Multiple scattering of terahertz pulses

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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.