Spectroscopy of Atmospheric Processes using a Terahertz Quantum Cascade Laser

Results from the National Aeronautics and Space Administration (NASA), Cosmic Background Explorer, Diffuse Infrared Background Experiment, and examinations of the spectral energy distributions of galaxies provides evidence that up to 98 % of photons emitted after the big bang are observable in the FIR/THz region. As a result of this, there has been an increasing amount of interest in and research into the THz frequency range in the last few decades. This has lead to rapid development in manufacturing efficient and compact THz sources and detectors that can better help our understanding of the chemical workings of our galaxy.

Through laboratory measurements, we can not only obtain the spectrum of a molecule, but we can also look at the kinetics and different reaction pathways of a range of molecular and atomic species in a controlled way. We are able to mimic the conditions typically found in our atmosphere and from this, gather more realistic and detailed data for atmospheric models.

The aim of this project was to develop instrumentation for the purpose of gas sensing in the THz region, with a particular focus on it being used for atmospheric and climate change studies. This project involved designing and building a THz quantum cascade laser (QCL) gas spectroscopy system that can achieve high enough sensitivity to allow for trace gas detection with potential future satellite applications. This required multiple experimental systems to be developed as various different spectroscopic methods were tried and tested to optimise the system to the best of its ability. It is hoped that the research in this project will contribute to the successful progression towards the first portable compact sensitive spectrometer with a THz QCL.

In this thesis, the development of a THz gas spectrometer that utilised self-mixing (SM) interferometry and later on, Michelson interferometry is presented. Both systems employed a multimode block integrated THz QCL and were successful in measuring the QCL emission spectrum between 3-4 THz. In addition, the singular species D2O and CH3OH were measured, as well as the first mixture of gaseous species with a THz QCL using SM interferometry.

Also presented in this work is the detailed characterisation of three QCLs, two at 3.4-THz and one at 4.7-THz. These QCLs were integrated into a copper block in order to improve the beam quality for atmospheric and space research purposes. The measurements in this thesis provided a thorough insight into the operating conditions and beam pattern of the QCLs, and generated key findings that will aid in future developments in employing this integration technique.