86 research outputs found
Green's function retrieval and fluctuations of cross density of states in multiple scattering media
In this article we derive the average and the variance of the
cross-correlation of a noise wavefield. The noise cross-correlation function
(NCF) is widely used to passively estimate the Green's function between two
probes and is proportional to the cross density of states (CDOS) in photonic
and plasmonic systems. We first explain from the ladder approximation how the
diffusion halo plays the role of secondary sources to reconstruct the mean
Green's function. We then show that fluctuations of NCF are governed by several
non-Gaussian correlations. An infinite-range NCF correlation dominates CDOS
fluctuations and proves that NCF is not a self averaging quantity with respect
to the plurality of noise sources. The link between these correlations and the
intensity ones is highlighted. These results are supported by numerical
simulations and are of importance for passive imaging applications and material
science.Comment: 5 pages, 4 figures, 1 supplemental materia
Statistics and control of waves in disordered media
Fundamental concepts in the quasi-one-dimensional geometry of disordered
wires and random waveguides in which ideas of scaling and the transmission
matrix were first introduced are reviewed. We discuss the use of the
transmission matrix to describe the scaling, fluctuations, delay time, density
of states, and control of waves propagating through and within disordered
systems. Microwave measurements, random matrix theory calculations, and
computer simulations are employed to study the statistics of transmission and
focusing in single samples and the scaling of the probability distribution of
transmission and transmittance in random ensembles. Finally, we explore the
disposition of the energy density of transmission eigenchannels inside random
media.Comment: 28 Pages, 18 Figures (Review
Transmission statistics and focusing in single disordered samples
We show in microwave experiments and random matrix calculations that in
samples with a large number of channels the statistics of transmission for
different incident channels relative to the average transmission is determined
by a single parameter, the participation number of the eigenvalues of the
transmission matrix, M. Its inverse, M-1, is equal to the variance of relative
total transmission of the sample, while the contrast in maximal focusing is
equal to M. The distribution of relative total transmission changes from
Gaussian to negative exponential over the range in which M-1 changes from 0 to
1. This provides a framework for transmission and imaging in single samples.Comment: 9 pages, 4 figure
Transmission eigenchannels and the densities of states of random media
We show in microwave measurements and computer simulations that the
contribution of each eigenchannel of the transmission matrix to the density of
states (DOS) is the derivative with angular frequency of a composite phase
shift. The accuracy of the measurement of the DOS determined from transmission
eigenchannels is confirmed by the agreement with the DOS found from the
decomposition of the field into modes. The distribution of the DOS, which
underlies the Thouless number, is substantially broadened in the Anderson
localization transition. We find a crossover from constant to exponential
scaling of fluctuations of the DOS normalized by its average value. These
results illuminate the relationships between scattering, stored energy and
dynamics in complex media.Comment: Supplementary Information included at the end of the documen
Focusing and amplification of electromagnetic waves by time reversal in an leaky reverberation chamber
Free Brick1 Is a Trimeric Precursor in the Assembly of a Functional Wave Complex
Background: The Wave complex activates the Arp2/3 complex, inducing actin polymerization in lamellipodia and membrane ruffles. The Wave complex is composed of five subunits, the smallest of which, Brick1/Hspc300 (Brk1), is the least characterized. We previously reported that, unlike the other subunits, Brk1 also exists as a free form. Principal Findings: Here we report that this free form of Brk1 is composed of homotrimers. Using a novel assay in which purified free Brk1 is electroporated into HeLa cells, we were able to follow its biochemical fate in cells and to show that free Brk1 becomes incorporated into the Wave complex. Importantly, incorporation of free Brk1 into the Wave complex was blocked upon inhibition of protein synthesis and incorporated Brk1 was found to associate preferentially with neosynthesized subunits. Brk1 depleted HeLa cells were found to bleb, as were Nap1, Wave2 or ARPC2 depleted cells, suggesting that this blebbing phenotype of Brk1 depleted cells is due to an impairment of the Wave complex function rather than a specific function of free Brk1. Blebs of Brk1 depleted cells were emitted at sites where lamellipodia and membrane ruffles were normally emitted. In Brk1 depleted cells, the electroporation of free Brk1 was sufficient to restore Wave complex assembly and to rescue the blebbing phenotype. Conclusion: Together these results establish that the free form of Brk1 is an essential precursor in the assembly of
Application du retournement temporel en micro-ondes Ă l'amplification d'impulsions et l'imagerie
Time Reversal (TR) methods are applied in microwaves to pulse amplification and imaging. First, we use a reverberation chamber with an aperture on the front face and we take advantage of the pulse compression property of time reversal. High amplitude peaks are generated outside the chamber thanks to the long spreading time of the signals inside. Our device is auto-adaptive in position and in polarization. The second part of the manuscript deals theoretically and experimentally with the DORT method (decomposition of the TR operator). The method is first applied to characterize a dielectric cylinder and work out its parameters. Imaging of two close scatterers separated by a subwavelength distance is then considered. A criterion is especially extracted to deduce the noise level above which the resolution fails. Furthermore, we use the DORT method to track experimentally people behind a wall. The wave propagation inside the wall is taken into account to localize a human being. This last part leads to the study of the invariants of the TR operator when a pointlike target is moving during the acquisition of the transfer matrix. Eventually, we introduce the first wideband ambient noise cross-correlation experiment in microwaves. The crosscorrelation yields the Green's function between two antennas and allows the passive detection and localization of targets. The analogy with a TR process is developed. Passive people localization is also achieved with the narrow bandwidth signals emitted by a WIFI router.Les méthodes de retournement temporel (RT) en micro-ondes sont appliquées à l'amplification d'impulsions et à l'imagerie. Lors d'une première partie, une chambre réverbérante ouverte sur sa face avant permet d'engendrer un champ diffus tout en laissant s'échapper l'énergie afin de focaliser le champ par RT à l'extérieur. La compression spatiotemporelle après RT produit une impulsion de forte amplitude et confère au dispositif un caractère auto-adaptatif en position et en polarisation. La seconde partie traite expérimentalement et théoriquement de la méthode DORT (décomposition de l'opérateur de RT). Le cas d'un cylindre diélectrique est examiné dans le but de retrouver ses paramètres. L'imagerie de deux cibles séparées d'une distance sub-longueur d'onde est alors abordée. Un critère de résolution déterminant le niveau de bruit à partir duquel la résolution des cibles échoue est notamment extrait. La méthode est ensuite appliquée à la localisation de personnes mobiles derrière un mur. La possibilité de suivre un déplacement est illustrée en prenant en compte la propagation à l'intérieur du mur. L'influence du déplacement d'une cible ponctuelle pendant l'acquisition de la matrice de transfert sur les invariants de l'opérateur de RT est aussi examinée. Enfin, une technique d'imagerie passive fondée sur les corrélations du bruit ambiant est expérimentalement mise en évidence en micro-ondes. Suivant une analogie avec le RT, la corrélation de signaux de bruit large bande mène à la fonction de Green entre deux antennes et ainsi à la localisation de cibles. La localisation passive d'une personne est aussi abordée en « bande étroite » grâce à l'émission d'une borne WIFI
- …