THz applications

Terahertz radiation is situated in the electromagnetic spectrum between microwaves and infrared and is characterised by a frequency range from 100 GHz to 10 THz, i.e. wavelengths between 3 mm and 30 μm. A peculiarity of THz radiation is that it penetrates many objects that are opaque to infrared and visible light. On the other hand, THz radiation still behaves ray-like, unlike longer wavelengths such as MHz and low GHz radiation. THz radiation is often produced and detected as short pulses as in THz time-domain spectroscopy (THz-TDS). Each pulse interacts with a material and its distorted waveform is detected.

Ever since the first laser-induced generation of THz radiation in the 1990s, scientists have enthusiastically embraced THz technology in academic studies. THz spectroscopy has a number of advantages that point to abundant industrial applications as well, in areas such quality control and security. Those advantages include detectability at room temperature; a non-ionizing nature; applicability to many materials; and, through THz-TDS, the ability to provide depth sensitivity, nondestructively. Consequently, THz technology is particularly suited for the inspection of sheet materials, such as paper-derived products, paint layers[1] and plastic sheets, where it has proven sensitivity to industrially relevant parameters.

Yet despite those advantages and application ideas, the search for a THz “killer application”—a novel, innovative use with a business case strong enough to bring the technology into the industrial mainstream—has remained fruitless, and even the hope of finding such an application has begun to falter.[2] Why has a killer app for THz radiation been so elusive? And how can the road to industrial application of this versatile technology be cleared?

The focus of our activities in the field of applications of THz spectroscopy is precisely aiming at answering this question: what are bottlenecks for THz applications to become widely adopted and have societal impact? Signal processing of raw THz signals in order to obtain reliable material parameters is a major hurdle. As mentioned under the tab 'Advanced Signal Processing', our research hereto focusses on the development of model-based[3], data-based[4] and hybrid approaches. Furthermore, we choose application domains with present societal interest, such as the determination of leaf wetness[4] for pest control and the inspection of goods in the field of security. Combining both aspects, we develop sensor technologies and build sensor demonstrators to evince the potential impact of THz technology for applications.

[1] van Mechelen, J.L.M., Frank, A. & Maas,D.J.H.C. Thickness sensor for drying paints using THz spectroscopy. Opt. Express 29, 7514 (2021).
[2] van Mechelen, D. An industrial THz killer application? Opt. Photon. News 26, 16–18 (2015).
[3] van Mechelen, J. L. M., Kuzmenko, A. B. & Merbold, H. Stratified dispersive model for material characterization using terahertz time-domain spectroscopy. Opt. Lett. 39, 3853–3856 (2014).
[4] Koumans, M., Meulendijks, D., Middeljans, Peeters, D., H., Douma, J.C., and, van Mechelen, D. (2024). Physics‐assisted machine learning for THz time‐domain spectroscopy: sensing leaf wetness, Sci. Reports 14:7034 (2024).