We report the first experimental demonstration of time-reversal focusing with electromagnetic waves. An antenna transmits a 1-micros electromagnetic pulse at a central frequency of 2.45 GHz in a high-Q cavity. Another antenna records the strongly reverberated signal. The time-reversed wave is built and transmitted back by the same antenna acting now as a time-reversal mirror. The wave is found to converge to its initial source and is compressed in time. The quality of focusing is determined by the frequency bandwidth and the spectral correlations of the field within the cavity.

We report the first experiments showing the reversibility of transient acoustic waves through highorder multiple scattering by means of an acoustic time-reversal mirror. A point source generates a pulse which scatters through 2000 steel rods immersed in water. The time-reversed waves are found to converge to their source and recover their original wave form, despite the high order of multiple scattering involved and the usual sensitivity to initial conditions of time-reversal processes. Surprisingly, the observed resolution was one-sixth of the theoretical limit for the mirror's aperture.

The objective of this paper is to show that time reversal invariance can be exploited in acoustics to create a variety of useful instruments as well as elegant experiments in pure physics.Section 1 is devoted to the description of time reversal cavities and mirrors together with a comparison between time reversal and phase conjugation. To illustrate these concepts, several experiments conducted in multiply scattering media, waveguides and chaotic cavities are presented in section 2. Applications of time reversal mirrors (TRMs) in hydrodynamics are then presented in section 3. Section 4 is devoted to the application of TRMs in pulse echo detection. A complete theory of the iterative time reversal mode is presented. It will be explained how this technique allows for focusing on different targets in a multi-target medium. Another application of pulse echo TRMs is presented in this section: how to achieve resonance in an elastic target? Section 5 explores the medical applications of TRMs in ultrasonic imaging, lithotripsy and hyperthermia and section 6 shows the promising applications of TRMs in nondestructive testing of solid samples.

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.Wave propagation in a multiple scattering or reverberating environment has been a subject of interest in a wide variety of domains ranging from solid state physics to optics or acoustics. Ultrasound is particularly interesting because it allows a direct measurement of the field fluctuations, both in amplitude and in phase. In connection with this, a remarkable work by Weaver and Lobkis 1-3 recently showed that the Green's function between two points could be recovered from the field-field correlation of a diffuse ultrasonic field. This amounts to doing ''ultrasonics without a source'' since they showed that thermal noise could be used instead of a direct pulse/echo measurement between the two points. The experiment was carried out in an aluminum block, and the theoretical analysis was based on discrete modal expansion of the field, with random modal amplitudes. Applications are promising: it would be possible to recover the Green's function of a complex medium just by correlating diffuse fields received on passive sensors ͑application to shallow water ocean acoustics, where the field is not diffuse but propagates in a wave guide, was also evoked 4 ͒.However, the basic assumption in the theoretical analysis is that the medium is closed and free of absorption. In a real medium, absorption will tend to cut out the longest scattering ͑or reverberating͒ paths, and discrete modes will not be resolved any more. Similar problems are expected if the medium is open rather than closed ͑actually, in an open medium, the fluctuation-dissipation theorem 3 establishes the result, as long as the field is diffuse in the thermal sense͒. The aim of this letter is to examine whether the Green's function can still be recovered from the correlations of an ultrasonic wave field in an open scattering medium, when a discrete expansion on orthogonal modes is no longer relevant and the field is not thermally diffuse.To that end, we present 2-D numerical experiments of acoustic scattering on rigid inclusions randomly located either in a closed cavity or in a open medium. The wave equation is solved by a finite differences simulation ͑centered scheme͒; the boundary conditions is implemented following Collino's work. 5 Naturally, a finite difference scheme shows numerical dispersion. However, the essential point is that the fundamental symmetries of reciprocity and time reversal still hold in the numerical experiments.To begin with, let us consider...

Active waveguide Green's function estimation with application to time-reversal focusing without a probe source in a range-independent waveguide

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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.

The singular values distribution of the propagation operator in a random medium is investigated in a backscattering configuration. Experiments are carried out with pulsed ultrasonic waves around 3 MHz, using an array of transducers. Coherent backscattering and field correlations are taken into account. Interestingly, the distribution of singular values shows a dramatically different behavior in the single and multiple-scattering regimes. Based on a matrix separation of single and multiple-scattered waves, an experimental illustration of imaging through a highly scattering slab is presented.