High-Resolution Terahertz Gas Spectroscopy

Earth’s climate is governed by the atmosphere which is a complex multi-phase
system with natural and anthropogenic contributions. One of the greatest challenges scientists of this generation face is better understanding the many factors
which influence Earth’s climate and weather. Complex reactions take place in
Earth’s upper atmosphere which is dominated by Ultraviolet (UV) radiation
where highly reactive radicals such as atomic oxygen are created by photolysis
of ozone and carry chemical potential energy before releasing heat in exothermic
reactions.
Atomic oxygen was not directly observable using field-based systems until
recently due to its spectral features lying within the Terahertz (THz) range and
instead intermediates in the exothermic reactions have been tracked to assume
the concentrations. The THz spectrum was known historically for difficulties
in fabricating coherent sources and detectors, but in recent decades the development of THz Quantum Cascade Laser (QCL) sources has made the spectrum
attractive for many technologies including astronomy and Earth observation.
Laboratory experiments in the THz region allow us to sample the spectrum
of a molecule and track the kinetics of a reaction by controlling parameters.
Fast and sensitive THz detection is needed to ensure that any short-lived or
low-concentration intermediate states are accurately recorded.
This project aims to further develop an already existing THz gas spectrometer which uses a QCL source and focuses on atmospheric gases. This work
was carried out by characterising a 3.4 THz QCL, measuring errors within the
system, characterising a fast THz detector and developing a multi-pass gas cell
to increase the minimum detection concentration.
The characterisation of a 3.4 THz QCL is presented which includes the
Light-Current (LI) characteristic and spectral information from Michelson interferometry. The results from the interferometry are then compared to known
spectral features from methanol (MeOH) spectra in the THz region to accurately acquire the QCL emission frequency. A fast room temperature THz
detector using Field-Effect Transistor (FET) technology (TeraFET) is characterised which is part of the Interest collaboration between the University of
Leeds and Goethe University Frankfurt. A QCL measured the noise characteristic of an array of FET detectors and tested the modulation bandwidth which
exceeded 100 Megahertz (MHz) modulation bandwidth. This chapter demonstrated the first investigation into fast modulation of THz signals utilising an array TeraFET detector.
Gas spectroscopy results taken throughout this thesis have been used to
calibrate and benchmark the system throughout development. MeOH and D2O
were used as calibration gases to obtain the lasing frequency and test the lowest
detectable concentration. Using a TeraFET detector allowed a high number
of averages and reduced noise floor, revealing previously undocumented MeOH
absorption features. After static measurements, deuterated methanol (MeOD)
was sampled in the gas cell which is an undocumented species in this spectrum.
The first spectra of MeOH and deuterated methanol at 3.4 THz are presented
in this work along with the first observation of a H/D exchange in methanol in
the 3.4 THz spectrum.
A multi-pass Herriott cell has been designed and fabricated to improve
the sensitivity of the spectrometer. By increasing the number of passes, low-concentration gases such as intermediates in a chemical reaction will be easier
to track due to an increased absorption strength. Additionally, the multi-pass
optics are set up for future photolysis experiments by including a path for UV radiation.
A Photonic Integrated Circuit (PIC) approach to stabilise the output power
of a QCL is presented. The device is a Racetrack Resonator (RTR) which
was designed and fabricated by Iman Kundu for QCL power modulation. The
RTR work in this thesis shows a control loop using both Proportional Integral
(PI) and Proportional Integral Derivative (PID) control to stabilise the power
of a 3.4 THz QCL for over 10 minutes, which is longer than a typical Earth
observation integration period. Utilising a PIC reduces weight and complexity
in a space mission where weight is critical in a satellite, this approach shows a
simple power modulation system without the need for additional optics.