Terahertz quantum-cascade laser instrumentation for high-precision gas spectroscopy

The terahertz (THz) band of the electromagnetic spectrum lies between infrared and microwave frequencies and offers unique capabilities for gas spectroscopy. These include the ability to study the reactions underpinning climate-change phenomena in the upper atmosphere, or to observe the mechanisms occurring within star and planet-forming nebulae far beyond our Solar system.
Until recently, however, there has been a lack of suitable instrumentation for trace gas sensing at THz frequencies. The output power of electronic oscillators is extremely low at > 1 THz, while conventional semiconductor lasers are limited to infrared wavelengths by the bandgaps of the materials. This project will overcome these limitations by exploiting THz quantum-cascade lasers (QCLs) - compact, yet powerful sources of THz radiation, based on intersubband transitions within multiple quantum-well structures.
To date though, THz-QCL gas spectroscopy has been based on simple, direct transmission schemes, which do not provide the sensitivity, resolution or speed needed for studying atmospheric reactions. This project will address these challenges by translating highly sensitive laser-spectroscopy techniques from the infrared to the THz band for the first time and demonstrating their capability to resolve key atmospheric gases. This work will underpin a new field of analytical chemistry, with potential impact in climate science, and wider fields including clean combustion, clinical breath analysis, plasma diagnostics and industrial process control.
This will deliver, for the first time, the ultra-high sensitivity, frequency precision and high speed needed for studying atmospheric reactions in the laboratory. Key objectives to be delivered will include:
1. Development of multi-pass optical cavities and detection schemes, enabling THz waves to pass many times through atmospheric gases, improving the sensitivity of analysis by a factor of ~100.
2. Development of spectroscopy schemes using a QCL, locked to a frequency-comb source, allowing part-per-trillion frequency resolution, and analysis of complex gas mixtures.
3. Closing the "THz gap" via in-depth studies of key atmospheric gases using widely tunable THz photomixers, providing the first detailed analysis of spectral fingerprints in the 1-3 THz band.
This project is inherently interdisciplinary, coupling high-frequency electronic and optical system design with sensitive analytical chemistry techniques. Multi-pass spectroscopy will be undertaken in collaboration with a project partner, UKRI-STFC Rutherford Appleton Laboratory (RAL), while applications in atmospheric chemistry will be supported by ongoing engagement with UAFs in the Atmospheric & Planetary Chemistry group within the School of Chemistry.
Year 1 will focus on development of custom cavities and gas cells, and spectral analysis of stable atmospheric species. Year 2 will develop multi-pass spectroscopy apparatus, with potential to collaborate with the RAL laser spectroscopy group. Frequency-locked spectroscopy schemes will be developed in Years 2 and 3, underpinning analysis of trace atmospheric species. 3-4 articles in primary archival journals (IEEE Trans. THz Sci. Technol., Opt. Lett., Phys. Chem. Chem. Phys.) can be expected, focusing on (i) multi-pass THz spectroscopy, (ii) frequency-locked THz spectroscopy, (iii) THz spectral analysis of key atmospheric species and (iv) THz analysis of atmospheric reaction products. This work will support and strengthen a UKRI Future Leaders Fellowship and leverage the outputs of the EPSRC "Hyper-THz" programme grant.