Coherent THz emission from an array of resonant tunneling diodes (RTDs) mediated by a strong-coupling Fabry-Perot cavity

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Terahertz radiation, whose frequency lies between those of infrared radiation and microwave radiation, has a broad range of applications, e.g. in non-destructive testing, medical imaging, security screening, as well as high-bit-rate wireless communications. However, the notorious “terahertz gap”, mainly due to the lack of cost-efficient, compact, high-power emitters at around 0.3-3 THz, has prevented the large-scale application of terahertz radiation. As the technology continues to advance, the main objective of this project is to help to fill the “terahertz gap” by an innovative interdisciplinary power-combing approach for emitters, which is based on two hitherto unconnected advanced technologies. This project involves two terahertz research groups: That of Prof. Masahiro Asada at the Tokyo Institute of Technology in Japan and that of Prof. Hartmut G. Roskos at Goethe-University Frankfurt am Main in Germany. Prof. Asada’s group develop and optimize resonant tunneling diodes (RTDs) as compact, chip-based electronic terahertz emitters, and they have become a world-leading laboratory for this type of terahertz radiation source. Their RTDs can now operate from tens of GHz up to 1.94 THz, and thus cover a large part of the “terahertz gap”. But the output power of the RTDs remains limited to tens of microwatt, which is not sufficient for most of the envisaged terahertz applications, and no good strategy has been visible to substantially increase the output power. Coming from a different area of research of terahertz photonics, we now suggest a novel synergistic approach to this problem. During the last few years, we have built and studied one-dimensional photonic crystal cavities with high quality factor in the terahertz frequency range, and demonstrated strong light-matter interaction by coupling the cavity photons with metamaterial plasmons within the cavity. It occurred to us that by combining a terahertz cavity with an array of RTDs, one should be able to substantially increase the output power of the RTDs via strong coupling leading to coherent superradiant emission from the RTD arrays. Funded by a grant of DFG to start an international collaboration, we were able to build the foundation for this ambitious endeavor by successful numerical simulations and joint experimental work in Japan. On this basis, we now enter into a full project to put into practice the idea of the combination of a strong-coupling cavity with state-of-the-art RTD arrays to form a powerful and compact photonic terahertz emitter.