The terahertz (THz) frequency region of the electromagnetic spectrum (100 GHz – 10 THz) has attracted significant interest over the past decade for the development of new imaging and sensing technologies across a range of applications, including non-destructive testing, industrial inspection, airport/security screening, medical imaging, and atmospheric science. Despite this huge potential, however, THz sensing systems have yet to achieve widespread adoption and currently rely on time-domain spectroscopy (TDS) systems, which are typically based on bulky and expensive ultrafast laser technology that provide limited THz power and only allow slow data acquisition rates.
During the ERC Advanced Grant ‘TOSCA’ (N° 247375), a very different approach was taken to THz generation, focused on the use of a small, compact, semiconductor source – the THz frequency quantum cascade laser (QCL), which was first demonstrated in 2002. A key breakthrough occurred in 2011 when my group in Leeds demonstrated, in collaboration with the University of Queensland, that it is possible to use a single QCL device as both a high power source of THz radiation, and also as a phase sensitive detector in a ‘self-mixing’ scheme. We subsequently made considerable advances to this technology, including not only the development and demonstration of new imaging modalities, but also the verification of the applicability of our technology across a range of application areas including non-destructive testing, materials analysis and biomedical imaging. This included measuring doped semiconductor samples, identifying explosives, and classifying different types of biological tissue. We had also developed techniques for swept-frequency feedback interferometry, three-dimensional imaging, and inverse synthetic aperture radar imaging.
Notwithstanding this potential, a number of developments were necessary to translate this technology to industry and other non-academic end-users – these were the focus of this proposal – Themis (N° 727451). They included: (a) the development of a portable optical-breadboard-based self-mixing imaging system, moving away from the existing laboratory-based optical-bench-top apparatus; (b) the demonstration of the technology in the field both with commercial end-users, and at conferences/workshops/trade shows; and, (c) the evaluation of new opportunities for THz self-mixing imaging, together with identification of end-user applications.
The project was highly successful. A portable imaging system was developed, and demonstrated at the UK’s Knowledge Transfer Network (KTN)’s ‘Showcasing Emerging Technologies 2018 – Photonics and Imaging’ in London, with terahertz research and its applications being discussed at the Innovate UK's KTN 2018 ‘Materials Research Exchange & Investor Showcase’ in London. The self-mixing imaging system was adapted to enable gas spectroscopy, incorporating a gas cell into the experimental geometry, and broad range of samples were tested with a specific focus on materials of relevance to the Electronics and Medical sectors. Further evaluation of the terahertz market was also undertaken, and a range of high performance THz QCLs were developed, operating over a range of frequencies, providing greater opportunities to potential end-users.
In part as a result of this success, further national funding was received from the UK’s Engineering and Physical Sciences Research Council, through a £6.5M Programme Grant ‘HyperTerahertz’ with University College London and the University of Cambridge to develop a breadth of terahertz instrumentation, including self-mixing imaging and near-field microscopy.
