671 research outputs found
Control of the temporal and polarization response of a multimode fiber
Control of the spatial and temporal properties of light propagating in
disordered media have been demonstrated over the last decade using spatial
light modulators. Most of the previous studies demonstrated spatial focusing to
the speckle grain size, and manipulation of the temporal properties of the
achieved focus. In this work, we demonstrate temporal control of the total
impulse response integrated over all the spatial and polarization modes
propagating through a multimode fiber. We notably demonstrate a global
enhancement of light intensity at a chosen arrival time, as well as attenuating
light intensity at an arbitrary delay. We also demonstrate the full
polarization control of such engineered states and a multiple control at
different delay times, which opens interesting perspectives for non-linear
imaging through complex systems and high power fiber lasers.Comment: 10 pages including main and supplemental documents. 5 figures in the
main manuscript, 4 figures in the supplementa
Cosmological Equations for a Thick Brane
Generalized Friedmann equations governing the cosmological evolution inside a
thick brane embedded in a five-dimensional Anti-de Sitter spacetime are
derived. These equations are written in terms of four-dimensional effective
brane quantities obtained by integrating, along the fifth dimension, over the
brane thickness. In the case of a Randall-Sundrum type cosmology, different
limits of these effective quantities are considered yielding cosmological
equations which interpolate between the thin brane limit (governed by
unconventional brane cosmology), and the opposite limit of an ``infinite''
brane thickness corresponding to the familiar Kaluza-Klein approach. In the
more restrictive case of a Minkowski bulk, it is shown that no effective
four-dimensional reduction is possible in the regimes where the brane thickness
is not small enough.Comment: 23 pages, Latex, 2 figure
Linear Amplifier Breakdown and Concentration Properties of a Gaussian Field Given that its -Norm is Large
In the context of linear amplification for systems driven by the square of a
Gaussian noise, we investigate the realizations of a Gaussian field in the
limit where its -norm is large. Concentration onto the eigenspace
associated with the largest eigenvalue of the covariance of the field is
proved. When the covariance is trace class, the concentration is in probability
for the -norm. A stronger concentration, in mean for the sup-norm, is
proved for a smaller class of Gaussian fields, and an example of a field
belonging to that class is given. A possible connection with Bose-Einstein
condensation is briefly discussed.Comment: REVTeX file, 11 pages, 1 added paragraph in the introduction, 2 added
references, minor modifications in the text and abstract, submitted to J.
Stat. Phy
Note on a diffraction-amplification problem
We investigate the solution of the equation \partial_t E(x,t)-iD\partial_x^2
E(x,t)= \lambda |S(x,t)|^2 E(x,t)$, for x in a circle and S(x,t) a Gaussian
stochastic field with a covariance of a particular form. It is shown that the
coupling \lambda_c at which diverges for t>=1 (in suitable units), is
always less or equal for D>0 than D=0.Comment: REVTeX file, 8 pages, submitted to Journal of Physics
Survival Probability of Random Walks and L\'evy Flights on a Semi-Infinite Line
We consider a one-dimensional random walk (RW) with a continuous and
symmetric jump distribution, , characterized by a L\'evy index , which includes standard random walks () and L\'evy flights
(). We study the survival probability, , representing the
probability that the RW stays non-negative up to step , starting initially
at . Our main focus is on the -dependence of for
large . We show that displays two distinct regimes as
varies: (i) for ("quantum regime"), the discreteness of the jump
process significantly alters the standard scaling behavior of and
(ii) for ("classical regime") the discrete-time nature of
the process is irrelevant and one recovers the standard scaling behavior (for
this corresponds to the standard Brownian scaling limit). The purpose
of this paper is to study how precisely the crossover in occurs
between the quantum and the classical regime as one increases .Comment: 20 pages, 3 figures, revised and accepted versio
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