关于时间反转镜
On time reversal mirrors
摘要：
从基本原理出发，重新审视了标量波时间反演的概念。介绍了五种不同的时间反转镜（TRM），分析了它们之间的关系。对于边界行为，对于傍轴波，只有双模式TR格式满足精确边界条件，而对于球面波，只有一个混合模式TR格式在乘以2后满足精确边界条件。给出了近场聚焦特性的渐近分析。结果表明，有一个亚波长的焦点的TRM应涉及偶极场。由于焦斑尺寸随光源和TRM之间的距离几何地减小，所以TRM对波长以下的聚焦是极其无效的。与类似于TR的匹配场处理和相位处理器相反，弱散射或非散射介质中的TR通常在纵向方向上偏置，特别是当TR在具有{有限}孔径的{\em单个}平面上进行时。这对于所有五种TR方案都是如此。另一方面，TR焦斑已被反复在文献中，理论和实验上，被集中在源点时，介质是多重散射。调和的两个看似矛盾的结果被发现在随机波动的强度的格林函数的多重散射介质和散射放大的有效孔径的概念。
英文摘要：
The concept of time reversal (TR) of scalar wave is reexamined from basic principles. Five different time reversal mirrors (TRM) are introduced and their relations are analyzed. For the boundary behavior, it is shown that for paraxial wave only the monopole TR scheme satisfies the exact boundary condition while for spherical wave only one of the mixed mode TR scheme, after multiplication by two, satisfies the exact boundary condition. The asymptotic analysis of the near-field focusing property is presented. It is shown that to have a subwavelength focal spot the TRM should involve dipole fields. The monopole TR is extremely ineffective to focus below wavelength as the focal spot size decreases logarithmically with the distance between the source and TRM. Contrary to the matched field processing and the phase processor, both of which resemble TR, TR in a weak- or non-scattering medium is usually biased in the longitudinal direction, especially when TR is carried out on a {\em single} plane with a {finite} aperture. This is true for all five TR schemes. On the other hand, the TR focal spot has been shown repeatedly in the literature, both theoretically and experimentally, to be centered at the source point when the medium is multiply scattering. A reconciliation of the two seemingly conflicting results is found in the random fluctuations in the intensity of the Green function for a multiply scattering medium and the notion of scattering-enlarged effective aperture.
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