Rare Earth:Photoconductors for Terahertz Generation and Detection

在太赫兹(0.1-10太赫兹)源和接收器概念的众多实现中，基于光电导体的系统以其前所未有的带宽而出类拔萃，其频率范围从几十GHz到几个太赫兹，只需一次设置。这使得多种应用成为可能，包括光谱学和通过渡越时间测量的无损检测，其中THz脉冲持续时间短至300 fS是一个独立的标准。对于未来的商业和工业应用，1550 nm兼容光电二极管是当前研究的重点，因为它们利用了多种负担得起的电信元件。这类光电导体是在前一个项目REPHCON的框架内进行研究的，基于Era：In(Al)GaAs材料体系。在连续波(CW)和脉冲工作时，其峰值动态范围都在100分贝左右，带宽为6.5太赫兹(脉冲)，在189 GHz连续工作时的噪声等效功率为1.8FW/Hz，载流子寿命低至470±50飞秒。这些光导体是迄今为止世界上最好的光导体之一。前一个项目的主要问题是缺乏材料。世界上只有几个小组能够合成这种复杂的材料。这个问题将在这个项目中解决：在德国慕尼黑大学的沃尔特·肖特基研究所将建立一个Erb池，在那里将建立未来的Era：In(Al)GaAs光导体。材料合成和分析将在德国慕尼黑大学进行，样品设计、加工和太赫兹表征(连续波和脉冲)将在德国达姆施塔特大学进行。在后续步骤中，材料将进一步完善。首先，层序将被优化，其次，少量Sb的加入将有助于增加电阻，同时改善载流子迁移率。在材料改进的同时，我们将研究新的电极概念，如石墨烯，以及改进的天线概念。该项目的目标是将动态范围增加至少两个数量级，甚至三个数量级。我们期待通过应用噪声压缩技术对两个光导体进行同步检测来进一步改进，以减少例如由激光或拾取杂散场引起的共同噪声。我们预计，噪声下限将降至中瓦级，这是迄今为止为低温探测器保留的一个领域。这样的低噪音地板应该允许被动太赫兹探测，这是本项目的最终目标。

Amongst the many realizations of Terahertz (0.1-10 THz) source and receiver concepts, photoconductor-based systems are preeminent in terms of their unprecedented bandwidth covering a frequency range of a few tens of GHz to several THz with a single setup. This enables a manifold of applications, including spectroscopy and non-destructive testing via transit time measurements, where the THz pulse duration as short as 300 fs is a stand-alone criterion. For future commercial and industrial applications, 1550 nm compatible photomixers are in the focus of current research as they make use of the manifold of affordable telecom components.Such photoconductors were investigated in the framework of the preceding project REPHCON, based on the ErAs:In(Al)GaAs material system. The outstanding performance was proven, e.g. by a peak dynamic range around 100 dB in both continuous-wave (CW) and pulsed operation, a bandwidth of 6.5 THz (pulsed), a noise equivalent power of the receivers of 1.8 fW/Hz at 189 GHz under CW operation and a carrier lifetime as low as 470±50 fs. These photoconductors are amongst the best world-wide to date.The major problem of the preceding project was the lack of availability of the material. Only a few groups world-wide are able to synthesize this sophisticated material. This problem shall be tackled within this project: An Erbium-cell shall be put in operation at the Walter Schottky Institute, TU Munich, where future growth of ErAs:In(Al)GaAs photoconductors shall be established. While material synthesis and –analysis will be performed at TU Munich, sample design, processing and THz characterization (CW and pulsed) will be performed at TU Darmstadt. In subsequent steps, the material will be further improved. First, the layer sequence will be optimized, second, the addition of a small fraction of antimony will assist in increasing the resistance while improving the carrier mobility at the same time. Parallel to material improvements, we will investigate new electrode concepts such as graphene, as well as improved antenna concepts. The project aims for an increase of the dynamic range of at least two, potentially three orders of magnitude. We expect further improvements by synchronous detection with two photoconductors by applying noise squeezing techniques in order to reduce common noise, e.g. caused by the lasers or by pick up of stray fields. We expect that the noise floor reduces to the mid attowatt-level, a domain so far reserved to cryogenic detectors. Such low noise floors should allow for passive THz detection, the final goal of this project.