Material characterization

PIs: Prof. Dr. Ullrich Pfeiffer (Universität Wuppertal), Dr. Jan Barowski (Ruhr-Universität Bochum), Dr. André Froehly (Fraunhofer FHR)

The imaging material characterization is one of the central research topics for the various network partners. Important pillars in this regard are the exploration of electronic and photonic technologies for hardware components in the terahertz range. The previous work of the Collaborative Research Center (CRC) MARIE plays a significant role in this context. While reflective measurement methods have primarily been used so far, taking surface effects into account, terahertz.NRW aims to focus on transmissive geometries, compact concepts, transceivers, and algorithms for penetrating terahertz measurements of materials and volumetric bodies, addressing highly relevant new aspects in the field of material characterization.


The goal of evolving from highly accurate reflective imaging and material characterization in the CRC MARIE to a transmissive tomographic approach serves as a bridge from semiconductor technology to photonic signal generation and signal processing, complementing the work of the existing Collaborative Research Center.


Transmissive tomographic imaging is currently mainly found in the medical field (ultrasound, X-ray CT, MRI) and in security applications (X-ray CT). It comes into play when materials need to be penetrated, and their internal properties need to be inferred. Millimeter-wave and THz tomography currently play a subordinate role in this context. However, when the use of ionizing X-ray radiation is not suitable, or when non-contact measurements with high dynamics are required compared to ultrasound, the use of THz radiation becomes an important alternative. This allows for spectrally resolved imaging with manageable integrated systems.


A relevant application field is the imaging of model plants for the study of microstructures or nutrient transport in indoor farming modules. This area has strong connections to the fields of medical technology and environmental monitoring. Furthermore, applications such as 3D material analysis (e.g., 3D metamaterials) or novel non-destructive testing methods from the Collaborative Research Center can be incorporated.


The network’s goal is a close integration of material characterization with the individual work packages. Joint workshops between users and technology representatives are offered as part of Work Package 1. While this exchange initially takes place based on the consortium’s preliminary work, in the first year, measurement capabilities can already be established, allowing for application-oriented tests for industrial users to significantly lower the barrier for THz imaging and enable application-oriented research. Within the network, numerous previous works and existing systems are available on the system side. Complete THz imaging systems (Czylwik, Kolpatzeck, Balzer) have already been realized and can be used for initial data collection. These systems can be further improved in terms of beamforming elements such as MEMS (M. Hoffmann). Additionally, systems based on electronic multipliers are available up to 300 GHz. Component-wise, individual transmitters, receivers, references, antennas, etc., with top performance have been developed within the CRC MARIE. Therefore, terahertz.NRW can draw on electronic components at 140 GHz, 300 GHz, and optoelectronic THz systems with a bandwidth of well over 1 THz. Together with MIMO radar technology at 120 GHz, 140 GHz, and 240 GHz, the network brings together a broad technological foundation that is invaluable for the entire consortium.


Building on this, Work Package 2 aims to explore system approaches and optimize them in collaboration with technology and application partners. These approaches allow for elegant transmissive imaging by physically separating the transmitter and receiver, enabled by the distribution of highly accurate reference signals in the GHz range (Musch, van Delden). An approach with optical signal generation and fiber-optic distribution (Balzer, Brenner, Saraceno) is also plausible, providing this capability intrinsically. The transceiver technologies from Work Package 3 enable unprecedented system dynamics and the exploration of new frequency bands above 275 GHz through compact electronic systems. In particular, the development of array-based solutions allows for highly compact transceiver modules in the THz range, enabling scanning speeds that bring industrial applications within reach.


Work Package 5 ultimately transforms data into images. Numerous researchers in the network are already active in the field of millimeter-wave and THz imaging. Both RUB (Rolfes, Barowski) and Fraunhofer FHR (Froehly) are engaged in radar imaging. The tomographic preliminary work (Balzer, Brenner) further establishes the planned research on an excellent scientific basis. Approaches for high-resolution transmissive imaging are being developed based on ultrasound imaging methods (Schmitz), with a focus on incorporating scattering and diffraction effects. By fusing with established reflective backscattering imaging, the resolution limit imposed by diffraction can be overcome by considering the complete scattering spectrum. Tomography provides an ideal platform for demonstrating new transceiver technologies and system concepts. Through improved imaging, in constant exchange among researchers in the individual work packages, the potential of Terahertz technology “made in NRW” can be presented to the public on a national and international level. By combining optically and electronically generated Terahertz data, not only high image resolution is achievable, but also 3D imaging in multiple frequency bands and polarization states. This allows for the visualization of static and dynamic volumetric permittivity distributions (moisture distribution in leaves, nutrient transport in indoor farming modules, measurement of exhaust filters, etc.) that were previously not directly accessible to biologists and materials scientists.


The possibilities of THz tomography thus open up further research fields that can initiate new research projects, applications, and spin-offs from the network. The collaborative work of scientists and students forms the crucial starting point for a sustainable and internationally visible research landscape in NRW.