Coherent high-power THz emission from arrays of resonant-tunneling-diode (RTD) oscillators and its application for imaging

• Applicant: Professor Dr. Hartmut G. Roskos
• Subject Area: Electronic Semiconductors, Components and Circuits, Integrated Systems, Sensor Technology, Theoretical Electrical Engineering ; Communication Technology and Networks, High-Frequency Technology and Photonic Systems, Signal Processing and Machine Learning for Information Technology
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 469064321

Terahertz (THz) 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 “THz gap”, mainly due to the lack of cost-efficient, compact, high-power emitters at around 0.3-3 THz, has delayed the large-scale application of THz radiation. The main objective of this project is to help fill the “THz gap” by innovative coherent power-combing approaches for emitters based on resonant tunneling diodes (RTDs). This project is a continuation project in the second phase of the Priority Program INTEREST and is a cooperation of the THz research group of Prof. Hartmut G. Roskos at Goethe-University Frankfurt am Main and that of Prof. Safumi Suzuki at the Tokyo Institute of Technology in Japan. In the first phase of the INTEREST project, we achieved coherent emission from line arrays of eleven RTD emitters reaching close to 1 mW of output power at about 750 GHz. This achievement was possible because we found a new way to reach in-phase coupling of neighboring oscillators. Before our study, it was believed that oscillators in a linear array always couple in the odd fundamental mode, with the oscillation in neighboring slots occurring with opposite phase, thus that the radiation destructively interferes in normal direction in the far field. We found, however, that asymmetrically-RTD-fed slot antennae coupled in a linear array can also exhibit even-mode operation if the mesa area of the RTDs is reduced: The odd mode prevails at large mesa areas, while the even mode dominates for small ones. The odd mode was found to run at lower frequencies than the even mode. Additionally, both odd and even modes exhibit constructive interference in the far field, but at different distinct radiation angles. For intermediate mesa areas, the RTD array can either run in even or odd mode, controlled by the bias current of the RTDs (the switching exhibiting a hysteresis). This finding opens the potential for current-controlled frequency and emission-direction switching. For the second phase of the project, we will now exploit these results, extend them to two dimensional oscillator arrays, and develop practically usable radiation sources with emission of narrow-band, single-mode radiation in normal direction at an output power of 5 mW or more. We aim for a beam profile closely approximating a radially symmetric power distribution. We target radiation frequencies in the 0.7-0.8 THz band and at or above 1.0 THz. We will then integrate such high-power RTD array emitters into THz imaging systems at Goethe-University and perform application tests of a) standard THz transmission imaging and b) heterodyne holographic imaging.

• DFG Programme: Priority Programmes
• Subproject of: SPP 2314: INtegrated TERahErtz sySTems Enabling Novel Functionality (INTEREST)
• International Connection: Japan
• Cooperation Partner: Professor Dr. Safumi Suzuki