Since the advent of Terahertz (THz) quantum cascade lasers (QCLs) in 2002, they have seen significant improvement in output power, operating temperature, and spectral coverage as well as having been widely applied to astronomical applications, imaging, and spectroscopy because of their high output power and narrow spectral linewidth. In this work, three techniques, self-mixing gas spectroscopy, THz amplitude modulation and THz nearfield imaging, will be experimentally studied and demonstrated. These techniques can either expand or increase the efficiency of their corresponding applications using other techniques. The dissertation starts with a brief introduction of the basic knowledge of THz QCLs and some characterization methods for a THz system, as well as a summary of their main applications and their current capabilities. When the lasing of a THz QCL is suppressed, it becomes a quantum cascade amplifier (QCA), which compared with a THz QCL can enhance the sensitivity in both spectroscopy and imaging applications. A THz QCA has been achieved by adhering an antireflection-coated Si lens to the QCL facet and is then used as the source for THz gas spectroscopy based on a self-mixing effect, in which the THz QCL functions as both the light source and the detector. This offers an approach to achieving a compact spectroscopy system with a fast response. For spectroscopy, communication and astronomical applications, the ability to actively control the amplitude of the THz radiation is desired. This has been demonstrated with a graphene loaded metamaterial device, with which and a PID feedback loop, the output power fluctuation of a THz QCL is reduced from 1.52% to 0.043% of the total power. An amplitude stabilised THz source is also essential for high resolution THz imaging, where amplitude fluctuations will distort the acquired THz images. A scattering type nearfield microscope (s-SNOM) with a better than λ/1000 resolution has been demonstrated with a tuning fork based atomic force microscope (AFM) and a partially suppressed THz QCL. The detection scheme is also harnessing the self-mixing effect. Compared with conventional detection methods, this can give a fast response and high sensitivity, which are essential for high-speed high-resolution imaging. The performance of this home-built THz s-SNOM has then been significantly improved by vibration isolation and electronic noise reduction and it has been used to examine a variety of samples. It is able to reveal the plasmonic resonance of resonant structures, spatially map the electric field distribution on a metamaterial device and image subsurface plasmons. By using tips of different materials, it has also been found that a gold coating can improve the THz sensitivity of the system. Afterwards, to optimise the design of metamaterials, a special metasurface has been designed to study the influence of the geometric parameters on the optical performance of it. This can be achieved by probing the electric field distribution of the metasurface with the THz s-SNOM. The dissertation is then concluded with all the results obtained and a brief overview of what can be done in the future in related research fields.