Terahertz-frequency sensors for atmospheric chemistry and space research

When we look into space with existing infrared, radio and microwave sensors, we see less than half the light in our galaxy. Most of this "missing" light lies in the terahertz (THz) or far-infrared part of the spectrum (1-10 THz, 30-300 micron wavelength). Indeed, the "invisible" gases in the Earth's atmosphere and the "dark" dust and gas clouds between stars all glow with distinctive THz fingerprints, providing a wealth of hidden information urgently needed by atmospheric and space scientists.

Despite this great potential, existing THz sensor systems are too large, fragile and complex for most applications outside the laboratory and lack the sensitivity needed for studying reactive gases. Furthermore, this lack of technological readiness limits the prospects for THz systems being deployed in space. A short time-window is available for the UK to invest in real-world demonstrations of key THz components and sensing techniques and secure a place in forthcoming space missions. Without this, the potential for a UK researcher to lead the world in this emerging area will be lost.

In this fellowship, I will overcome the previous limitations of THz gas sensors by developing high-sensitivity systems based on quantum-cascade lasers (QCLs) - QCLs are highly compact sources of THz radiation, which yield >1000 times the power of any similar-sized device.

Unlike previous THz-QCL-based gas-sensing schemes, I will use high-precision analytical chemistry techniques. This will include the use of custom-made reflective cavities and multi-pass gas cells in which THz radiation passes repeatedly through the gas under study, yielding an estimated 100x improvement in sensitivity. I will also work with my project partners in RAL Space to control the frequency of my THz sensors precisely, giving 10x improvement in the resolution of spectral "fingerprints" compared with free-running QCLs. Together these advancements will allow the first laboratory observation of key atmospheric reactions and underpin future deployment in satellite applications.

Specifically, I will develop the first high-precision gas sensors operating in the THz band, with the robustness, compact size and sensitivity needed for installation on a satellite or the International Space Station. The UK and European Space Research communities have taken great interest in THz gas sensing, with at least two in-flight systems currently under consideration. These include "TARDiS" - a UK-led THz astrophysics system for the International Space Station, and "LOCUS" - an Earth-observing THz satellite for studying the climate and space-weather effects in the upper atmosphere.

The same THz instrumentation would allow ultra-precise monitoring of chemical processes in industry or in research laboratories. Trace gases will be revealed, together with the composition of highly-reactive gas species, which cannot be distinguished reliably using existing techniques. Key examples include allowing manufacturers to ensure that vehicles meet strict new emission targets (Euro 6) or allowing atmospheric scientists to understand chemical processes that occur in the upper atmosphere, providing critical missing pieces of information needed to model the Earth's changing climate.

I shall also bring these high-sensitivity techniques into relevant application environments for the first time, through the development of a compact and portable "industrial evaluator" system, containing the complete optical, electronic and cooling systems needed for high-precision THz gas sensing. I will undertake a series of industrial placements with my project partners in the STFC RAL Space division to prove key applications within long-path atmospheric gas sensing. Furthermore, I will build a wider network of academic and industrial beneficiaries to establish and sustain THz gas-sensing as a UK strength within analytical chemistry. Through this, I will initiate the first commercialisation of this new technology.

This programme is designed to have considerable impact far beyond Academic research and will be sustained beyond the duration of my Fellowship. The beneficiaries of this work include:

1) The UK Space Industry: The UK Industrial Strategy aims to grow this sector to 10% of the £400bn global market by 2030, and Climate applications represent a potential return of >£1bn to UK-based suppliers. This programme will demonstrate key THz components with applications in atmospheric research. This will accelerate their technology readiness level (TRL), enabling their use in space missions such as the LOCUS Upper Atmospheric Satellite, and the TARDiS International Space Station instrument. Specifically, high-precision THz gas sensing will play a key part in international efforts to understand and manage climate change through at least two routes: (i) The first direct mapping of reactive upper-atmospheric species (e.g., O, OH), giving a sensitive indicator of climate change effects. (ii) The first laboratory measurements of short-lived upper-atmospheric species (peroxy radicals and Criegee intermediates) that dictate "greenhouse gas" lifetimes. Earth Observation data and improved climate models will reduce uncertainty in scientific assessment of future emission pathways and climate risk (e.g. through the Intergovernmental Panel on Climate Change Assessment Reports) thereby supporting policy-makers in developing strategies to meet the Paris Agreement targets.

2) Vehicle manufacturers, environmental regulators and testing services: Environmental legislation imposes strict limits on vehicle emissions (e.g., CO). As such, manufacturers and regulators require technology for accurate trace-gas measurements to ensure compliance with targets. The latest European standards (Euro-6) already challenge the spectral-resolution capability of conventional emission-measurement systems. The THz sensors developed in this work will yield environmental impact by providing the sensitivity and resolution needed for such measurements and enable stricter future targets to be monitored. This will impact on the automotive industry by enabling production of commercial portable emissions measurement systems, and their use by testing services.

3) Chemical and process industry: THz sensors will enable precise analysis of gases in industrial process control, e.g., within the petrochemical, biofuels and manufacturing sectors. By monitoring key gas species in emissions, or within reaction vessels, processes can be made cleaner, and more efficient, leading to lower wastage and improved yields, benefiting industry and in turn the UK economy. Urban populations will benefit from the above, through improved air quality and ensuing health benefits.

4) Public health: The development of THz biomedical diagnostics will help improve healthcare. For example, rapid exhaled breath analysis has the potential to replace invasive and slow blood tests or biopsies, with key gas species having been identified as markers for many conditions including asthma, diabetes and gastro-intestinal diseases.

5) Public safety and security: The use of THz trace-gas sensors will enable detection of explosives or chemical agents. Within industrial safety, hazardous gas species (e.g., CO or H2S) will be detectable in factories, mines or confined areas at safe stand-off distances.

Additional economic benefits may be realised through ultra-fast THz transceivers for satellite-to-satellite communications or low-latency links for high-frequency financial trading.