| Summary: | THz QCLs, based on a three-well resonant-phonon depopulation active region design, have been studied in depth using semi-insulating surface plasmon waveguides. Lasing frequencies between 2.3 to 4.0 THz have been investigated, together with a detailed evaluation of the effect of cavity length on devices performance. The effect of reducing the thickness of the THz QCL active region from 10 to 5 IJm was investigated, and the influence of active region thickness on lattice temperatures compared. Thinner active regions potentially make it possible to obtain continuous-wave (CW) operation from three-well, resonant-phonon depopulation QCLs, which is a requirement for many applications. It was demonstrated that the laser cavity of edge-emitting THz QCLs could be defined using an etched rather than a cleaved facet. Three emission frequencies (- 3.0, - 3.5 and - 4.0 THz) with two different facet angles (et:> = - 50° and et:> = - 42°) were studied and device performance were compared. The new THz QCL devices with sloped facets could open up the possibility of monolithic integration of QCLs into terahertz circuits. They also remove the requirement for THz QCLs to be fabricated perpendicular to cleavage planes of the underlying crystal structure. From the demonstration of lasing from THz QCLs with angled facets, the possibility of coupling surface acoustic waves (SAWs) with THz QCLs is investigated. The aim was to use SAWs propagating through upper surface of the waveguide of the THz QCL to introduce modulation of the gain region. This would have significant application to spectroscopy at THz frequencies. SAW propagate across a QCL mesa was demonstrated, and fully investigated devices were fabricated and tested.
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