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.

https://iopscience.iop.org/article/10.1088/0034-4885/63/12/202/pdf

Time-reversed acoustics

A major challenge in neuroscience is to reliably activate individual neurons, particularly those in deeper brain regions. Current optogenetic approaches require invasive surgical procedures to deliver light of specific wavelengths to target cells to activate or silence them. Here, we demonstrate the use of low-pressure ultrasound as a non-invasive trigger to activate specific ultrasonically sensitized neurons in the nematode, Caenorhabditis elegans. We first show that wild-type animals are insensitive to low-pressure ultrasound and require gas-filled microbubbles to transduce the ultrasound wave. We find that neuron-specific misexpression of TRP-4, the pore-forming subunit of a mechanotransduction channel, sensitizes neurons to ultrasound stimulus, resulting in behavioural outputs. Furthermore, we use this approach to manipulate the function of sensory neurons and interneurons and identify a role for PVD sensory neurons in modifying locomotory behaviours. We suggest that this method can be broadly applied to manipulate cellular functions in vivo.

/pdf/sonogenetics-is-a-non-invasive-approach-to-activating-1pb6nxslsg.pdf

Sonogenetics is a non-invasive approach to activating neurons in Caenorhabditis elegans

In this paper, we present an overview of the time-reversal (TR) wireless paradigm for green Internet of Things (IoT) It is shown that the TR technique is a promising technique that focuses signal waves in both time and space domains The unique asymmetric architecture significantly reduces the cost of the terminal devices, the total number of which is expected to be very large for IoT The focusing effect of the TR technique can harvest the energy of all the multi-paths at the receiver, which improves the energy efficiency of the wireless transmission and thus the battery life of terminal devices in IoT Facilitated by the high-resolution spatial focusing, the TR division multiple access scheme leverages the uniqueness of the multi-path profiles in the rich-scattering environment and maps them into location-specific signatures, so that spatial multiplexing can be achieved for multiple users operating on the same spectrum In addition, the TR system can easily support heterogeneous terminal devices by providing various quality-of-service (QoS) options through adjusting the waveform and rate backoff factor Finally, the unique location-specific signature in TR system can provide additional physical-layer security and thus can enhance the privacy and security of customers in IoT All the advantages show that the TR technique is a promising paradigm for IoT

/pdf/time-reversal-wireless-paradigm-for-green-internet-of-things-4t6utlevrf.pdf

Time-Reversal Wireless Paradigm for Green Internet of Things: An Overview

Recent results on the effect of the nanocrystalline state on the mechanical, thermal, and magnetic properties and microscopic structure of metals and solid-phase compounds are reviewed Basic methods for preparing compact nanocrystalline materials are briefly discussed Size effects in such materials are discussed in detail and the importance of interfaces in determining the structure and properties of compact nanomaterials is demonstrated Models explaining the structural features and anomalous properties of nanocrystalline solids are analyzed

https://iopscience.iop.org/article/10.1070/PU1998v041n01ABEH000329/pdf

Effects of the nanocrystalline state in solids

The state of the art in the modification of iron, steel, and titanium alloys and some metal multilayers by medium intensity (106–109 W cm−2) pulsed electron and ion beams is summarized, including the contribution from the authors. Many of the processes that determine the defect structure and its properties are described and such problems as the formation of elastoplastic and shock waves, mixing of multilayers, thin film deposition, and nanophase powder synthesis are discussed. The high potential of particle beam technology — in particular, for improving the operating characteristics of materials — has been demonstrated amply by research centers worldwide.

Metal modification by high-power pulsed particle beams

The current state of the acoustic wave phase conjugation (PC) problem is reviewed. The generation of phase conjugate ultrasonic waves is discussed with emphasis on the parametric method using electromagnetic pumping in solids. The giant-amplification supercritical parametric PC mode is considered in detail. Ultrasonic PC with a gain in excess of 80 dB is demonstrated for a soft magnetic ceramics-based amplifier. The high quality of supercritical parametric PC sound is confirmed by acoustic-optical visualization. Acoustic PC effects, such as anomalous sound reflection at the PC mirror and the autofocusing and self-targeting of phase conjugate sound beams incident on a scatterer in a liquid, are shown. Recent experimental results demonstrating the potential of PC for applications are presented.

https://iopscience.iop.org/article/10.1070/PU1998v041n08ABEH000429/pdf

Wave phase conjugation of ultrasonic beams

A shift of the band envelope of liquid water stretching vibrations in the field of an ultrasonic acoustic pulse is experimentally observed; this shift is indicative of modification of the water cluster structure in the acoustic field. Simple evaluations show that the observed shift can be caused by a change in the average distance between oxygen atoms in a molecular cluster by about 0.05 A.

Spectroscopy of Spontaneous Raman Scattering of a Liquid-Water Local Structure in the Field of an Intense Ultrasound Pulse

Nonlinear propagation of a quasi-plane conjugate ultrasonic beam

The propagation of a nonlinear phase-conjugate ultrasonic wave through a layer introducing random phase aberrations is studied experimentally. The wave is generated by an overthreshold parametric phase-conjugating ultrasonic amplifier. It is shown that, with the extent of nonlinearity achieved for the conjugate wave, the phase locking of harmonics is retained and, as a consequence, a compensation of the distortions introduced by the layer takes place. The possibility of an automatic focusing of a nonlinear phase-conjugate wave propagating in an inhomogeneous medium is demonstrated, which is important for practical applications.

Focusing of a nonlinear phase-conjugate ultrasonic wave transmitted through a phase-inhomogeneous layer

The effect of phase conjugation for the second harmonic of a focused ultrasonic beam was investigated experimentally and by numerical simulation. An ultrasonic pulse with the carrier frequency f=3 MHz was emitted into water and focused at a point between the source and the phase conjugating system. The phase conjugation for the second harmonic of the incident wave (2f=6 MHz) was performed in a magnetostrictive ceramic as a result of the parametric interaction of the incident wave with the pumping magnetic field (the pumping frequency was f
p=4f=12 MHz). The axial and focal distributions of sound pressure in the incident and conjugated beams were measured using a broadband PVDF membrane hydrophone. The corresponding calculations were performed by solving numerically the Khokhlov-Zabolotskaya-Kuznetsov (KZK) equation allowing for the nonlinearity, diffraction, and thermoviscous absorption. The results of measurements agreed well with the calculations and showed that the field of a conjugate wave adequately reproduces the field of the second harmonic of the incident wave. A certain advantage of focusing with the phase conjugation for the second harmonic was demonstrated in comparison with the operation at the doubled frequency of the incident wave. The results of this study can serve as a basis for the utilization of the phase conjugation of harmonics in ultrasonic tomography and nondestructive testing.

L. M. Krutyanskii

Papers

Wave phase conjugation of ultrasonic beams

Spectroscopy of Spontaneous Raman Scattering of a Liquid-Water Local Structure in the Field of an Intense Ultrasound Pulse

Nonlinear propagation of a quasi-plane conjugate ultrasonic beam

Focusing of a nonlinear phase-conjugate ultrasonic wave transmitted through a phase-inhomogeneous layer

Parametric phase conjugation for the second harmonic of a nonlinear ultrasonic beam