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Convergence of Electronics and Photonics Technologies for Enabling Terahertz Applications

Periodic Reporting for period 2 - CELTA (Convergence of Electronics and Photonics Technologies for Enabling Terahertz Applications)

Reporting period: 2018-03-01 to 2020-02-29

Societal relevance: securing that the food we consume is free of poisonous substances, securing that medicines are not falsified and being able to early detect diseases such as cancer, are a few examples of applications of Terahertz technologies. The ITN CELTA aims to produce the next generation of researchers who will enable Europe to take a leading role in the multidisciplinary area of utilizing Terahertz technology for applications involving components and complete systems for sensing, instrumentation, imaging, spectroscopy, and wireless communications. Within CELTA, fifteen early stage researchers are working towards the realization of three demonstrators: beam steering technology for communication applications, a photonic vector analyzer for spectroscopy and materials characterization, and a THz imager for sensing applications.

The problem tackled. For the three demonstrators in CELTA, a common technological challenge is achieving high enough power level of Terahertz radiation at the transmitter side and performing high sensitivity detection at the receiver side. Another challenge of Terahertz technologies is compact, small size and portable solutions, preferable with user friendly interfaces. Increasing the resolution and reliability of imaging realized by Terahertz cameras is a timely and relevant challenge. All the three above-mentioned challenges are tackled within the ITN CELTA.

Overall objectives: To face the challenge of power levels for THz generation and high sensitivity of THz receivers, the following objectives are pursued in CELTA. Firstly, transmitter and receiver configurations including combination of beam steering and beamforming using antenna arrays, phase shifting, envelope detectors, and an optical true time delay network are considered. Furthermore, high power amplification and multiple stages of frequency multiplication are also considered. A free-space THz vector network analyzer (VNA) using optical fiber coupled and compact THz elements is developed as a small size and portable solution. A fine tunable THz transmitter as well as a compact and highly sensitive THz detector are developed serving the objective of compactness and high-resolution for imaging and THz-VNA analysis. Finally, the challenge of increasing resolution and reliability of THz cameras is tackled by a combination of large pixel array THz cameras in combination with advanced signal processing algorithms.

Conclusion: the ITN CELTA has achieved all its objectives both the technical goals and the training activities of 15 ESRs are successfully completed.
The beamforming demonstrator includes advances in integrated circuits for optical true time delay beamforming. A dielectric rod waveguide-based phase-shifters with integrated carbon nanotube layers have been simulated, fabricated and measured in the frequency range 75–500 GHz. Circularly polarized antennas in a substrate integrated waveguide technology has also been designed, analyzed and manufactured. Another component that has been designed is a leaky wave antenna (LWA) with frequency/electrically scanning capability of main beam over a wide angular range. Two highlight, invited presentations were delivered by two ESRs at the 2019 IRmmw-THZ conference.

Regarding the sub-systems of THz electronics for the vector network analyzer (THz-VNA) and THz imager, new results are related to the design of frequency doublers at 280 GHz and frequency triplers at 549 GHz with high rejection of undesired harmonics. A solid-state power amplifier based a common emitter configuration at 160 GHz was design. It exhibits remarkable features: high output power up to 15 dBm, power efficiency of 11% and gain of 26 dB. New progress has also been made developing electro-optical comb scheme for signal generation a detection in the sub-THz range. Schemes based on phase modulation, dual comb generation and mode-locking/mode-picking through optical injection locking were demonstrated within a remote dual comb detection. For the CELTA graphene-hybrid electrode based photomixers devices, a packaging architecture for plug-and-play operation has been developed as well as experiments for determining their noise equivalent power (for receivers) and their emitted power (as transmitters) with archive journal publication as highlight output of this activity.

The progress towards the Terahertz imager demonstrator is highlighted by the design and manufacturing of two different types of THz arrays (6x7 pixels and 12x12 pixels) . They were tested at room temperature and at 80 K showing state-of-the-art performance for quasi-optical detectors. THz camera electronics has also been developed. A new concept of a compact sensor based on interferometric transmission measurement applicable for near-field scanning microwave microscopy measurements was suggested and experimentally verified in frequency range 45 MHz – 20 GHz. Also, a continuous-wave Terahertz off-axis digital holographic system has been built. And sparsity based compression technique has been introduced before numerical data reconstruction in order to reduce the dataset required for hologram reconstruction. Two highlighted archival journal publications report on these advancements.
Hybrid photonic/millimeter wave systems with beam-steering capabilities are validated showing the way for a new optical beamformer chip design, that together with the new design of a dielectric rod waveguide-based antenna array are remarkable novel results towards the implementation of an agile beam-forming system. Hence, the results we expect by the end of the project is a system with broadband operation, small size, that is also energy efficient and reconfigurable. Already today, the whitepapers on the technolgoy needed for 6G generation of wireless communications system refer to these advancements.

Free space photonic solutions offer larger bandwidths than their electronic counterparts. The second demonstrator of the project, is a free space THz VNA. Research directions include novel graphene-hybrid electrode based photomixers constituting a hybrid electrode structure, rare earth-based photoconductors, and continuous-wave optical frequency combs as narrow linewidth sources. These technologies will be combined in an architecture of a complete free space THz-VNA system with performance beyond the-state-of-the-art.

THz imaging is the third main experimental demonstrator of the project. Within this area, two THz detection arrays were designed, fabricated and successfully tested as well as electronics for the cameras showing performance at the edge of what is possible today, including holographic capabilities. Further achievements include using plasmonic detectors to achieve high resolution THz cameras, MIMO radars in the THz range, new SiGe BJTs amplifiers, interferometry in the sub-THz region, as well as signal processing software to retrieve and interpret what the imaging system captures, particularly to include the holographic feature.
All three demonstrators have the potential to be taken over for higher technology readiness levels (TRL) with view of test in realistic end-user conditions and therefore be considered for commercialization. The prospects of use of such THz products in metrology, sensing and communications would be of large impact due to their compact, potential low cost and high performance parameters.
Experimental setup for demonstration of THz wireless data communication
Optical beamformer chip under characterization at TU/e lab.
Logo ITN CELTA project