Coherent detection and manipulation of terahertz quantum cascade lasers

电磁波谱的太赫兹（THz）区域跨越微波和中红外之间的频率范围。从历史上看，这是频谱中最虚幻和最少探索的区域，主要是由于缺乏合适的太赫兹频率辐射实验室源，特别是高功率，紧凑，室温固态设备。尽管如此，在过去的十年中，太赫兹频率辐射已经吸引了新的成像和光谱技术的发展，由于其化学区分样品，识别晶体结构的变化，太赫兹光子学领域最近最重要的发展之一是太赫兹量子级联激光器（TQCL）。这些高功率紧凑型半导体源在太赫兹光子学领域开辟了许多新的机会，并吸引了全球范围内的重大研究兴趣。然而，需要开发用于测量从TQCL发射的辐射场的相位的技术，从而为当前建立的非相干检测方案提供补充技术。此外，有必要充分探索通过控制和操纵TQCL发射的THz场的相位可以取得的进展。我的愿景是启动一系列研究计划，目的是探测，操纵和利用TQCL辐射的相干性。这将为太赫兹光子学的大量研究机会奠定基础，并促进太赫兹技术在基础科学和现实世界中的应用。我将开发用于TQCL发射场的相干检测/测量的光学和电子技术。实现光学相干检测的一种手段是通过电光（EO）晶体将THz场的相位和振幅上转换到近红外波段。这种方法还将允许利用TQCL的大场振幅和窄线宽，使QCL辐射能够使用宽面积EO晶体和标准光学CCD进行采样。这将为在太赫兹科学中利用发展良好的可见/近红外探测器和CCD技术开辟一系列重要的机会。与此同时，我将开发相干检测技术的太赫兹场下变频到无线电频率。我将通过使用肖特基二极管对两个TQCL的场进行外差锁相来实现这一点。我将研究在TQCL中使用自混合的相干检测。该方法依赖于感测由辐射场反馈到TQCL腔中引起的结电压扰动，使得能够使用单个TQCL设备作为源和检测器来相干检测场。使用这种方法，TQCL的线宽变窄将进行调查，以及三维“无探测器”成像和Tomography.I的技术还将建立一个程序集中在太赫兹场的射频控制和操纵，通过使用动态和静态光栅，产生和控制通过表面声波（SAW）与TQCL设备的相互作用。这种方法将被用来提供一个非接触的手段来施加一个潜在的调制TQCL设备，从而提供了一个分布式的反馈机制的太赫兹波。作为该项目的一部分，我将开发有源区厚度减小的TQCL和TQCL梅萨结构。所有这些技术的组合将被结合起来，展示第一个使用QCL的2D相敏THz层析成像系统，第一个结合TQCL和商业CCD技术的全场成像系统，以及高分辨率THz气体光谱。

The terahertz (THz) region of the electromagnetic spectrum spans the frequency range between microwaves and the mid-infrared. Historically, this is the most illusive and least-explored region of the spectrum, predominantly owing to the lack of suitable laboratory sources of THz frequency radiation, particularly high-power, compact, room-temperature solid-state devices. Nevertheless, over the past decade, THz frequency radiation has attracted much interest for the development of new imaging and spectroscopy technologies, owing to its ability to discriminate samples chemically, to identify changes in crystalline structure, and to penetrate dry materials enabling sub-surface or concealed sample investigation.One of the most significant recent developments within the field of THz photonics has been the THz quantum cascade laser (TQCL). These high-power compact semiconductor sources have opened up a host of new opportunities in the field of THz photonics and have attracted significant research interest world-wide. However, there is the need to develop techniques for measurement of the phase of the radiation field emitted from TQCLs, thereby providing a complementary technology to currently established incoherent detection schemes. Furthermore, there is a need to explore fully the advances that can be made through control and manipulation of the phase of the THz field emitted by TQCLs.My vision is to initiate a range of research programmes with the aim of probing, manipulating and utilising the coherent nature of TQCL radiation. This will lay the foundations for a wealth of research opportunities in THz photonics, as well as facilitating the exploitation of THz technology for fundamental science and also for real-world applications.I will develop both optical and electronic techniques for coherent detection/measurement of the field emitted by TQCLs. One means of achieving optical coherent detection is through the up-conversion of the phase and amplitude of the THz field into the near-infrared band with an electro-optic (EO) crystal. This approach will also allow the large field amplitudes and narrow line-widths of TQCLs to be exploited, enabling QCL radiation to be sampled using a broad-area EO crystal and a standard optical CCD. This will open up a significant range of opportunities for exploiting well developed visible/near infrared detector and CCD technologies within THz science. In parallel, I will develop coherent detection techniques by down-conversion of the THz field to radio frequencies. I will accomplish this through heterodyne phase-locking the fields from two TQCLs using a Schottky diode.I will investigate coherent detection using self-mixing in TQCLs. This method relies on sensing junction voltage perturbations induced by feedback of the radiation field into the TQCL cavity, enabling coherent detection of the field using a single TQCL device as both source and detector. Using this approach, linewidth narrowing in TQCLs will be investigated, as well as techniques for three-dimensional 'detector-less' imaging and tomography.I will also establish a programme concentrating on the radio-frequency control and manipulation of the THz field through the use of dynamic and static gratings, generated and controlled via the interaction of surface acoustic waves (SAWs) with TQCL devices. This approach will be used to provide a non-contact means to apply a potential modulation to TQCL devices, thereby providing a distributed feedback mechanism for the THz wave. As part of this I will develop TQCLs with reduced active regions thicknesses and TQCL mesa structures.The combination of all these technologies will be combined to demonstrate the first 2D phase-sensitive THz tomography system using QCLs, the first full-field imaging system combining TQCLs and commercial CCD technology, and high-resolution THz gas spectroscopy.