Terahertz-frequency sensors for atmospheric chemistry and space research (renewal)

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, for example, via the ESA Earth Explorer 12 (or 13) programmes. Without this, the potential for a UK researcher to lead the world in this emerging area will be lost.

In this fellowship, I am overcoming limitations of THz gas sensors by developing high-sensitivity systems based on quantum-cascade lasers (QCLs) - highly compact sources of THz radiation, which yield >1000 times the power of any similar-sized device. I have developed new project partnerships to exploit extremely fast and stable TeraFET detectors, enabling tiny changes in gas concentrations to be measured in real time. Unlike previous THz-QCL-based gas-sensing schemes, I am developing high-precision analytical chemistry techniques, and have developed the first custom-made multi-pass gas cell in which THz radiation passes repeatedly through the gas under study, yielding an estimated 100x improvement in sensitivity.

In this Renewal phase of the fellowship, I will adapt my internationally-leading THz gas sensing instrumentation to use an ultraviolet (UV) laser to simulate the behaviour of gases in the upper atmosphere, and "trigger" chemical reactions at a precise time. This will allow me to study the behaviour of volatile organic compounds (VOCs) such as formaldehyde as they react in the atmosphere, and resolve the huge uncertainties in the effect of these reactions on climate change. By developing fast detection schemes, I will provide the means to study the concentrations of industrial and agricultural pollutants in real time, and I will investigate the potential for UV-pump/THz-probe "step-scan" detection technique to probe the dynamics of upper-atmospheric reactions on microsecond timescales.

Through my partnership with RAL Space, I have demonstrated the world's first integration of THz QCLs with precision-micromachined waveguides, antennas, and "on-chip" stabilisation subsystems. This Renewal phase will provide a further step-change in capability, by developing the first satellite-compatible THz laser stabilisation schemes, through the use of integrated power and frequency control systems, within "sugarcube"-sized satellite-compatible modules. I will work with RAL Space, and TK Instruments to demonstrate this capability within a space-qualified cryocooler, including bespoke THz optics and calibration targets, on a satellite-test "breadboard", underpinning its future deployment on a satellite platform.

To sustain my research vision, and establish THz sensing as a key tool for atmospheric and space research, I will work closely with my project partners to secure follow-on funding for THz chemistry, Earth observation and critical satellite payload instrumentation. I will produce a roadmap for in-orbit deployment, and commercialisation, including developing the science and technology case for a European Space Agency satellite mission through the Earth Explorer programme.