Since their first demonstration in 2002, terahertz (THz) quantum cascade lasers (QCLs) have proven to be one of the most reliable sources of this type of radiation. Advances have been made in increasing maximum operating temperature, shaping the beam and tuning the frequency and bandwidth. Waveguide engineering has been a particular area of interest due to the large wavelength of the THz radiation (> 100 μm). This thesis introduces several improvements related to THz QCL waveguide technology.

We begin by describing the THz radiation range, its applications and available sources. We then focus on the theoretical framework used to describe and design QCLs. We also review the two main waveguides employed in THz QCLs: the surface plasmon (SP) and the metal-metal (MM) waveguides. We outline the main fabrication steps for both waveguides.

Firstly, a novel photonic crystal-based QCL is presented. It comprises a defect line in a triangular lattice of active region (GaAs/AlGaAs) pillars embedded in a polymer matrix. The defects are made of pillars larger than the lattice pillars. Lasing is possible in this device on specific frequency levels (defect modes) within the band gap of the photonic crystal. Emitted frequency can be finely tuned by changing the size of the pillars in the lattice. A non-linear QCL geometry (a 90° junction), which allows for arbitrary emission direction, is also presented. Such waveguides may play an important role in integrated THz circuits.

Secondly, a new hybrid plasmonic waveguide for THz QCL is presented. It employs a polymer, benzocyclobutene (BCB), which solidifies upon heating and acts as a bonding agent. This waveguide serves as an alternative to MM and allows for arbitrarily thick metal deposition below the active region. Simulations are presented with an aim to explain the observed beam shape. For two different active regions and different thicknesses of bottom gold layer, a full light-current-voltage, spectral, and far-field characterisation is presented. The BCB-bonded device may be another element crucial in developing integrated THz elements on chip as it is more flexible than the rigid, substrate-based MM waveguide fabrication.

Thirdly, another novel concept of a THz QCL waveguide is presented, which employs etched pockets in the substrate directly below the active region. The air pockets allow for reducing the bottom plasmon layer doping and improving the ratio between the mode overlap (with the gain medium) and waveguide loss. The process development is presented, as well as simulations corroborating the hypothesis that this device has a potential to outperform surface plasmon waveguides.

Finally, potential applications of this work are discussed and suggestions are made for further work that could be done to improve the presented concepts.