Terahertz technology has numerous potential applications in trace-gas analysis, and in atmospheric and space research. A new Earth observation system satellite concept, ‘Keystone’, has been proposed, which aims to explore and study the distribution of key gas species in the Earth’s upper atmosphere using terahertz-frequency (THz) heterodyne spectrometers, based upon quantum cascade lasers (QCLs) as compact, yet powerful local oscillators (LOs).
In this work, THz QCLs have been fabricated and characterized, in both semi-insulating single plasmon and double metal waveguides. Their performance has been assessed against the requirements for a spaceborne LO in terms of power, operating temperature, and their spectral resolution. Improvement of the output power and operating temperature has been achieved through the development of a high reflectivity coating applied to the QCL rear facet, and a silver-based waveguide.
The second part of this work focuses on the integration of the QCLs with other components of the THz detection system. This has been done through electromagnetic-field analysis of a THz QCL integrated with a mechanically micro-machined waveguide cavity and diagonal feedhorn. A hybrid finite-element/Fourier transform approach enables analysis of both the near-field and far-field regions and is shown to agree well with experimental observations. The far-field antenna patterns show enhancement of the beam profile when compared with an unmounted QCL, in terms of beam divergence and side-lobe suppression ratio. Furthermore, we demonstrate integration of the QCL with dual diagonal feedhorns, enabling simultaneous access to both facets of the QCL, underpinning future integration with a satellite-based receiver and frequency-stabilization subsystem.
