the coherent backscattering interference effect, we investigate
experimentally and theoretically how coherent transport of light inside a cold
atomic vapour is affected by the residual motion of atomic scatterers. As the
temperature of the atomic cloud increases, the interference contrast
dramatically decreases emphazising the role of motion-induced decoherence for
resonant scatterers even in the sub-Doppler regime of temperature. We derive
analytical expressions for the corresponding coherence time.Comment: 4 pages - submitted to Physical Review Letter
Supersymmetric field theory of local light diffusion in semi-infinite media
A supersymmetric field theory of light diffusion in semi-infinite disordered
media is presented. With the help of this technique we justify--at the
perturbative level--the local light diffusion proposed by Tiggelen, Lagendijk,
and Wiersma [Phys. Rev. Lett. \textbf{84}, 4333 (2000)], and show that the
coherent backscattering line shape of medium bar displays a crossover from
two-dimensional weak to quasi-one-dimensional strong localization.Comment: 14 pages, 1 figure. accepted for publication in Phys. Rev.
Coherent Backscattering of Ultracold Atoms
We report on the direct observation of coherent backscattering (CBS) of
ultracold atoms, in a quasi-two-dimensional configuration. Launching atoms with
a well-defined momentum in a laser speckle disordered potential, we follow the
progressive build up of the momentum scattering pattern, consisting of a ring
associated with multiple elastic scattering, and the CBS peak in the backward
direction. Monitoring the depletion of the initial momentum component and the
formation of the angular ring profile allows us to determine microscopic
transport quantities. The time resolved evolution of the CBS peak is studied
and is found a fair agreement with predictions, at long times as well as at
short times. The observation of CBS can be considered a direct signature of
coherence in quantum transport of particles in disordered media. It is
responsible for the so called weak localization phenomenon, which is the
precursor of Anderson localization.Comment: 5 pages, 4 figure
Hanle effect in coherent backscattering
We study the shape of the coherent backscattering (CBS) cone obtained when
resonant light illuminates a thick cloud of laser-cooled rubidium atoms in
presence of a homogenous magnetic field. We observe new magnetic
field-dependent anisotropies in the CBS signal. We show that the observed
behavior is due to the modification of the atomic radiation pattern by the
magnetic field (Hanle effect in the excited state).Comment: 4 pages, 3 figure
Coherent backscattering of light by atoms in the saturated regime
We present the first calculation of coherent backscattering with inelastic
scattering by saturated atoms. We consider the scattering of a
quasi-monochromatic laser pulse by two distant atoms in free space. By
restricting ourselves to scattering of two photons, we employ a perturbative
approach, valid up to second order in the incident laser intensity. The
backscattering enhancement factor is found to be smaller than two (after
excluding single scattering), indicating a loss of coherence between the doubly
scattered light emitted by both atoms. Since the undetected photon carries
information about the path of the detected photon, the coherence loss can be
explained by a which-path argument, in analogy with a double-slit experiment.Comment: 16 pages, 10 figure
Observation of coherent backscattering of light by cold atoms
Coherent backscattering (CBS) of light waves by a random medium is a
signature of interference effects in multiple scattering. This effect has been
studied in many systems ranging from white paint to biological tissues.
Recently, we have observed CBS from a sample of laser-cooled atoms, a
scattering medium with interesting new properties. In this paper we discuss
various effects, which have to be taken into account for a quantitative study
of coherent backscattering of light by cold atoms.Comment: 25 pages LaTex2e, 17 figures, submitted to J. Opt. B: Quant. Semicl.
Op
Instabilities of waves in nonlinear disordered media
We develop a self-consistent theory of temporal fluctuations of a speckle
pattern resulting from the multiple scattering of a coherent wave in a weakly
nonlinear disordered medium. The speckle pattern is shown to become unstable if
the nonlinearity exceeds a threshold value. The instability is due to a
feedback provided by the multiple scattering and manifests itself in
spontaneous fluctuations of the scattered intensity. The development of
instability is independent of the sign of nonlinearity.Comment: 6 pages, 2 PostScript figures, accepted to Phys. Rev. Let
Correlations in Transmission of Light through a Disordered Amplifying Medium
The angular and frequency correlation functions of the transmission
coefficient for light propagation through a strongly scattering amplifying
medium are considered. It is found that just as in the case of an elastic
scattering medium the correlation function consists of three terms. However,
the structure of the terms is rather different. Angular correlation has a
power-law decay and exhibits oscillations. There is no "memory effect" as in
the case of an elastic medium. Interaction between diffusion modes is strongly
enhanced near the lasing threshold. Frequency correlation scale decreases close
to the lasing threshold.
We also consider time correlations of the transmission in the case of
nonstationary inhomogeneities. We find short- and long-range time correlations.
The scale of the short-range correlation decreases, while the long-range
correlation scale becomes infinite near the threshold.Comment: 16 pages, 7 postscript figure
Coherent Backscattering of light in a magnetic field
This paper describes how coherent backscattering is altered by an external
magnetic field. In the theory presented, magneto-optical effects occur inside
Mie scatterers embedded in a non-magnetic medium. Unlike previous theories
based on point-like scatterers, the decrease of coherent backscattering is
obtained in leading order of the magnetic field using rigorous Mie theory. This
decrease is strongly enhanced in the proximity of resonances, which cause the
path length of the wave inside a scatterer to be increased. Also presented is a
novel analysis of the shape of the backscattering cone in a magnetic field.Comment: 27 pages, 5 figures, Revtex, to appear in Phys. Rev.
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