Rare Earth:Photoconductors for Terahertz Generation and Detection

在太赫兹（0.1-10 THz）源和接收器概念的许多实现中，基于光电导体的系统在其前所未有的带宽方面是卓越的，其带宽覆盖了几十GHz到几个THz的频率范围。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兼容的光混合器是当前研究的焦点，因为它们利用了多种经济实惠的电信组件，这样的光电导体在前面的项目REPHCON的框架内进行了研究，基于ErAs：In（Al）GaAs材料系统。其出色的性能得到了证明，例如，在连续波（CW）和脉冲操作中，峰值动态范围均为100 dB左右，带宽为6.5 THz（脉冲），在CW操作下，接收器的噪声等效功率为1.8 fW/Hz（189 GHz），载流子寿命低至470±50 fs。这些光电导体是迄今为止世界上最好的光电导体之一。前一个项目的主要问题是缺乏材料。全世界只有少数几个小组能够合成这种复杂的材料。这个问题将在本项目中解决：铒电池将在慕尼黑工业大学的沃尔特·肖特基研究所投入使用，在那里将建立ErAs：In（Al）GaAs光电导体的未来增长。虽然材料合成和分析将在慕尼黑工业大学进行，但样品设计、加工和THz表征（CW和脉冲）将在达姆施塔特工业大学进行。在后续步骤中，材料将进一步改进。首先，层序列将被优化，其次，添加一小部分锑将有助于增加电阻，同时提高载流子迁移率。与材料改进平行，我们将研究新的电极概念，如石墨烯，以及改进的天线概念。该项目旨在将动态范围增加至少两个，可能是三个数量级。我们期望通过应用噪声压缩技术，以减少常见的噪声，例如，由激光器或拾取杂散场所引起的同步检测与两个光电导体的进一步改进。我们预计，噪声地板降低到中等attowatt水平，一个域到目前为止保留给低温探测器。这种低噪声地板应该允许被动太赫兹探测，这是该项目的最终目标。

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.