Arnaud Tourin

Focusing beyond the diffraction limit with far-field time reversal.

We present an approach for subwavelength focusing of microwaves using both a time-reversal mirror placed in the far field and a random distribution of scatterers placed in the near field of the focusing point. The far-field time-reversal mirror is used to build the time-reversed wave field, which interacts with the random medium to regenerate not only the propagating waves but also the evanescent waves required to refocus below the diffraction limit. Focal spots as small as one-thirtieth of a wavelength are described. We present one example of an application to telecommunications, which shows enhancement of the information transmission rate by a factor of 3.

Recovering the Green’s function from field-field correlations in an open scattering medium (L)

The possibility of recovering the Green’s function from the field-field correlations of coda waves in an open multiple scattering medium is investigated. The argument is based on fundamental symmetries of reciprocity, time-reversal invariance, and the Helmholtz–Kirchhoff theorem. A criterion is defined, indicating how sources should be placed inside an open medium in order to recover the Green’s function between two passive receivers. The case of noise sources is also discussed. Numerical experiments of ultrasonic wave propagation in a multiple scattering medium are presented to support the argument.The possibility of recovering the Green’s function from the field-field correlations of coda waves in an open multiple scattering medium is investigated. The argument is based on fundamental symmetries of reciprocity, time-reversal invariance, and the Helmholtz–Kirchhoff theorem. A criterion is defined, indicating how sources should be placed inside an open medium in order to recover the Green’s function between two passive receivers. The case of noise sources is also discussed. Numerical experiments of ultrasonic wave propagation in a multiple scattering medium are presented to support the argument.

ULTRASONIC PULSE COMPRESSION WITH ONE-BIT TIME REVERSAL THROUGH MULTIPLE SCATTERING

We present experimental results that demonstrate the feasibility of one-bit time reversal even through high-order multiple scattering. A short ultrasonic pulse is transmitted and propagates through a random set of steel rods. The scattered waves are recorded on a 128-channel array, time reversed, and retransmitted through the same medium. The time-reversal mirror takes advantage of multiple scattering to compress the scattered waves into a pulse and focus it back onto the source. Paradoxically, we show that the results are even better when the scattered signals are digitized over one bit. Both temporal and spatial resolutions remain unchanged, while the compressed pulse is amplified by 12 dB, and the signal-to-noise ratio is lowered by 1.2 dB. A statistical model is developed, and its predictions are found to be in good agreement with the experimental results.

Time‐reversal imaging of seismic sources and application to the great Sumatra earthquake

The increasing power of computers and numerical methods (like spectral element methods) allows continuously improving modelization of the propagation of seismic waves in heterogeneous media and the development of new applications in particular time reversal in the three-dimensional Earth. The concept of time-reversal (hereafter referred to as TR) was previously successfully applied for acoustic waves in many fields like medical imaging, underwater acoustics and non destructive testing. We present here the first application at the global scale of TR with associated reverse movies of seismic waves propagation by sending back long period time-reversed seismograms. We show that seismic wave energy is refocused at the right location and the right time of the earthquake. When TR is applied to the Sumatra-Andaman earthquake (26 Dec. 2004), the migration of the rupture from the south towards the north is retrieved. Therefore, TR is potentially interesting for constraining the spatio-temporal history of complex earthquakes.

Time reversal of wideband microwaves

In this letter, time reversal is applied to wideband electromagnetic waves in a reverberant room. To that end a multiantenna time reversal mirror (TRM) has been built. A 150MHz bandwidth pulse at a central frequency of 2.45GHz is radiated by a monopolar antenna, spread in time due to reverberation, recorded at the TRM, time reversed, and retransmitted. The time-reversed wave converges back to its source and focus in both time and space. The time compression is studied versus the number of antennas in the TRM and its bandwidth. The focal spot is also measured thanks to an eight-channel receiving array.

Imaging of Complex Media with Acoustic and Seismic Waves

Time-Reversal Invariance and the Relation between Wave Chaos and Classical Chaos.- Acoustic Time-Reversal Mirrors.- Ocean Acoustics, Matched-Field Processing and Phase Conjugation.- Time Reversal, Focusing and Exact Inverse Scattering.- Detection and Imaging in Complex Media with the D.O.R.T. Method.- Ultrasound Imaging and Its Modeling.- Nondestructive Acoustic Imaging Techniques.- Seismic Anisotropy Tomography.- Elastic-Wave Propagation in Random Polycrystals: Fundamentals and Application to Nondestructive Evaluation.- Imaging the Viscoelastic Properties of Tissue.- Estimation of Complex-Valued Stiffness Using Acoustic Waves Measured with Magnetic Resonance.- A New Approach for Traveltime Tomography and Migration Without Ray Tracing.- Simple Models in the Mechanics of Earthquake Rupture.

Design and characterization of bubble phononic crystals

We report the practical realization of phononic crystals with gas inclusions, using soft lithography techniques. Ultrasonic experiments from 0.3 to 5 MHz confirm the existence of deep and wide minima of transmission through the crystal. We show that the first gap is due to the combined effects of Bragg reflections and bubble resonances. We propose a simple layered model that gives a reasonable prediction of the ultrasonic transmission.

Multiple scattering of sound

Abstract We present a topical review which summarizes the main contributions to ‘multiple scattering of acoustic and elastic waves’ including the most recent advances. The review is divided into five main parts. In the first part, the effects of multiple scattering on ultrasonic propagation are illustrated on the basis of three experimental examples. In the second and third parts, we present the two possible descriptions for the propagation of an acoustic wave in a random medium. The first one is based on the study of the coherent wave, i.e. the wave amplitude averaged over disorder, whereas the second one deals with the propagation of the incoherent intensity, i.e. the intensity averaged over disorder. We especially insist on the microscopic basis for the phenomenological radiative transfer equation and show how it can be solved in the diffusion approximation. The theory is illustrated with experimental results obtained on a two-dimensional multiple-scattering prototype made of thousands of steel rods randomly distributed and immersed in water. In the fourth part, we present experimental evidence that the diffusion equation fails in describing all the aspects of the propagation of an acoustic wave in a random medium: e.g. the coherent backscattering effect recently observed for ultrasonic waves. We show that this effect arises as a consequence of reciprocity. Finally, in the fifth part, we discuss another property which is not taken into account in the radiative transfer theory: the reversibility of an acoustic wave propagating in a disordered medium.

Limits of time-reversal focusing through multiple scattering: Long-range correlation

Experimental results of time-reversal focusing in a high-order multiple scattering medium are presented and compared to theoretical predictions based on a statistical model. The medium consists of a random collection of parallel steel rods. An ultrasonic source (3.2 MHz) transmits a pulse that undergoes multiple scattering and is recorded on an array. The time-reversed waves are sent by the array back to the source through the scattering medium. The quality of temporal focusing is very well predicted by a simple statistical model. However, for thicker samples, persistent temporal side-lobes appear. We interpret these side-lobes as a consequence of the growing number of crossing paths in the sample due to high-order multiple scattering. As to spatial focusing, the resolution is practically independent from the arrays aperture. With a 16-element array, the resolution was found to be 30 times finer than in a homogeneous medium. Resolutions of the order of the wavelength (0.5 mm) were attained. These results are discussed in relation with the statistical properties of time-reversal mirrors in a random medium.

Optimal spatiotemporal focusing through complex scattering media

We demonstrate, based on spatial and frequency resolved wave front shaping of ultrasound with a nonlinear feedback signal, how to achieve optimal spatiotemporal focusing through a complex scattering medium.