Active THz polarization laser imaging

光是一种电磁波，其性质不仅由波的幅度(即其强度)和波长(即其颜色)描述，而且还由其偏振(即电场方向)来描述。在大多数类型的成像使用强度和/或波长作为对比机制而忽略偏振的情况下，通常通过检查从场景反射或传输的光的偏振状态来获得许多信息。与电磁光谱的其他区域相比，在太赫兹频率范围(300 GHz-10 THz)，偏振作为成像的对比度机制一直没有得到充分利用。太赫兹辐射的一个主要应用是用于非破坏性评估，因为它能够穿透许多明显不透明的材料，例如药片涂层、泡沫、涂料、被覆盖的艺术品和古董。由于其能够提供关于材料双折射、表面特征、形状、粗糙度、边缘细节和手性的信息，偏振可以在太赫兹成像用于非破坏性评估方面提供显著的改进。本研究的主题是开发一种新型的太赫兹激光器，这种激光器可以在不需要移动部件的情况下，通过电子控制快速选择和切换发射光的偏振。这种方法是基于对微波频率范围内天线设计概念的适应，并将其与太赫兹激光器设计相结合。然后，开发的激光将被用作偏振成像的照明源，特别强调评估其在非破坏性评估应用中的适用性。作为该项目的一部分，该研究将培训研究生和本科生，并将通过PI参与一个有针对性的研究项目课程来支持招募和保留代表不足的少数族裔到工程学。这项研究的目标是开发基于太赫兹量子级联激光器(QCL)作为照明源的太赫兹主动偏振成像。首先，提出了一种QCL的输出极化可以电子调制的创新方案。这一方案基于太赫兹超材料天线或激光相控阵，这些天线被设计成以重叠的波束辐射，但具有正交极化。通过控制激光辐射的相对相位，可以在各种线偏振态和圆偏振态之间动态切换输出偏振。与基于旋转偏振器和波片的当前方案相比，这将允许对输出极化进行更快速和更稳定的调制，这反过来将允许以更低的噪声更快地收集数据，并且系统更紧凑和更轻。其次，开发的QCL将在几种方案中应用于偏振成像，并将与传统技术相比其性能将被量化。开发的技术有可能影响许多无损评估应用，包括腐蚀检测和监测、生物成像、安全筛选和非法材料检测以及制造过程质量控制。

Light is an electromagnetic wave whose properties are described not only by the amplitude of the wave (i.e. its intensity) and its wavelength (i.e. its color), but also its polarization (i.e. direction of the electric field). Where most types of imaging use the intensity and/or wavelength as contrast mechanisms and ignore the polarization, there is often much information to be obtained by examining the polarization state of the light reflected or transmitted from a scene. Compared to other regions of the electromagnetic spectrum, in the terahertz frequency range (300 GHz-10 THz), polarization has been underutilized as a contrast mechanism for imaging. A major application of terahertz radiation is for non-destructive evaluation due its ability to penetrate many visibly opaque materials for example pharmaceutical tablet coatings, foams, paints, covered art and antiquities. Polarization can potentially provide significant improvement in the utility of terahertz imaging for non-destructive evaluation due to its ability provide information about material birefringence, surface features, shape, roughness, edge detail, and chirality. The theme of this research is the development of a new type of terahertz laser in which the polarization of the emitted light can be chosen and switched rapidly, by electronic control, without moving parts. The approach is based upon the adaptation of antenna design concepts from the microwave frequency range, and combining them with terahertz laser design. The developed lasers will then be used as illumination sources for polarization-based imaging, with a particular emphasis on evaluating their suitability for non-destructive evaluation applications. As a part of the project, the research will train graduate and undergraduate students, and will support recruitment and retention of underrepresented minorities to engineering through the PI's participation in a targeted research project course.The objective of this research is the development of terahertz active polarization imaging based upon THz quantum-cascade lasers (QCLs) as an illumination source. First, an innovative scheme is proposed where the output polarization of QCLs can be modulated electronically. This scheme is based upon terahertz metamaterial antennas or phased arrays of lasers that are designed to radiate with overlapping beams, but with orthogonal polarization. By controlling the relative phase of the laser radiation, the output polarization can be dynamically switched between various linear and circular polarization states. This will allow more rapid and stable modulation of the output polarization compared to current schemes based upon rotating polarizers and waveplates, which in turn will allow faster data collection with reduced noise and a more compact and lightweight system. Second, the developed QCLs will be applied to polarimetric imaging in several schemes, and their performance will be quantified versus conventional techniques. The developed technology has the potential to impact many nondestructive evaluation applications including corrosion detection and monitoring, bioimaging, security screening and illicit material detection, and manufacturing process quality control.