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

La résistance des rites traditionnels dans l’Afrique moderne

Contrairement à certains pronostics, la modernité ne fait pas disparaître les rites traditionnels en Afrique. À partir du cas significatif d’un médecin camerounais qui se fait soigner « à l’indigène », E. de Rosny réfléchit sur les causes de la vitalité de la vie rituelle dans les sociétés africaines aujourd’hui. Il montre qu’en deçà de raisons historiques, thérapeutiques et même culturelles, il convient de se placer au niveau anthropologique pour comprendre cette longévité. C’est dans sa conception de l’homme centrée sur le corps et la relation au groupe que l’Africain puise son besoin persistant d’une vie rituelle. Dans ce domaine, il peut donc mieux résister qu’un Occidental

Comment on ``Multiple scattering in a reflecting cavity: Application to fish counting in a tank [J. Acoust. Soc. Am. {\bf 109}, 2587-2597 (2001)]"

This paper presents a comment on the recent work on fish counting in a tank (J. Acoust. Soc. Am. 109, 2587-2597 (2001)). It is pointed out that there are ambiguities with the counting method.Comment: 2 page

MIMO feedback and application to detection

International audienceThe feedback effect is well known but unwanted, by sound engineers. It results from a feedback loop between a microphone and a loudspeaker. Recently, it has been shown that we can take benefit of this effect to estimate with a very good accuracy some parameters such as sound speed. More recently, some experimental results has shown the effect of a local perturbation on the top of an ultrasonic wavewguide. Here we generalize the concept to MIMO (Multiple Input Multiple Output) system where the feedback effect occurs between an array of emitters and an array of receivers. We propose to model the MIMO feedback effect by introducing a feedback matrix. Thanks to the singular decomposition of this matrix times the transfert matrix, we are able to predict the spatial dependence of the feedback effect either on the emitting array and on the receiving array. In a second part, we present experimental results that are obtained with an array of about 10 microphones and an array of about 10 loudspeakers. Several feedback matrices have been tested. One of them is inspired from time reversal. We have applied this technique to detect a person who goes across this acoustic barrier

Turning Optical Complex Media into Universal Reconfigurable Linear Operators by Wavefront Shaping

Performing linear operations using optical devices is a crucial building block in many fields ranging from telecommunication to optical analogue computation and machine learning. For many of these applications, key requirements are robustness to fabrication inaccuracies and reconfigurability. Current designs of custom-tailored photonic devices or coherent photonic circuits only partially satisfy these needs. Here, we propose a way to perform linear operations by using complex optical media such as multimode fibers or thin scattering layers as a computational platform driven by wavefront shaping. Given a large random transmission matrix (TM) representing light propagation in such a medium, we can extract a desired smaller linear operator by finding suitable input and output projectors. We discuss fundamental upper bounds on the size of the linear transformations our approach can achieve and provide an experimental demonstration. For the latter, first we retrieve the complex medium's TM with a non-interferometric phase retrieval method. Then, we take advantage of the large number of degrees of freedom to find input wavefronts using a Spatial Light Modulator (SLM) that cause the system, composed of the SLM and the complex medium, to act as a desired complex-valued linear operator on the optical field. We experimentally build several $16\times16$ complex-valued operators, and are able to switch from one to another at will. Our technique offers the prospect of reconfigurable, robust and easy-to-fabricate linear optical analogue computation units