Active THz polarization laser imaging

光是一种电磁波，它的性质不仅由波的振幅（即它的强度）和波长（即它的颜色）来描述，而且还由它的偏振（即电场的方向）来描述。当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时，通过检查从场景反射或传输的光的偏振状态通常可以获得许多信息。与电磁频谱的其他区域相比，在太赫兹频率范围内（300 GHz-10太赫兹），极化作为成像的对比机制尚未得到充分利用。太赫兹辐射的一个主要应用是用于非破坏性评价，因为它能够穿透许多明显不透明的材料，例如制药片剂涂层、泡沫、油漆、有盖艺术品和古董。由于偏振能够提供有关材料双折射、表面特征、形状、粗糙度、边缘细节和手性的信息，因此它可以为太赫兹成像的非破坏性评估提供潜在的重大改进。这项研究的主题是开发一种新型的太赫兹激光器，它可以通过电子控制快速选择和切换发射光的偏振，而不需要移动部件。该方法是基于微波频率范围的天线设计理念，并将其与太赫兹激光设计相结合。然后，开发的激光器将用作基于偏振成像的照明源，特别强调评估其非破坏性评估应用的适用性。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参与有针对性的研究项目课程，支持招募和留住代表性不足的少数民族。本研究的目的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有源偏振成像。首先，提出了一种可以通过电子方式调制量子液晶输出偏振的创新方案。该方案基于太赫兹超材料天线或激光相控阵，设计成以重叠光束辐射，但具有正交偏振。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态。与目前基于旋转偏振器和波片的方案相比，这将允许更快速、更稳定的输出偏振调制，从而可以更快地收集数据，降低噪音，并使系统更紧凑、更轻。其次，所开发的量子级联激光器将以多种方案应用于偏振成像，并将其性能与传统技术进行量化。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.