Multiple scattering of terahertz pulses

本提案的主要目的是加深对宽带太赫兹脉冲在随机介质中传播的理解。在之前NSF拨款结果的基础上，PI将寻求两种不同的研究途径。其中第一个涉及发展扩散光子成像的新范例，基于这样一个概念，即对完整散射电场的相干测量比对时间平均强度的测量包含的信息要多得多。第二个推力将探索在极强多次散射极限下脉冲传播的本质，在这种极限下，扩散近似被打破。这项工作应该会产生第一个明确的观察到光子在块状三维随机介质中的局部化。在过去的十年里，已经有大量关于光散射的研究。这项工作的主要动机是生物医学的潜在应用，因为人体组织在近红外范围内是弱吸收者，但是强散射者。尽管多次散射过程中固有的信息明显丢失，但仍有可能形成浸没在混浊介质中的物体的有用图像。为了克服信息的损失，人们必须进行许多测量，例如，通过测量空间中许多位置的散射(例如，扩散)场。因此，采集速度和图像质量之间总是存在权衡，这限制了这些技术的适用性。在这里，PI将开发一种新的漫射光子成像范例，基于对电场(即强度和相位)的直接测量将极大地简化成像问题。他将使用太赫兹时域光谱学来演示这一点，这是一种用于宽带相干脉冲测量的有用试验台。此外，这项研究将极大地扩大太赫兹成像的用途，因为它将确立散射在成像中的重要性。这也将导致这项新兴技术在散射不可避免的情况下得到新的应用。这部分研究的智力价值在于开发了专门为测量(随机)电场而不是随机强度的情况设计的成像程序。这应该能够以更少的空间尺寸形成有用的图像。更广泛的影响在于这项新技术的应用，包括太赫兹频率和其他学科。PI还计划发展对多次散射短脉冲统计的透彻理解。在早期的工作中，他已经确定人们可以使用太赫兹时域技术来观察扩散的光子。在这项提议中，他将把这项工作扩展到一个全新的制度。他将研究在散射变得非常强的情况下多次散射太赫兹脉冲的特性。在这种情况下，人们预计用扩散理论来描述传播必须被打破，因为强烈的多次散射会导致相干效应。最终，人们期望传播波的完全局域化，这直接类似于众所周知的无序固体中电子的Anderson局域化现象。观察三维随机介质中的光子局域化一直是光学研究中的一个长期目标。事实证明，要对这种效应做出明确的证明是极其困难的，这在很大程度上是因为光吸收所起的关键作用。通过使用太赫兹脉冲进行这些实验，将有可能完全绕过这一最令人烦恼的问题。这个项目的智力优势将是在受控的实验环境中清楚地观察到波的局部化。这将对波的局部化和强多次散射波的统计有一个新的认识。这项工作的更广泛的影响将最明显地体现在我们对随机介质中激光现象的理解上，这种现象涉及到由局域态形成的激光模式。另一个影响将体现在扩大一个已经成功的计划，以吸引更多的女性进入科学和工程专业的研究生学习。

0401349MittlemanThe primary aim of this proposal is to develop an understanding of the propagation of broadband THz pulses in random media. Building on the results of the previous NSF grant, the PI will pursue two distinct avenues of research. The first of these involves the development of a new paradigm in diffuse photon imaging, based on the notion that a coherent measurement of the complete scattered electric field contains much more information than a measurement of the time-averaged intensity. The second thrust will explore the nature of pulse propagation in the limit of extremely strong multiple scattering, where the diffusion approximation breaks down. This work should produce the first unambiguous observation of photon localization in a bulk three-dimensional random medium.In the last decade, there has been a tremendous amount of research involving light scattering. Much of this work has been motivated by the potential applications to biomedicine, since human tissue is a weak absorber but a strong scatterer of radiation in the near infrared range. Despite the apparent loss of information inherent in the process of multiple scattering, it is still possible to form useful images of objects immersed in a turbid medium. In order to overcome the loss of information, one must make many measurements, for example, by measuring the scattered (e.g., diffusing) field at many locations in space. Therefore, there is always a trade-off between acquisition rate and image quality, which has limited the applicability of these techniquesHere, the PI will develop a new paradigm for diffuse photon imaging, based on the idea that a direct measurement of the electric field (i.e., both intensity and phase) should vastly simplify the imaging problem. He will demonstrate this using terahertz time-domain spectroscopy, a useful test bed for broadband coherent pulse measurements. In addition, this research will vastly broaden the utility of THz imaging, as it will establish the significance of scattering in image formation. It will also result in new applications for this emerging technology, in situations where scattering is inevitable.The intellectual merit of this portion of the research lies in the development of imaging procedures designed specifically for the situation where a (random) electric field, rather than a random intensity, is measured. This should enable the formation of a useful image with many fewer spatial measurements. The broader impacts lie in the applications of this new technique, both at terahertz frequencies and in other disciplines.The PI also plans to develop a thorough understanding of the statistics of multiply scattered short pulses. In earlier work, he has established that one can use the THz time-domain technique for observing diffusing photons. In this proposal, he will extend this work to an entirely new regime. He will study the properties of multiply scattered THz pulses in the case where the scattering becomes extremely strong. In this case, one expects that the description of the propagation in terms of a diffusion theory must break down, since strong multiple scattering can lead to coherent effects. Ultimately, one expects a complete localization of the propagating wave, in direct analogy to the well-known phenomenon of Anderson localization of electrons in disordered solids.Observing photon localization in a three-dimensional random medium has been a long-standing goal in optics research. It turns out to be extremely difficult to produce an unambiguous demonstration of this effect, in large part because of the crucial role played by optical absorption. By performing these experiments using THz pulses, it will be possible to completely circumvent this most vexing of problems. The intellectual merit of this project will be a clear observation of wave localization, in a controlled experimental environment. This will lead to a new understanding of wave localization and the statistics of strongly multiply scattered waves. The broader impacts of this work will be most evident in the implications for our understanding of the phenomenon of lasing in random media, which involves laser modes formed by localized states.An additional impact will be seen in the broadening of an already successful program to attract more women to graduate studies in science and engineering.