Despite numerous potential application areas, the terahertz frequency (1-10 THz) region in the electromagnetic spectrum still remains relatively unharnessed -because of the lack of powerful, compact and coherent sources working at room temperature. This project is dedicated to developing a compact solid state semiconductor laser based on quantum cascade technology operating in the terahertz (THz) frequency range. The main emphasis has bee.!1_~.!LJh~ ._' ~~-,---,---,----------- --~--,-----~---~------~-----~- ---- .-- --_._-----.. _--~. --------- establishment and optimisation of a new molecular beam epitaxy (MBE) facility, based on the GaAs/AIGaAs material system, for growing high quality THz quantum cascade lasers (QCLs). The initial focus of this thesis is on the setting up of the new MBE system at Leeds. Pyrometric interferometry was used as the principal technique for calibrating the gallium and aluminium growth rates. The chamber quality ,(cleanliness) was assessed from electron mobility measurements of twodimensional electron gas (2DEG) test samples. This thesis describes results from two growth campaigns. Although the first growth campaign did not give high electron mobility owing to a poor batch ofgallium, -the material grown helped to develop and optimise the standard growth and single-metal fabrication procedures for THz QCLs at Leeds. The second growth campaign gave 2DEG electron mobilities > Ixl06 cm2 y-Is-1 (after illumination) at 1.5 K and subsequently led to a commensurate five-fold increase in the laser output powers. A record maximum operating t~mperature was achieved Jor a THz QCL with a three-well active region design using a novel copper-copper metal-metal waveguide. The work then investigated the effect ofdifferent waveguides (semiinsulating surface-plasmon compared with metal-metal),' barrier thicknesses, growth temperature and VillI group ratios on the performance of a threequantum- well active region THz QCL. For the first time, electrical tunability of terahertz quantum cascade laser was achieved by changing the thickness of successive active region periods during the growth. This was undertaken using a three-quantum-well active region design. The emission frequency covered a range from 2.9 THz to 3.45 THz in a single device. The work also investig<;tted the effect on device performance, offlux drifts through the active region. Pyrometric interferometry was used, for the first time, to monitor and then estimate the thickness of the actual material depo~ited during a QCL growth. 75% of the THz QCLs grown were estimated to be within ±2% deviation from the actual design. Comparing pyrometric data between two nominally identical wafers, it was seen that a 4.5% thicker active region resulted in a 5% decrease in the lasing frequency in a bound-to-continuum QCL structure. This thesis concludes with the description of a number of different techniques that were implemented (working in collaboration) for improving the mode profile and output coupling ofa THz QCL. A discussion ofpossible future ___~r~~~_arch _~ir~~!ions ._ ts. th~11. _giY~I1, _tog~theL~ith._a_.few preliminary_results__ .__. .. __ . associated with broadband/multi-coloured THz QCLs, novel staircase THz QCL designs (based on a 45% Al mole fraction), and three-well active region designs incorporating a two-phonon depopulation scheme.