Terahertz near-field microscopy of nanostructures and 2D materials

太赫兹频率范围位于电磁频谱的微波和中红外区域之间，但由于制造方便的源和探测器的困难，长期以来一直拒绝开发；太赫兹辐射的频率太高，不能由移动电话中使用的电子技术产生，但太赫兹的频率太低，不能由例如CD播放器激光器中利用的光学技术产生。然而，在过去的二十年里，由于创新的光源、探测器和成像系统的发展，该领域取得了显著的增长--特别是量子级联激光器。这些发展使广泛的成像和光谱学研究成为可能，在这些研究中，太赫兹辐射的选择性吸收或传输提供了关于这一相对未被探索的光谱区域中材料的物理和化学性质的独特和基本信息。最近太赫兹仪器的商业应用在制药和汽车工业，以及半导体器件的高分辨率故障隔离和集成电路的3D成像等方面得到了应用。这个博士项目将通过结合两项激动人心的技术来解决这一不足：原子力显微镜(AFM)中的散射尖端近场成像和太赫兹量子级联激光的自混合干涉测量。自混合干涉术是我们开发的一种技术，其中发射激光腔本身被用作散射/反射辐射的异常灵敏的相干自探测器。来自远距离样品的散射信号在激光内部电场中引起扰动，并通过激光电压的可测量波动来表现。我们将把它与近场成像方法结合起来，在近场成像方法中，入射的太赫兹辐射通过AFM针局部聚焦在样品上。通过结合原子力显微镜的纳米级分辨率和自混合检测技术的高灵敏度和紧凑性，我们将开发出一种太赫兹纳米显微镜，并将其应用于太赫兹范围内的一系列纳米结构和2D材料的研究。

The terahertz frequency range sits between the microwave and mid-infrared regions of the electromagnetic spectrum, but has long resisted exploitation owing to difficulties in fabricating convenient sources and detectors; terahertz radiation is too high in frequency to be generated by the electronic techniques used in mobile telephones, but too low in frequency to be produced by the optical techniques exploited in, for example, CD player lasers. However, the last twenty years have witnessed a remarkable growth in the field owing to the development of innovative sources, detectors, and imaging systems-and in particular, the quantum cascade laser. These developments have enabled a wide range of imaging and spectroscopy studies in which the selective absorption or transmission of terahertz radiation has provided unique and fundamental information about the physical and chemical properties of materials in this relatively unexplored region of the spectrum. Recent commercial application of terahertz instrumentation is now finding application in the pharmaceutical and automotive industries, and in high-resolution fault isolation in semiconductor devices and 3D imaging of integrated circuits, inter alia.This PhD project will address this shortfall by combining two exciting technologies: scattering tip near-field imaging in an atomic force microscope (AFM), and self-mixing interferometry with terahertz quantum cascade lasers. Self-mixing interferometry is a technique that we have developed in which the emitting laser cavity itself is used as an exceptionally sensitive coherent self-detector of scattered/reflected radiation. Signals scattered from remote samples induce perturbations in the intra-laser electric field, and are manifest by measurable fluctuations in the laser voltage. We will combine this with a near-field imaging approach in which the incident terahertz radiation is focused locally on the sample by an AFM needle. By combining the nanometer-scale resolution of the AFM with the high sensitivity and compact footprint of the self-mixing detection technique, we will develop a terahertz nanoscale microscope and apply it to the investigation of a range of nanostructures and 2D materials in the terahertz range.