Damping-Driven Time Reversal for Waves.

Damping is usually associated with irreversibility. Here, we present a counterintuitive concept to achieve time reversal of waves propagating in a lossless medium using a transitory dissipation pulse. Applying a sudden and strong damping in a limited time generates a time-reversed wave. In the limit of a high damping shock, this amounts to "freezing" the initial wave by maintaining the wave amplitude while canceling its time derivative. The initial wave then splits in two counterpropagating waves with half of its amplitude and time evolutions in opposite directions. We implement this damping-based time reversal using phonon waves propagating in a lattice of interacting magnets placed on an air cushion. We show with computer simulations that this concept also applies to broadband time reversal in complex disordered systems.

Freeze-Dried Microfluidic Monodisperse Microbubbles as a New Generation of Ultrasound Contrast Agents.

We succeeded in freeze-drying monodisperse microbubbles without degrading their performance, that is, their monodispersity in size and echogenicity. We used microfluidic technology to generate cryoprotected highly monodisperse microbubbles (coefficient of variation [CV] <5%). By using a novel retrieval technique, we were able to freeze-dry the microbubbles and resuspend them without degradation, that is, keeping their size distribution narrow (CV <6%). Acoustic characterization performed in two geometries (a centimetric cell and a millichannel) revealed that the resuspended bubbles conserved the sharpness of the backscattered resonance peak, leading to CVs ranging between 5% and 10%, depending on the geometry. As currently observed with monodisperse bubbles, the peak amplitudes are one order of magnitude higher than those of commercial ultrasound contrast agents. Our work thus solves the question of storage and transportation of highly monodisperse bubbles. This work might open pathways toward novel clinical non-invasive measurements, such as local pressure, impossible to carry out with the existing commercial ultrasound contrast agents.

Drastic slowdown of the Rayleigh-like wave in unjammed granular suspensions.

We present an experimental investigation of Rayleigh-like wave propagation along the surface of a dense granular suspension. Using an ultrafast ultrasound scanner, we monitor the softening of the shear modulus via the Rayleigh-like wave velocity slowdown in the optically opaque medium as the driving amplitude increases. For such nonlinear behavior two regimes are found when increasingthe driving amplitude progressively: First, we observe a significant shear modulus weakening due to the microslip on the contact level without macroscopic rearrangements of grains. Second, there is a clear macroscopic plastic rearrangement accompanied by a modulus decrease up to 88%. A friction model is proposed to describe the interplay between nonlinear elasticity and plasticity, which highlights the crucial effect of contact slipping before contact breaking or loss. Investigation of this nonlinear Rayleigh-like wave may bridge the gap between two disjoint approaches for describing the dynamics near unjamming: linear elastic soft modes and nonlinear collisional shock.

Optimal spatiotemporal focusing through complex scattering media.

We present an alternative approach for spatiotemporal focusing through complex scattering media by wave front shaping. Using a nonlinear feedback signal to shape the incident pulsed wave front, we show that the limit of a spatiotemporal matched filter can be achieved; i.e., the wave amplitude at the intended time and focus position is maximized for a given input energy. It is exactly what is also achieved with time reversal. Demonstrated with ultrasound experiments, our method is generally applicable to all types of waves.

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.

Influence of correlations between scatterers on the attenuation of the coherent wave in a random medium.

Experimental measurements of the coherent wave transmission for ultrasonic waves propagating in water through a random set of scatterers (metallic rods) are presented. Though the densities are moderate (6% and 14%) the experimental results show that the mean-free path deviates from the classical first-order approximation due to the existence of correlations between scatterers. Theoretical results for the mean free path obtained from different approaches are compared to the experimental measurements. The best agreement is obtained with the second-order diagrammatic expansion of the self-energy.

Weak localization and time reversal of ultrasound in a rotational flow.

A one-channel time-reversal (TR) experiment is performed inside a rough reverberating cavity in the presence of a rotational flow. The amplitude of the TR wave is plotted versus the distance between the TR channel and the initial source: when they coincide, it exhibits a "time-reversal enhancement" (TRE). With no flow, the TRE is the same as the coherent backscattering enhancement (CBE). But contrary to CBE, the TRE peak is shown to be insensitive to the breaking down of reciprocity due to the flow. This new effect of weak localization is sustained by a diagrammatic approach.

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.

Time reversed wave propagation experiments in chaotic micro-structured cavities.

The elastic wave propagation in strongly scattering solid-state cavity consisting of a thin micro-patterned silicon wafer is studied experimentally. The chaotic behavior is induced by the irregular boundary of the cavity and/or by fabricating patterns of small holes in the wafer by laser machining. The pattern and hole size are designed with length scales matching the wavelength

Taking advantage of multiple scattering to communicate with time-reversal antennas.

We present an experimental demonstration showing that, contrary to first intuition, the more scattering a mesoscopic medium is, the more information can be conveyed through it. We used a multiple input-multiple output configuration: a multichannel ultrasonic time-reversal antenna is used to transmit random series of bits simultaneously to different receivers which were only a few wavelengths apart. Whereas the transmission is free of error when multiple scattering occurs in the propagation medium, the error rate is huge in a homogeneous medium.

Time reversal versus phase conjugation in a multiple scattering environment.

We present experimental results on the reversibility of ultrasound in a multiple scattering medium. An ultrasonic pulsed wave is transmitted from a point source to a 128-element receiving array through 2D samples with various thickness. The samples consist of random collections of parallel steel rods immersed in water. The scattered waves are recorded, time reversed and sent back into the medium. The time-reversed waves are converging back to their source and the quality of spatial and temporal focusing on the source is related to the second-order moments of the scattered wave (correlation) in time and in space. Experimental results show that it is possible to obtain a robust estimation of the correlations on a single realisation of disorder, taking advantage of the wide frequency bandwidth. The spatial resolution of the system is only limited by the correlation length of the scattered field, and no longer by diffraction. Moreover, successful time-reversal focusing using a single element instead of an array is possible, whereas a one-channel monochromatic phase conjugation fails. The efficiency of broad-band time reversal compared to monochromatic phase conjugation lies in the number of 'information grains' in the frequency bandwidth.