Acoustical imaging through a multiple scattering medium using a time-reversal mirror

Acoustical imaging is based on the ability to focus an acoustic beam inside the zone of interest. This remains an issue through a high-order multiple scattering medium because the electronic delay lines that enable one to focus through a multiple scattering medium are a priori unknown. Using time-reversal principles, we show that images can be obtained through a very disordered medium. Surprisingly, the images are better than those obtained in a homogeneous medium with a classical imaging device.     

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.     

Sound focusing in rooms: the time-reversal approach

New perspectives in audible range acoustics, such as virtual sound space creation and active noise control, rely on the ability of the rendering system to recreate precisely a desired sound field. This ability to control sound in a given volume of a room is directly linked to the capacity to focus acoustical energy both in space and time. However, sound focusing in rooms remains a complicated problem, essentially because of the multiple reflections on obstacles and walls occurring during propagation. In this paper, the technique of time-reversal focusing, well known in ultrasound, is experimentally applied to audible range acoustics. Compared to classical focusing techniques such as delay law focusing, time reversal appears to considerably improve quality of both temporal and spatial focusing. This so-called super-resolution phenomenon is due to the ability of time reversal to take into account all of the different sound paths between the emitting antenna and the focal point, thus creating an adaptive spatial and temporal matched filter for the considered propagation medium. Experiments emphasize the strong robustness of time-reversal focusing towards small modifications in the medium, such as people in motion or temperature variations. Sound focusing through walls using the time-reversal approach is also experimentally demonstrated.     

Exploiting the time-reversal operator for adaptive optics, selective focusing, and scattering pattern analysis

We report on the experimental measurement of the backscattering matrix of a weakly scattering medium in optics, composed of a few dispersed gold nanobeads. The decomposition of the time-reversal operator is applied to this matrix and we demonstrate selective and efficient focusing on individual scatterers, even through an aberrating layer. Moreover, we show that this approach provides the decomposition of the scattering pattern of a single nanoparticle. These results open important perspectives for optical imaging, characterization, and selective excitation of nanoparticles.     

四元裂解位相调制实现相干光通过散射介质聚焦

光在不均匀介质中传播会受到散射的干扰,在这些散射材料中,例如粉末、生物组织、亚波长颗粒对入射光多次散射使得出射光无法聚焦,从而在接收平面形成散斑.本文提出四元裂解位相调制方法对入射相干光场进行调制,使其通过散射介质聚焦.此方法利用入射光场全场调制,充分考虑光场单元之间的干涉作用,从整个空间光调制器的调制面开始,逐层进行四元裂解及位相优化.运用此方法在实验中实现了相干光的前向散射和后向散射有效聚焦,这为生物医学领域中通过散射介质成像提供了新的思路和方法.     

An angular correlation test of time-reversal invariance

A test of time-reversal invariance in electromagnetic transitions is proposed in which all possible two-gamma angular correlations in a triple cascade are measured. The results can be combined to yield a term proportional to cos η.     

Parallel wavefront optimization method for focusing light through random scattering media

A parallel wavefront optimization method is demonstrated experimentally to focus light through random scattering media. The simultaneous modulation of multiple phase elements, each at a unique frequency, enables a parallel determination of the optimal wavefront. Compared to a pixel-by-pixel measurement, the reported parallel method uses the target signal in a highly efficient way. With 441 phase elements, a high-quality focus was formed through a glass diffuser with a peak-to-background ratio of ～270. The accuracy and repeatability of the system were tested through experiments.     

基于时间反转理论的聚焦Lamb波结构损伤成像

从理论和实验上研究了时间反转法在频散和多模式的Lamb波结构健康检测方面的应用.当Lamb波在包含有损伤的板类结构中传播时,损伤的存在表现为一个被动波源.采用分布式传感器网络,基于传递函数的观点,通过推导由损伤这个被动波源产生的时间反转波场幅值的表达式,证实了当观察点位于损伤位置时,时间反转波场的幅值最大.为验证时间反转方法的聚焦效应,提出了一种适合于分布的激励/接收传感器网络的成像方法,该方法可以对损伤定位并近似确定损伤尺寸.结合有限元的实验结果显示了Lamb波检测信号的能量可在损伤处聚焦,表明时间反转     

Light intensity correlation measurements in the presence of multiple scattering

We discuss the results of a model calculation of the intensity correlation function for multiply scattered light and compare our analytic predictions with experiments.     

Three-dimensional spatial multi-point uniform light focusing through scattering media based on feedback wavefront shaping

We use feedback wavefront shaping technology to realize the multi-point uniform light focusing in three-dimensional(3 D) space through scattering media only by loading the optimal mask once.General 3 D spatial focusing needs to load the optimal mask multiple times to realize the spatial movement of the focal point and the uniformity of multi-point focusing cannot be guaranteed.First,we investigate the effects of speckle axial correlation and different axial distances on 3 D spatial multi-point uniform focusing and propose possible solutions.Then we use our developed non-dominated sorting genetic algorithm suitable for 3 D spatial focusing(S-NSGA) to verify the experiment of multi-point focusing in 3 D space.This research is expected to have potential applications in the fields of optical manipulation and optogenetics.     

Attracted by long-range electron correlation: adenine on graphite

The adsorption of adenine on graphite is analyzed from first-principles calculations as a model case for the interaction between organic molecules and chemically inert surfaces. Within density-functional theory we find no chemical bonding due to ionic or covalent interactions, only a very weak attraction at distances beyond the equilibrium position due to the lowering of the kinetic energy of the valence electrons. Electron exchange and correlation effects are much more important for the stabilization of the adsystem. They are modeled by the local density or generalized gradient approximation supplemented by the London dispersion formula for the van der Waals interaction.     

HMSC: simultaneously detected heteronuclear shift correlation through multiple and single bonds

A new 2D pulse sequence HMSC (heteronuclear multiple-bond and single-bond coupling connectivities) for the simultaneous detection of long-range and one-bond heteronuclear connectivities is proposed which allows the two types of responses to be separated and the corresponding (n)J(CH) and (1)J(CH) connectivity maps to be calculated. (n)J(CH) coherences are selectively labeled in the course of the pulse sequence, the correspondingly acquired data are separately stored, and a simple add/subtract procedure is applied to disentangle and edit (n)J(CH) and (1)J(CH) responses prior to final data processing. Unlike standard methods, which are designed to measure one single type of heteronuclear spin-spin interactions and to efficiently suppress the other, both (n)J(CH) and (1)J(CH) are measured simultaneously in a single experiment with the HMSC pulse sequence. Compared to the common strategy with two standard experiments applied one after the other, e.g., HMBC and HMQC, valuable measuring time may be saved with this single experiment approach. The efficiency of the new pulse sequence and the quality of the corresponding spectra are demonstrated using strychnine. Features such as sensitivity, lineshapes, and the suppression of (1)J(CH) residual peaks in the final (n)J(CH) subspectra are investigated and compared with the corresponding results obtained with standard methods. The attractive and unique single experiment approach, its high efficiency, and its easy experimental setup together with straightforward data processing make HMSC a valuable experimental alternative for the today's more time-consuming "two-step" practice and makes it suitable for standard routine applications.     

Imaging and multiple scattering through media containing optically active particles

This paper examines the behaviour of polarized light scattered by a medium containing small chiral spheroidal particles. We show that for single scattering the observed phenomena of optical activity may be interpreted in terms of an averaged Mueller matrix and describe how the degree of polarization is affected by such a medium. The polarization properties of multiply scattered light by chiral particles are considered through the use of Monte Carlo simulations. It is shown that the effects of chirality under multiple scattering can be interpreted as an order-preserving influence in a disordered system and that this influence can, in principle, be exploited for the purposes of imaging.     

Imaging and multiple scattering through media containing optically active particles

Abstract

This paper examines the behaviour of polarized light scattered by a medium containing small chiral spheroidal particles. We show that for single scattering the observed phenomena of optical activity may be interpreted in terms of an averaged Mueller matrix and describe how the degree of polarization is affected by such a medium. The polarization properties of multiply scattered light by chiral particles are considered through the use of Monte Carlo simulations. It is shown that the effects of chirality under multiple scattering can be interpreted as an order-preserving influence in a disordered system and that this influence can, in principle, be exploited for the purposes of imaging.     

Elastic scattering of slow particles by some long-range potentials

We investigate the elastic s-scattering of slow particles by some model long-range potentials, obtained by joining the Coulomb potential to a polarization potential in the form-A · (r + a)^–4.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 73–79, August, 1972.     

A generalized Milne solution for the correlation effects of multiple light scattering with polarization

The Wiener-Hopf method is used to obtain a generalized Milne solution for scalar and electromagnetic fields with single-scattering anisotropy. For the scalar field, the solution found for the time correlation function and the interference component of backscattering is in good agreement with available experimental data. A solution of the Milne problem is constructed for the electromagnetic field. Given anisotropy, the generalized Milne equation is solved in the P₁ approximation for the quantity that describes the degree of depolarization of the scattered light. It is shown that the depolarization of the scattered light can change sign at large anisotropies.