Derode A, Tourin A, Fink M
Laboratoire Ondes et Acoustique, Université Denis Diderot-Paris VII, ESPCI-CNRS (UMR 7587), 10 rue Vauquelin, 75005 Paris, France.
Phys Rev E Stat Nonlin Soft Matter Phys. 2001 Sep;64(3 Pt 2):036606. doi: 10.1103/PhysRevE.64.036606. Epub 2001 Aug 28.
This is the second article in a series of two dealing with the statistical moments of ultrasonic waves transmitted through a disordered medium with resonant multiple scattering. Second-order moments in time and space are considered here. An ultrasonic pulsed wave is transmitted from a point source to a 128-element receiving array through two-dimensional samples with various thicknesses. 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 on a single realization 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 the array aperture. As the sample thickness is increased, the quality of focusing saturates, which we believe is linked to the Thouless factor g. In the thickest sample, g approximately 30, which is still well above the localization threshold.
这是关于通过具有共振多重散射的无序介质传输的超声波统计矩的两篇系列文章中的第二篇。本文考虑了时间和空间上的二阶矩。一个超声脉冲波从点源通过具有不同厚度的二维样本传输到一个128元接收阵列。样本由浸没在水中的平行钢棒的随机集合组成。散射波被记录下来，进行时间反转，然后再发送回介质中。时间反转波汇聚回其源点，并且在源点处空间和时间聚焦的质量与散射波在时间和空间上的二阶矩（相关性）有关。实验结果表明，利用宽频带，有可能在单次无序实现上获得稳健的估计。系统的空间分辨率仅受散射场相关长度的限制，而不再受阵列孔径的限制。随着样本厚度增加，聚焦质量达到饱和，我们认为这与 Thouless 因子 g 有关。在最厚的样本中，g 约为30，仍远高于局域化阈值。
