High-speed Terahertz Imaging using Rydberg Atoms & Quantum Cascade Lasers

电磁频谱的太赫兹（THz）区域（频率约为10^12赫兹的辐射）传统上被认为是一个难以工作的区域，因为它福尔斯陷入了技术空白，电子，微波源在较低频率，光子，红外设备在较高频率。近几十年来，人们已经做出了相当大的努力来开发在太赫兹范围内工作的技术，以便利用太赫兹波所表现出的独特特性组合。例如，许多日常材料，如塑料，纸张，布料等对太赫兹波是透明的，这意味着我们可以深入样品。然而，与我们更熟悉的X射线不同，太赫兹波是安全的，因为它们是低能量和非电离的。为此，太赫兹成像技术被提出用于医学扫描、无损检测、安全、生产线检测和药品质量扫描等广泛的应用。然而，尽管付出了相当大的努力，太赫兹相机仍然远远低于其光学同行和太赫兹成像的应用是有限的，因此，在达勒姆，我们最近开发了一种新的方法，太赫兹成像，使用原子蒸气转换成容易检测到的光频率难以检测太赫兹波。使用激光束将原子蒸气激发到高位（里德伯）态，一旦处于这些里德伯态，原子对太赫兹波的扰动非常敏感，并发出可见光。这种有效的太赫兹光转换过程使我们能够有效地捕捉太赫兹图像使用标准的光学相机和观察帧速率超过3000帧每秒，远远超过其他太赫兹成像技术的能力。QCL是能够在太赫兹频段发射高功率的半导体激光器-使用QCL将导致更清晰的空间分辨率，并能够在我们的成像应用中成像更大的区域和/或探测更厚的样品。为了提高我们技术的图像质量，我们还将开发太赫兹范围的自适应光学技术。OA技术广泛用于光学和红外范围，以校正成像系统中的像差。以前尝试在太赫兹范围内执行AO受到可变形反射镜的小范围移动和太赫兹相机的慢图像采集速率的限制。我们将开发大行程可变形反射镜，以允许在太赫兹范围内进行有效的AO校正。这将使我们的THz成像过程能够进行深度选择并消除成像伪影和像差。此外，我们将通过添加第二种原子种类（Rb 87 + Cs 133）为外部成像仪添加光谱（频率相关）功能，从而提供类似于彩色摄影的光谱灵敏度，扩展我们的THz成像仪的能力，以包括光谱响应的材料区分。一旦我们构建并表征了我们的QCL照明，AO校正，双色THz成像仪，我们将在我们的工业项目合作伙伴的启发和指导下将其应用于一系列工业相关应用。

The terahertz (THz) region of the electromagnetic spectrum (radiation with frequencies around 10^12 Hertz) has traditionally been considered a difficult region to work in because it falls into a technology gap, with electronic, microwave sources at lower frequencies and photonic, infrared devices at higher frequencies. In recent decades, considerable efforts have been made to develop technologies that operate in the THz range in order to take advantage of the unique combination of properties exhibited by terahertz waves. For example, many everyday materials, such as plastics, paper, cloth etc. are transparent to THz waves, meaning that we can penetrate deeply into samples. However, unlike the more familiar X-rays, THz waves are safe to use because they are low energy and non-ionising. For this reason, terahertz imaging techniques are proposed for applications as broad as medical scanning, non-destructive testing, security, production line testing and medicine quality scanning. However, despite considerable efforts, terahertz cameras are still far slower and less sensitive than their optical counterparts and THz imaging applications are limited as a result.At Durham we have recently developed a novel approach to THz imaging that uses atomic vapour to convert difficult to detect terahertz waves into easy to detect optical frequencies. The atomic vapour is excited to high-lying (Rydberg) states using laser beams and once in these Rydberg states the atoms are very sensitive to perturbation by terahertz waves and emit optical light. This efficient THz to optical conversion process allows us to effectively capture terahertz images using standard optical cameras and observe frames rates exceeding 3000 frames per second, far exceeding the capabilities of other THz imaging techniques.This proposal intends to develop further our atom-based THz camera by using Quantum Cascade Lasers (QCLs) to provide the illumination. QCLs are semiconductor lasers capable of emitting high power in the terahertz frequency band - using QCLs will result in sharper spatial resolution and the ability to image larger areas and/or probe thicker samples in our imaging applications.In order to improve the image quality of our technique, we will also develop adaptive optics technology for the terahertz range. OA technologies are used extensively in the optical and infrared range to correct for aberrations in an imaging system. Previous attempts to perform AO in the THz range have been limited by the small range of movement of deformable mirrors and the slow image acquisition rates of THz cameras. We will develop large-stroke deformable mirrors to allow effective AO correction in the THz range. This will enable depth-selection in our THz imaging process and the removal of imaging artefacts and aberrations.Furthermore, we will add spectral (frequency dependent) functionality to out imager by adding a second atomic species (Rb87 + Cs133) thereby offering spectral sensitivity analogous to colour photography, expanding the capability of our THz imager to include material distinction by spectral response. Once we have constructed and characterised our QCL illuminated, AO corrected, 2-colour THz imager, we will apply it to a range of industrially relevant applications inspired and guided by our industrial project partners.