PhD Studentship: Real-Time Gas Sensing Using Terahertz Quantum-Cascade Lasers

This project will develop fast detection systems to provide the first real-time gas sensing in the terahertz (THz) band of the electromagnetic spectrum.

The THz band lies between the infrared and microwave regions and represents a meeting between electronic and optical technologies.  Although numerous potential applications for THz sensing exist, including atmospheric and space research, security and biomedical imaging, and industrial inspection, there has been limited practical use of THz systems outside specialised laboratories.

One key reason for this is the reliance on relatively slow thermal detectors to measure and analyse THz signals.  These are inadequate for studying rapidly changing systems, such as chemical reactions.  They are also highly susceptible to background thermal noise, which limits the accuracy and dynamic range of measurements.  In this project, the student will develop new high-speed THz gas-sensing techniques, taking advantage of recent developments in fast THz detector technology.

They will initially demonstrate THz spectroscopy using a multi-pass optical cavity, enabling THz waves to pass many times through gases, improving sensitivity by a factor of ~100.  They will integrate a fast, and sensitive TeraFET detector into this system, in collaboration with Goethe University Frankfurt, to detect and analyse rapidly changing chemical concentrations for the first time.  They will then integrate an ultraviolet laser or flashlamp into the system to "trigger" photochemical reactions, and analyse the reaction processes on sub-microsecond timescales.

The studentship enables a step-change in the speed and accuracy of THz sensing and imaging techniques, which have historically been limited by the availability of good THz detectors. Ultimately, this will allow direct THz analysis of chemical reactions for the first time.  This would provide key missing links in atmospheric chemistry (e.g., impact of volatile-organic compounds on the lifetimes of greenhouse gases), which currently introduce order-of-magnitude uncertainties in climate models.  There are far wider potential impacts though, in terms of high-speed industrial emission monitoring and control, video-rate biomedical sensing, and the first potential satellite deployment of fast and low-noise THz receiver systems.

Key objectives include:

Real-time analysis of chemical reactions using THz radiation: The PGR will use a broadband TeraFET detector to undertake analysis of gas reactions (e.g., deuteration of methanol) and flux rates (e.g., ammonia concentrations), leading ultimately to development of the first UV-pump/THz-probe reaction studies of atomic oxygen concentrations as an analytical chemistry technique.

Multi-pass THz gas analysis: The PGR will employ a bespoke multi-pass gas cell, allowing two orders of magnitude improvement in the sensitivity of gas measurements.  This will, for the first time, allow THz analysis of trace reactive gases, at the concentrations found within the Earth’s upper atmosphere, allowing reaction products to be studied to high precision.

UV pump-THz probe photolysis studies: The student will use ultraviolet-pumped photolysis techniques to study the reaction pathways of important atmospheric gas-phase species.  This will allow the fate of volatile organic compounds in the Earth’s upper atmosphere, and their impact on climate to be determined.

The project will include working closely with our UK and international project partners, with the opportunity to travel to partner sites and international conferences.

Please state your entry requirements plus any necessary or desired background

First or Upper Second Class UK Bachelor (Honours) or equivalent in Engineering, Chemistry, Physics, or a related discipline, and must be able to demonstrate successful experience in independent technical or scientific project work. 

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