The development and applications of terahertz quantum cascade lasers

Terahertz frequency quantum cascade lasers (THz QCLs) are compact, semiconductor sources of coherent THz radiation, and have numerous potential applications in chemical sensing and industrial inspection, as well as security and biomedical imaging. In this thesis, the development of QCLs as sources of THz radiation is explored, together with their application in self-mixing (SM) imaging systems.
The effect of reducing the etch depth of the THz QCL active region was explored, and its influence on QCL performance evaluated. This was aimed of improving the thermal management in QCLs, as well as optimizing the structures for integration with electrical photonic components.
The reliance of THz QCLs on slow and incoherent thermal detectors has limited their practical use in THz systems. This was addressed by using a THz QCL as both the radiation source and an interferometric detector. THz sensing and imaging through SM interferometry in a QCL was demonstrated, in which radiation is reflected from an object back into the QCL cavity, causing changes in the laser properties, depending on the amplitude and phase of the reflection. This allows simple, ‘detector-free’, sensing of displacement and reflectivity, with high-sensitivity owing to the coherent nature of the detection. The equivalence between SM-perturbations to the THz power and the laser voltage was shown. Owing to the high SM sensitivity, high-resolution stand-off imaging at round-trip distances of up to 21 m through air was demonstrated - the longest range interferometric sensing with a THz QCL to date. Coherent three-dimensional (3D) terahertz imaging through SM in a THz QCL was also performed, in which the surface height was extracted from the phase of the SM signal.
To achieve tunable single mode THz QCL emission, which is highly beneficial for imaging and sensing applications, surface acoustic waves (SAWs) propagation across the sloped etched facets of a QCL mesa was demonstrated. The work also investigated the effect on device performance of SAW propagation over the QCL active region. This demonstration could pave the way for monolithic integration of QCLs into terahertz circuits.