Multiple scattering of terahertz pulses

0401349Mittleman本提案的主要目的是了解宽带THz脉冲在随机介质中的传播。 在先前NSF拨款的基础上，PI将追求两种不同的研究途径。 其中第一个涉及到一个新的范例在扩散光子成像的基础上，完整的散射电场的相干测量包含更多的信息比测量的时间平均强度的概念。 第二个重点将探索脉冲传播的性质，在极端强的多重散射的限制，扩散近似打破。 这项工作应该产生第一个明确的观察光子本地化在一个散装的三维随机medium.In过去十年中，有大量的研究涉及光散射。 由于人体组织对近红外范围的辐射是弱吸收体，但却是强散射体，因此这项工作的大部分动机是其在生物医学上的潜在应用。 尽管多次散射过程中固有的信息明显丢失，但仍然可以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，必须进行许多测量，例如，通过测量散射（例如，扩散）场。 因此，在采集速率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。在此，PI将基于电场的直接测量（即，强度和相位两者）应当极大地简化成像问题。 他将使用太赫兹时域光谱来证明这一点，这是一个有用的宽带相干脉冲测量测试平台。 此外，这项研究将大大拓宽太赫兹成像的实用性，因为它将建立散射在图像形成中的重要性。 这也将导致在散射是不可避免的情况下，这一新兴技术的新的应用。这部分研究的智力价值在于专门为测量（随机）电场而不是随机强度的情况设计的成像程序的发展。 这应该能够以少得多的空间测量形成有用的图像。 更广泛的影响在于这种新技术的应用，无论是在太赫兹频率和其他学科。PI还计划发展一个全面的理解的统计多散射短脉冲。 在早期的工作中，他已经确定可以使用THz时域技术来观察扩散光子。 在这个建议中，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，人们预期，传播的扩散理论的描述必须打破，因为强大的多重散射可以导致相干效应。 最终，人们期望传播波的完全局域化，直接类比于众所周知的无序固体中电子的安德森局域化现象。观察三维随机介质中的光子局域化一直是光学研究的长期目标。 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学吸收所起的关键作用。 通过使用THz脉冲进行这些实验，将有可能完全规避这个最令人烦恼的问题。 这个项目的智力价值将是在受控的实验环境中对波的局部化进行清晰的观察。 这将导致对波局部化和强多次散射波统计的新的理解。 这项工作的更广泛的影响将是最明显的影响，我们的随机介质中的激光现象的理解，其中涉及激光模式形成的局域化状态的影响。一个额外的影响将被视为在扩大一个已经成功的计划，以吸引更多的女性研究生学习科学和工程。

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.