381 research outputs found
Mutual information between reflected and transmitted speckle images
We study theoretically the mutual information between reflected and
transmitted speckle patterns produced by wave scattering from disordered media.
The mutual information between the two speckle images recorded on an array of N
detection points (pixels) takes the form of long-range intensity correlation
loops, that we evaluate explicitly as a function of the disorder strength and
the Thouless number g. Our analysis, supported by extensive numerical
simulations, reveals a competing effect of cross-sample and surface spatial
correlations. An optimal distance between pixels is proven to exist, that
enhances the mutual information by a factor Ng compared to the single-pixel
scenario.Comment: 5 pages, 4 figures, + S
Near-field interactions and non-universality in speckle patterns produced by a point source in a disordered medium
A point source in a disordered scattering medium generates a speckle pattern
with non-universal features, giving rise to the so-called C_0 correlation. We
analyze theoretically the relationship between the C_0 correlation and the
statistical fluctuations of the local density of states, based on simple
arguments of energy conservation. This derivation leads to a clear physical
interpretation of the C_0 correlation. Using exact numerical simulations, we
show that C_0 is essentially a correlation resulting from near-field
interactions. These interactions are responsible for the non-universality of
C_0, that confers to this correlation a huge potential for sensing and imaging
at the subwavelength scale in complex media
Cooperative emission of a pulse train in an optically thick scattering medium
An optically thick cold atomic cloud emits a coherent flash of light in the
forward direction when the phase of an incident probe field is abruptly
changed. Because of cooperativity, the duration of this phenomena can be much
shorter than the excited lifetime of a single atom. Repeating periodically the
abrupt phase jump, we generate a train of pulses with short repetition time,
high intensity contrast and high efficiency. In this regime, the emission is
fully governed by cooperativity even if the cloud is dilute.Comment: 5 pages, 3 figure
Coherent flash of light emitted by a cold atomic cloud
When a resonant laser sent on an optically thick cold atomic cloud is
abruptly switched off, a coherent flash of light is emitted in the forward
direction. This transient phenomenon is observed due to the highly resonant
character of the atomic scatterers. We analyze quantitatively its
spatio-temporal properties and show very good agreement with theoretical
predictions. Based on complementary experiments, the phase of the coherent
field is reconstructed without interferometric tools.Comment: Submitted to Phys. Rev. Let
Correlations between reflected and transmitted intensity patterns emerging from opaque disordered media
The propagation of monochromatic light through a scattering medium produces
speckle patterns in reflection and transmission, and the apparent randomness of
these patterns prevents direct imaging through thick turbid media. Yet, since
elastic multiple scattering is fundamentally a linear and deterministic
process, information is not lost but distributed among many degrees of freedom
that can be resolved and manipulated. Here we demonstrate experimentally that
the reflected and transmitted speckle patterns are correlated, even for opaque
media with thickness much larger than the transport mean free path, proving
that information survives the multiple scattering process and can be recovered.
The existence of mutual information between the two sides of a scattering
medium opens up new possibilities for the control of transmitted light without
any feedback from the target side, but using only information gathered from the
reflected speckle.Comment: 6 pages, 4 figure
Cooperative Emission of a Coherent Superflash of Light
We investigate the transient coherent transmission of light through an
optically thick cold stron-tium gas. We observe a coherent superflash just
after an abrupt probe extinction, with peak intensity more than three times the
incident one. We show that this coherent superflash is a direct signature of
the cooperative forward emission of the atoms. By engineering fast transient
phenomena on the incident field, we give a clear and simple picture of the
physical mechanisms at play.Comment: 4 Fig., 5 page
Radiative and non-radiative local density of states on disordered plasmonic films
We present numerical calculations of the Local Density of Optical States
(LDOS) in the near field of disordered plasmonic films. The calculations are
based on an integral volume method, that takes into account polarization and
retardation effects, and allows us to discriminate radiative and non-radiative
contributions to the LDOS. At short distance, the LDOS is dominated by
non-radiative channels, showing that changes in the spontaneous dynamics of
dipole emitters are driven by non-radiative coupling to plasmon modes. Maps of
radiative and non-radiative LDOS exhibit strong fluctuations, but with
substantially different spatial distributions
Spatial coherence in complex photonic and plasmonic systems
The concept of cross density of states characterizes the intrinsic spatial
coherence of complex photonic or plasmonic systems, independently on the
illumination conditions. Using this tool and the associated intrinsic coherence
length, we demonstrate unambiguously the spatial squeezing of eigenmodes on
disordered fractal metallic films, thus clarifying a basic issue in plasmonics
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