Experimental investigation of distributions of the off-diagonal elements of the scattering matrix and Wigner's K[over ̂] matrix for networks with broken time reversal invariance.

We present an extensive experimental study of the distributions of the real and imaginary parts of the off-diagonal elements of the scattering matrix S[over ̂] and the Wigner's reaction K[over ̂] matrix for open microwave networks with broken time (T) reversal invariance. Microwave Faraday circulators were applied in order to break T invariance. The experimental distributions of the real and imaginary parts of the off-diagonal entries of the scattering matrix S[over ̂] are compared with the theoretical predictions from the supersymmetry random matrix theory [A. Nock, S. Kumar, H.-J. Sommers, and T. Guhr, Ann. Phys. (NY) 342, 103 (2014)10.1016/j.aop.2013.11.006]. Furthermore, we show that the experimental results are in very good agreement with the recent predictions for the distributions of the real and imaginary parts of the off-diagonal elements of the Wigner's reaction K[over ̂] matrix obtained within the framework of the Gaussian unitary ensemble of random matrix theory [S. B. Fedeli and Y. V. Fyodorov, J. Phys. A: Math. Theor. 53, 165701 (2020)1751-811310.1088/1751-8121/ab73ab]. Both theories include losses as tunable parameters and are therefore well adapted to the experimental verification.

Observation of multifractality in Anderson localization of ultrasound.

We report the experimental observation of strong multifractality in wave functions below the Anderson localization transition in open three-dimensional elastic networks. Our results confirm the recently predicted symmetry of the multifractal exponents. We have discovered that the result of multifractal analysis of real data depends on the excitation scheme used in the experiment.

Observation of multiple scattering of kHz vibrations in a concrete structure and application to monitoring weak changes.

In this paper, we present two experimental studies of mechanical wave propagation in a concrete building around 1 kHz. The first experiment is devoted to the observation of the coherent backscattering enhancement, which demonstrates the presence of multiple diffractions in the late part of the wave records. An application of multiple diffraction and reverberations is proposed in a second experiment. Thanks to their sensitivity to weak changes of the medium, the late records are used to monitor weak change in concrete wave velocity induced by thermal variations. The velocity change measurements have a precision of deltac/c=10(-4). Such a precision is difficult to obtain with direct waves. This experiment is the first step to other applications like stress, damage, aging, or crack monitoring in concrete structures.

Relation between time reversal focusing and coherent backscattering in multiple scattering media: a diagrammatic approach.

In this paper, we revisit one-channel time reversal (TR) experiments through multiple scattering media in the framework of the multiple scattering theory. The hyperresolution and the self-averaging property are retrieved. The developed formalism leads to a deeper understanding of the role of the ladder and most-crossed diagrams in a TR experiment and also establishes the link between TR and coherent backscattering (CBS). Especially, we show that when the initial source and the time reversal point are at the same location, the time-reversed amplitude is twice higher. Surprisingly, this enhancement is due to the ladder diagrams and not to the most-crossed ones, contrary to CBS. These theoretical predictions are confirmed by experimental results. The experiments are performed with ultrasonic waves propagating through a random collection of parallel steel rods.

Extracting the Green function from diffuse, equipartitioned waves.

A ballistic pulse launched in a strongly scattering random medium becomes diffusive after a few mean-free times. In this regime of diffusive propagation there is a net flux of energy away from the source. Eventually the flux goes to zero, in the equipartitioned regime, in which the signal consists of equal amounts of energy propagating in all directions. In this regime the two-point, two-time correlation of the wave-field should equal the sum of the advanced and retarded Green functions associated with the average medium. We observe the emergence of the Green function from this correlation at about 9 mean-free times in a highly heterogeneous rock sample.

Light transport in chiral and magnetochiral random media.

We present a microscopic approach to study electromagnetic wave propagation in media with broken mirror symmetry. We introduce and calculate the transport mean free path l*C associated with the residual polarization of diffuse light in chiral systems. In chiral media subject to an external magnetic field B, all symmetry requirements exist to create a macroscopic "super" light current in the direction of B that persists even in the absence of a spatial photon density gradient. However, we show that such a current is identically zero in our model. We finally show the existence of a linear magnetotransmission in magnetochiral media.

Quasi-two-dimensional transfer of elastic waves.

A theory for multiple scattering of elastic waves is presented in a heterogeneous plate bounded by two flat free surfaces, whose horizontal size is infinite and whose transverse size is smaller than the mean free path of the waves. We derive a time-dependent, quasi-two-dimensional radiative transfer equation (i.e., two horizontal dimensions with a finite number of vertical mode) that describes the coupling of the eigenmodes of the layer (surface Rayleigh waves, shear horizontal waves, and Lamb waves). The fundamentally different element is that the traction-free boundary conditions are treated on the level of the wave equation, whereas at the same time elastic transfer can be considered over macroscopic horizontal distances. Expressions are found that relate the small-scale fluctuations to the lifetime of the modes and to their mode-coupling rates. We discuss the diffusion approximation that simplifies the mathematics of this model significantly, and which should apply at large lapse times. Finally, this model facilitates a study of coherent backscattering near the plate surface for different sources and for different detection configurations.