Rare Earth:Photoconductors for Terahertz Generation and Detection

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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.